Hume Causation Essay

1. Life and Works

Born in Edinburgh, Hume spent his childhood at Ninewells, his family's modest estate in the border lowlands. He came from a “good family” (MOL 2)—socially well connected but not wealthy. His father died just after David's second birthday, leaving him and his elder brother and sister in

the care of our Mother, a woman of singular Merit, who, though young and handsome, devoted herself entirely to the rearing and educating of her Children. (MOL 3)

Katherine Falconer Hume realized that David was uncommonly precocious, so when his older brother went up to Edinburgh University, Hume went with him, although he was only 10 or 11. There he studied Latin and Greek, read widely in history and literature, ancient and modern philosophy, and also did some mathematics and natural philosophy—what we now call natural science.

The education David received, both at home and at the university, aimed at training pupils to a life of virtue regulated by stern Scottish Calvinist strictures. Prayers and sermons were prominent aspects of his home and university life. At some point, Hume read The Whole Duty of Man, a widely circulated Anglican devotional tract that details our duties to God, our fellow human beings, and ourselves.

Hume's family thought him suited for a legal career, but he found the law “nauseous”, preferring to read classical texts, especially Cicero. He decided to become a “Scholar and Philosopher”, and followed a rigorous program of reading and reflection for three years until “there seem'd to be open'd up to me a new Scene of Thought” (HL 3.2). The intensity of developing his philosophical vision precipitated a psychological crisis in the isolated scholar.

The crisis eventually passed, and Hume remained intent on articulating his “new Scene of Thought”. As a second son, his inheritance was meager, so he moved to France, where he could live cheaply, and finally settled in La Flèche, a sleepy village in Anjou best known for its Jesuit college where Descartes and Mersenne had studied a century before. Here he read French and other continental authors, especially Malebranche, Dubos, and Bayle, and occasionally baited the Jesuits with arguments attacking their beliefs. By this time, Hume had not only rejected the religious beliefs with which he was raised, but was also opposed to organized religion in general, an opposition that remained constant throughout his life. In 1734, when he was only 23, he began writing A Treatise of Human Nature.

Hume returned to England in 1737 to ready the Treatise for the press. To curry favor with Joseph Butler (1692–1752), he “castrated” his manuscript, deleting his controversial discussion of miracles, along with other “nobler parts” (HL 6.2). Book I, “Of the Understanding”, and Book II, “Of the Passions”, appeared anonymously in 1739. The next year saw the publication of Book III, “Of Morals” as well as his anonymous “Abstract” of Books I and II.

The Treatise was no literary sensation, but it didn't fall “deadborn from the press” (MOL 6), as Hume disappointedly described its reception. Despite his surgical deletions, it attracted enough of “a Murmour among the Zealots” (MOL 6) to fuel his lifelong reputation as an atheist and a sceptic. When he applied for the Chair of Ethics and Pneumatical (“Mental”) Philosophy at Edinburgh in 1745, his reputation provoked vocal and ultimately successful opposition. Six years later, he stood for the Chair of Logic at Glasgow, only to be turned down again. Hume never held an academic post.

In 1745, he accepted a position as a young nobleman's tutor, only to discover that his charge was insane. A year later he became secretary to his cousin, Lieutenant General James St Clair, eventually accompanying him on an extended diplomatic mission in Austria and Italy.

In 1748, An Enquiry concerning Human Understanding appeared, covering the central ideas of Book I of the Treatise and his discussion of liberty and necessity from Book II. He also included material he had excised from the Treatise. In 1751, he published An Enquiry concerning the Principles of Morals, a “recasting” of Book III of the Treatise, which he described as “incomparably the best” of all his work (MOL 10). More essays, the Political Discourses, appeared in 1752, and Hume's correspondence reveals that a draft of the Dialogues concerning Natural Religion was also underway at this time.

An offer to serve as Librarian to the Edinburgh Faculty of Advocates gave Hume the opportunity to begin another project, a History of England, using the law library's excellent resources. Published in six volumes between 1754 and 1762, his History was a bestseller well into the next century, finally giving him the financial independence he had long sought. But even as a librarian, Hume's reputation as an atheist and sceptic dogged him. One of his orders for “indecent Books” prompted an unsuccessful move for his dismissal and excommunication from the Kirk. Friends and publishers persuaded him to suppress some of his more controversial writings on religion during his lifetime.

In 1763, Hume accepted a position as private secretary to the British Ambassador to France. During his three-year stay in Paris, he became Secretary to the Embassy, and eventually its chargé d'affaires. He became the rage of the Parisian salons, enjoying the conversation and company of famous European intellectuals. He was known for his love of good food and wine, as well as his enjoyment of the attentions and affections of women.

Hume returned to Edinburgh in 1769. He built a house in Edinburgh's New Town, and spent his autumnal years quietly and comfortably, dining and conversing with friends, not all of whom were “studious and literary”, for he also found that his “company was not unacceptable to the young and careless” (MOL 21). He spent considerable time revising his works for new editions of his Essays and Treatises, which contained his collected Essays, the two Enquiries, A Dissertation on the Passions, and The Natural History of Religion, but —significantly—not A Treatise of Human Nature.

In 1775, Hume was diagnosed with intestinal cancer. Before his death in 1776, he arranged for the posthumous publication of his most controversial work, the Dialogues concerning Natural Religion, and composed a brief autobiography, “My Own Life”. Although there was much curiosity about how “the great infidel” would face his death, his friends agreed that he prepared himself with the same peaceful cheer that characterized his life.

2. The relation between the Treatise and the Enquiries

In 1775, as he was readying a revised edition of his Essays and Treatises for the press, Hume sent his publisher an “Advertisement”, asking that it be included in this and any subsequent edition of his works. He complains that his critics focused “all their batteries” on the Treatise, “that juvenile work”, which he published anonymously and never acknowledged. He urges his readers to regard the Enquiries “as containing his philosophical sentiments and principles”, assuring his publisher that they provide “a compleat answer” to his critics.

Hume's apparent disavowal of the Treatise and his regard for the Enquiries raise a question about how we should read his work. Should we take his statements literally and let the Enquiries represent his considered view, or should we ignore his “Advertisement” and take the Treatise as the best statement of his position?

Both options presuppose that the differences between the Treatise and the Enquiries are substantial enough to warrant taking one or the other as best representing Hume's views, but there are good reasons for doubting this. Even in the “Advertisement”, Hume says, “Most of the principles, and reasonings, contained in this volume, were published” in the Treatise. He repeats his conviction that he was guilty of “going to press too early”, and that his aim in the Enquiries was to “cast the whole anew … where some negligences in his former reasoning and more in the expression, are … corrected”.

Hume's description of his aims suggests another option. Rather than repudiating the Treatise, perhaps his recasting of it represents a shift in the way he presents his “principles and reasoning” rather than a substantive change in what he has to say. He reinforces this option when he says of the first Enquiry that the “philosophical Principles are the same in both” and that “By shortening & simplifying the Questions, I really render them much more complete” (HL 73.2). He also comments in “My Own Life” that the Treatise's lack of success “proceeded more from the manner than the matter”—more from the structure of what he said than what he said (MOL 8). It is not unreasonable to conclude that Hume's recasting of the Treatise was designed to address this issue, which suggests that we might understand him best by reading both works, despite their differences, together.

3. Philosophical Project

As the title of the Treatise proclaims, Hume's subject is human nature. He summarizes his project in its subtitle: “an attempt to introduce the experimental method into moral subjects”. In his day, “moral” meant anything concerned with human nature, not just ethics, as he makes clear at the beginning of the first Enquiry, where he defines “moral philosophy” as “the science of human nature” (EHU 1.1/5). Hume's aim is to bring the scientific method to bear on the study of human nature.

Hume's study of philosophical “systems” convinced him that philosophy was in a sorry state and in dire need of reform. When he was only 18 years old, he complains in a letter that anyone familiar with philosophy realizes that it is embroiled in “endless Disputes” (HL 3.2). The ancient philosophers, on whom he had been concentrating, replicated the errors their natural philosophers made. They advanced theories that were “entirely Hypothetical”, depending “more upon Invention than Experience”. He objects that they consulted their imagination in constructing their views about virtue and happiness, “without regarding human Nature, upon which every moral Conclusion must depend”. The youthful Hume resolved to avoid these mistakes in his own work, by making human nature his “principal Study, & the Source from which I would derive every Truth” (HL 3.6).

Even at this early stage, the roots of Hume's mature approach to the reform of philosophy are evident. He was convinced that the only way to improve philosophy was to make the investigation of human nature central—and empirical (HL 3.2). The problem with ancient philosophy was its reliance on hypotheses—claims based on speculation and invention rather than experience and observation.

By the time Hume began to write the Treatise three years later, he had immersed himself in the works of the modern philosophers, but he found them disturbing, not least because they made the same mistakes the ancients did, while professing to avoid them. Why, Hume asks, haven't philosophers been able to make the spectacular progress in understanding human nature that natural philosophers—whom we now call “scientists”—have recently achieved in the physical sciences? His answer is that while scientists have cured themselves of their “passion for hypotheses and systems”, philosophers haven't yet purged themselves of this temptation. Their theories were too speculative, relying on a priori assumptions, and paying too little attention to what human nature is actually like. Instead of helping us understand ourselves, modern philosophers were mired in interminable disputes—evident even to “the rabble without doors”—giving rise to “the common prejudice against metaphysical reasonings of all kinds”, that is, “every kind of argument which is in any way abstruse, and requires some attention to be comprehended” (T xiv.3).

To make progress, Hume maintains, we need to “reject every system … however subtle or ingenious, which is not founded on fact and observation”. These systems, covering a wide range of entrenched and influential metaphysical and theological views, purport to have discovered principles that give us a deeper and more certain knowledge of ultimate reality. But Hume argues that in attempting to go beyond anything we can possibly experience, these metaphysical theories try to “penetrate into subjects utterly inaccessible to the understanding” (EHU 1.11/11), which makes their claims to have found the “ultimate principles” of human nature not just false, but unintelligible. These “airy sciences”, as Hume calls them, have only the “air” of science (EHU 1.12/12).

Worse still, these metaphysical systems are smokescreens for “popular superstitions” that attempt to overwhelm us with religious fears and prejudices (EHU 1.11/11). Hume has in mind a variety of doctrines that need metaphysical cover to look respectable—arguments for the existence of God, the immortality of the soul, and the question of the nature of God's particular providence. Metaphysics aids and abets these and other superstitious doctrines.

But he insists that because these metaphysical and theological systems are objectionable doesn't mean we should give up doing philosophy. Instead, we need to appreciate “the necessity of carrying the war into the most secret recesses of the enemy”. The only way to resist the allure of these pseudo–sciences is to engage with them, countering their “abstruse … metaphysical jargon” with “accurate and just reasoning” (EHU 1.12/12).

This means that the initial phase of Hume's project must be critical. A prominent part of this aspect of his project is to “discover the proper province of human reason”—determining the extent and limits of reason's powers and capacities (EHU 1.12/12). He believes that his investigation will show that metaphysics as the quest for understanding the ultimate nature of reality is beyond reason's scope.

Scholars once emphasized this critical phase at the expense of the rest of Hume's project, encouraging the charge that he was just a negative skeptic, who rejects the views of others without defending any positive position himself. But while he is indeed skeptical about the possibility of metaphysical insights that go deeper than science can, investigating the proper province of reason isn't only a critical activity. His critique of metaphysics clears the way for the constructive phase of his project—the development of an empirical science of human nature—and Hume is not at all skeptical about its prospects.

In his “Introduction” to the Treatise, Hume launches the constructive phase of his project by proposing nothing less than “a compleat system of the sciences, built on a foundation entirely new” (T xvi.6). The new foundation is the scientific study of human nature. He argues that all the sciences have some relation to human nature, “even Mathematics, Natural Philosophy, and Natural Religion” (T xv.4). They are all human activities, so what we are able to accomplish in them depends on understanding what kinds of questions we are able to handle and what sorts we must leave alone. If we have a better grasp of the scope and limits of our understanding, the nature of our ideas, and the operations we perform in reasoning about them, there is no telling what improvements we might make in these sciences.

We should expect even more improvement in the sciences that are more closely connected to the study of human nature: “Logic, Morals, Criticism, and Politics”. Many longstanding philosophical debates are about the nature of our ideas—causation, liberty, virtue and beauty—so getting clear about their content should help us cut through these “endless disputes”.

As the science of human nature is the only solid foundation for the other sciences, “the only solid foundation we can give to this science itself must be laid on experience and observation” (T xvi.7). Although Hume does not mention him by name, Newton (1642–1727) is his hero. He accepts the Newtonian maxim “Hypotheses non fingo”, roughly, “I do not do hypotheses”. Any laws we discover must be established by observation and experiment.

Hume is proposing an empiricist alternative to traditional a priori metaphysics. His empiricism is naturalistic in that it refuses to countenance any appeal to the supernatural in the explanation of human nature. As a naturalist, he aims to account for the way our minds work in a manner that is consistent with a Newtonian picture of the world.

Hume portrays his scientific study of human nature as a kind of mental geography or anatomy of the mind (EHU 1.13/13; T 2.1.12.2/326). In the first section of the first Enquiry, he says that it has two principal tasks, one purely descriptive, the other explanatory. Mental geography consists in delineating “the distinct parts and powers” of the mind (EHU 1.13/3). While everyone can make some sense of the basic distinctions among the mind's contents and operations, more fine–grained distinctions are harder to grasp.

Hume, however, wants to go much further. He wants to explain how the mind works by discovering its “secret springs and principles”. He reminds us that astronomers, for a long time, were content with proving the “motions, order, and magnitude of the heavenly bodies”. But then “a philosopher”—Newton— went beyond them and determined “the laws and forces, by which the revolutions of the planets are governed and directed” (EHU 1.15/14). Newton's example led other natural philosophers to similar explanatory successes. Hume believes he will be equally successful in finding the fundamental laws governing our “mental powers and economy”, if he follows the same caution Newton exhibited in carrying out his inquiries.

Newton's scientific method provides Hume with a template for introducing the experimental method into his investigation of the mind. In An Enquiry concerning the Principles of Morals, he says he will follow “a very simple method” that he believes will bring about a transformation in the study of human nature. Following Newton's example, he argues that we should “reject every system … however subtile or ingenious, which is not founded on fact and observation”, and accept only arguments derived from experience. When we inquire about human nature, since we are asking “a question of fact, not of abstract science”, we must rely on experience and observation (EPM 1.10/173–174).

As the fledgling Newton of the moral sciences, Hume wants to find a set of laws that explain how the mind's contents—perceptions, as he calls them—come and go in the mind and how simple perceptions combine to form complex perceptions in ways that explain human thought, belief, feeling and action.

Newton's achievement was that he was able to explain diverse and complex physical phenomena in terms of a few general principles. Like him, Hume proposes to explain “all effects from the simplest and fewest causes” (T xvii.8). He predicts that it is likely that one “principle of the mind depends on another” and that this principle may in turn be brought under another principle even “more general and universal” (EHU 1.15/15). But he emphasizes that while he will try to find the most general principles, rendering them as universal as possible, all of his explanations must be based completely on experience.

Although philosophy, as an empirical enterprise, is itself bound by experience, this is not a defect in the science of human nature. The same is true for all the sciences: “None of them can go beyond experience, or establish any principles which are not founded on that authority” (T Intro 10). Explanations must come to an end somewhere. When we see that we have “arriv'd at the utmost extent of human reason, we sit down contented”, for the only reason we can give for our most general principles is “our experience of their reality” (T 9).

Hume is Newtonian in much more than method. He sees that Newton is significantly different from John Locke (1632–1704) and the other Royal Society natural philosophers, because he rejects their mechanist picture of the world. Newton's greatest discovery, the Law of Gravitation, is not a mechanical law. Hume explicitly models his account of the fundamental principles of the mind's operations—the principles of association—on the idea of gravitational attraction. By appealing to these same principles throughout, Hume gives an explanation of these diverse phenomena that enable him to provide a unified and economical account of the mind.

4. Account of the Mind

To explain the workings of our minds with the economy Newton displayed in his physics, Hume introduces the minimal amount of machinery he thinks is necessary to account for the mind's operations. Each piece is warranted by experience.

The early modern period was the heyday of the investigation of the ideas of causation, moral good and evil, and many other philosophically contested ideas. Every modern philosopher accepted some version of the theory of ideas—the view that we immediately perceive certain mental entities called ideas, but don't have direct access to physical objects. Hume holds an empiricist version of the theory, because he thinks that everything we believe is ultimately traceable to experience.

He begins with an account of perceptions, because he believes that any intelligible philosophical question must be asked and answered in those terms. He uses perception to designate any mental content whatsoever, and divides perceptions into two categories, impressions and ideas.

Impressions include sensations as well as desires, passions, and emotions. Ideas are “the faint images of these in thinking and reasoning” (T 1.1.1.1/1). He thinks everyone will recognize his distinction, since everyone is aware of the difference between feeling and thinking. It is the difference between feeling the pain of your present sunburn and recalling last year's sunburn.

Hume distinguishes two kinds of impressions: impressions of sensation, or original impressions, and impressions of reflection, or secondary impressions. Impressions of sensation include the feelings we get from our five senses as well as pains and pleasures, all of which arise in us “originally, from unknown causes” (T 1.1.2.1/7). He calls them original because trying to determine their ultimate causes would take us beyond anything we can experience. Any intelligible investigation must stop with them.

Impressions of reflection include desires, emotions, passions, and sentiments. They are essentially reactions or responses to ideas, which is why he calls them secondary. Your memories of last year's sunburn are ideas, copies of the original impressions you had when the sunburn occurred. Recalling those ideas causes you to fear that you'll get another sunburn this year, to hope that you won't, and to want to take proper precautions to avoid overexposure to the sun.

Perceptions—both impressions and ideas—may be either simple or complex. Complex impressions are made up of a group of simple impressions. My impression of the violet I just picked is complex. Among the ways it affects my senses are its brilliant purple color and its sweet smell. I can separate and distinguish its color and smell from the rest of my impressions of the violet. Its color and smell are simple impressions, which can't be broken down further because they have no component parts.

Hume initially distinguishes impressions and ideas in terms of their degree of force and vivacity. Impressions are more forceful and vivacious than ideas. My impression of this ripe tomato's bright red color is as vivid as anything could be. Last year's tomatoes were just as vivid when I was looking at them, but now my idea of them is much less vivid than my impressions of the tomato in front of me. Since last year's tomatoes were the same color, the difference can't be that they are different shades of red; the difference must lie in the sharpness, clarity, and brightness of my impressions—their force and vivacity. At various times, Hume tries other ways of characterizing the difference between impressions and ideas, but he was never completely satisfied with them. Still, what he says works well enough to give us a handle on the felt differences between impressions and ideas.

When Hume distinguishes impressions and ideas in terms of their relative force and vivacity, he is pointing out something that is generally true of them as a matter of fact. On occasion, in dreams or a high fever, ideas may approach the force and vivacity of impressions, but these are exceptions that prove the—empirical—rule. In general, impressions and ideas are so different that no one can deny the distinction.

Although nothing seems freer than the power of thought, which isn't “restrained within the limits of nature and reality” (EHU 2.4/18), Hume insists that our imagination is in fact “confined within very narrow limits”. We can separate and combine our ideas in new and even bizarre ways, imagining creatures we've never seen or faraway galaxies, but all the materials of thinking are ultimately derived from our impressions. Since “all our ideas or more feeble perceptions are copies of our impressions or more lively ones”; we are restricted to “compounding, transporting, augmenting, or diminishing the materials afforded us by the senses and experience” (EHU 2.5/19).

4.1 The Copy Principle

In the Treatise, Hume qualifies his claim that our ideas are copies of our impressions, making clear that it applies only to the relation between simple ideas and simple impressions. He offers this “general proposition”, usually called the Copy Principle, as his “first principle … in the science of human nature”:

All our simple ideas in their first appearance are deriv'd from simple impressions, which are correspondent to them, and which they exactly represent. (T 1.1.1.7/4)

He presents the principle as something that everyone's experience confirms, but he also gives an argument to establish it.

He argues first that there is a one–to–one correspondence between simple ideas and simple impressions. He can't prove that this correspondence holds universally, since he can't examine every individual impression and idea. But he is so confident the correspondence holds that he challenges anyone who doubts it to produce an example of a simple impression without a corresponding simple idea, or a simple idea without a corresponding simple impression. Since he is certain they will fail, he concludes that there is a constant conjunction between simple impressions and simple ideas.

Next, he maintains that this constant conjunction is so universal that the correspondence can't be a matter of chance. There must be a causal connection between them, but do ideas cause impressions or do impressions cause ideas?

Finally, he argues that experience tells us that simple impressions always precede and thus cause their corresponding ideas. To support this claim, he appeals to two sorts of cases. First, if you want to give a child an idea of the taste of pineapple, you give her a piece of pineapple to eat. When you do, you are giving her an impression of the pineapple's taste. You never go the other way round. His other case involves a person born blind, who won't have ideas of color because he won't have impressions of color.

The Copy Principle is an empirical thesis, which he emphasizes by offering “one contradictory phenomenon” as an empirical counterexample to the principle. He imagines someone who has had the same sorts of experiences of colors most of us have had, but has never experienced a certain shade of blue. Hume thinks that if he orders all the shades of blue he has experienced from the darkest to the lightest, he will see immediately that there is a gap where the missing shade should be. Then he asks

Whether ‘tis possible for him, from his own imagination, to … raise up to himself the idea of that particular shade, tho’ it had never been convey'd to him by his senses? I believe there are few but will be of opinion that he can; and this may serve as a proof, that the simple ideas are not always deriv'd from the correspondent impressions; tho’ the instance is so particular and singular, that ‘tis scarce worth our observing, and does not merit that for it alone we shou'd alter our general maxim. (T 1.1.1.10/6)

Hume repeats the case of the missing shade almost verbatim in the first Enquiry. While scholars have wondered exactly how the person might supply the missing shade, he seems unconcerned with the details. For Hume, once again the exception proves the—empirical—rule.

4.2 Hume's Account of Definition

Although Hume's distinctive brand of empiricism is often identified with his commitment to the Copy Principle, his use of the principle's reverse in his account of definition is perhaps the more innovative element of his system.

As his diagnosis of traditional metaphysics reveals, Hume believes that

the chief obstacle … to our improvement in the moral or metaphysical sciences is the obscurity of the ideas, and ambiguity of the terms. (EHU 7.1.2/61)

Conventional definitions—replacing terms with their synonyms—merely replicate philosophical confusions and never break out of a narrow definitional circle. Getting clear about the content of the ideas and the meanings of the terms we are investigating requires something else.

Hume argues that we must “pass from words to the true and real subject of the controversy”—ideas. He believes he has found a way to accurately determine their content—his account of definition. He touts it as “a new microscope or species of optics”, predicting that it will produce equally dramatic results in the moral sciences as its hardware counterparts—telescopes and microscopes—have produced in natural philosophy (EHU 7.1.4/62).

Hume's account of definition uses a simple series of tests to determine cognitive content. Begin with a term. Ask what idea is annexed to it. If there is no such idea, then the term has no cognitive content, however prominently it figures in philosophy or theology. If there is an idea annexed to the term, and it is complex, break it down into the simple ideas that compose it, and trace them back to their original impressions. If the process fails at any point, the idea in question lacks cognitive content. When carried through successfully, however, it yields a “just definition”—a precise account of the troublesome idea's content.

Hume uses his account of definition in his critical phase to show that many of the central concepts of traditional metaphysics lack intelligible content. He also uses it in his constructive phase to determine the exact meaning of our terms and ideas.

4.3 Principles of Association

Although we are capable of separating and combining our simple ideas as we please, there is, nevertheless, a regular order to our thoughts. If ideas occurred to us completely randomly, so that all our thoughts were “loose and unconnected”, we wouldn't be able to think coherently (T 1.1.4.1/10). This suggests that

There is a secret tie or union among particular ideas, which causes the mind to conjoin them more frequently, and makes the one, upon its appearance, introduce the other. (Abstract 35)

Hume explains this “tie or union” in terms of the mind's natural ability to associate certain ideas. Association is not “an inseparable connexion”, but rather “a gentle force, which commonly prevails”, by means of which one idea naturally introduces another (T 1.1.4.1/10).

In the first Enquiry, Hume says that even though it is obvious to everyone that our ideas are connected in this way, he is the first philosopher who has “attempted to enumerate or class all the principles of association” (EHU 3.2/24). He regards his use of these “universal principles” as so distinctive that in the Abstract he advertises them as his most original contribution—one that entitles him to be called an “inventor” (Abstract 35).

Hume identifies three principles of association: resemblance, contiguity in time and place, and causation. When someone shows you a picture of your best friend, you naturally think of her because the picture resembles her. When you're reminded of something that happened in the 1960s—miniskirts, for example—you may think of the Vietnam War, because they are temporally contiguous. Thinking of Sausalito may lead you to think of the Golden Gate Bridge, which may also lead you to think of San Francisco, since they are spatially contiguous. Causality works both from cause to effect and effect to cause: meeting someone's father may make you think of his son; encountering the son may lead you to thoughts of his father.

Of the three associative principles, causation is the strongest, and the only one that takes us “beyond our senses” (T 1.3.2.3/74). It establishes links between our present and past experiences and our expectations about the future, so that “all reasonings concerning matters of fact seem to be founded on the relation of Cause and Effect” (EHU 4.1.4/26). Taking aspirin in the past has relieved my headaches, so I expect that the aspirin I just took will soon relieve my present headache. Hume also makes clear that causation is the least understood of the associative principles, but he tells us, “we shall have occasion afterwards to examine it to the bottom” (T 1.1.4.2/11).

Like gravitational attraction, the associative principles are original, and so can't be explained further. Although the associative principles' “effects are everywhere conspicuous” their causes “are mostly unknown, and must be resolv'd into original qualities of human nature, which I pretend not to explain”. Accordingly, we should curb any “intemperate desire” to account further for them, for doing so would take us illegitimately beyond the bounds of experience (T 1.1.4.6/12–13).

Hume doesn't try to explain why we associate ideas as we do. He is interested only in establishing that, as a matter of fact, we do associate ideas in these ways. Given that his claim that the associative principles explain the important operations of the mind is an empirical one, he must admit, as he does in the first Enquiry, that he cannot prove conclusively that his list of associative principles is complete. Perhaps he has overlooked some additional principle. We are free to examine our own thoughts to determine whether resemblance, contiguity, and causation successfully explain them. The more instances the associative principles explain, the more assurance we have that Hume has identified the basic principles by which our minds work.

In the Abstract, Hume concludes that it should be “easy to conceive of what vast consequences these principles must be in the science of human nature”. Since they “are the only ties of our thoughts, they are really to us the cement of the universe, and all the operations of the mind must, in great measure, depend on them” (Abstract 35). Just what these “vast consequences” are will become clear when we examine Hume's revolutionary accounts of our causal inferences and moral judgments.

5. Causation

The medieval synthesis Thomas Aquinas (1224–74) forged between Christian theology and Aristotle's science and metaphysics set the terms for the early modern causation debate. Aristotle (384–322 BCE) drew an absolute categorical distinction between scientific knowledge (scientia) and belief (opinio). Scientific knowledge was knowledge of causes and scientific explanation consisted in demonstration—proving the necessary connection between a cause and its effect from intuitively obvious premises independently of experience.

Modern philosophers thought of themselves as scientific revolutionaries because they rejected Aristotle's account of causation. Even so, they accepted his distinction between knowledge and belief, and regarded causal inference as an exercise of reason, which aimed at demonstrating the necessary connection between cause and effect. Malebranche (1638–1715), and others following Descartes (1596–1650), were optimistic about the possibility of demonstrative scientific knowledge, while those in the British experimental tradition were more pessimistic. Locke was sufficiently sceptical about what knowledge we can attain that he constructed one of the first accounts of probable inference to show that belief can meet standards of rationality that make experimental natural philosophy intellectually respectable.

When Hume enters the debate, he translates the traditional distinction between knowledge and belief into his own terms, dividing “all the objects of human reason or enquiry” into two exclusive and exhaustive categories: relations of ideas and matters of fact.

Propositions concerning relations of ideas are intuitively or demonstratively certain. They are known a priori—discoverable independently of experience by “the mere operation of thought”, so their truth doesn't depend on anything actually existing (EHU 4.1.1/25). That the interior angles of a Euclidean triangle sum to 180 degrees is true whether or not there are any Euclidean triangles to be found in nature. Denying that proposition is a contradiction, just as it is contradictory to say that 8×7=57.

In sharp contrast, the truth of propositions concerning matters of fact depends on the way the world is. Their contraries are always possible, their denials never imply contradictions, and they can't be established by demonstration. Asserting that Miami is north of Boston is false, but not contradictory. We can understand what someone who asserts this is saying, even if we are puzzled about how he could have the facts so wrong.

The distinction between relations of ideas and matters of fact is often called “Hume's Fork”, and is generally used with the negative implication that Hume may be illicitly ruling out meaningful propositions that don't fit into these two categories or fit into both of them. To defuse this objection, however, it is important to bear in mind that Hume's categories are his translations of a traditional absolute categorical classificatory distinction between knowledge and belief that all of his contemporaries and immediate predecessors accepted.

Hume's method dictates his strategy in the causation debate. In the critical phase, he argues that his predecessors were wrong: our causal inferences aren't determined by “reason or any other operation of the understanding” (EHU 5.1.2/41). In the constructive phase, he supplies an alternative: the associative principles are their basis.

Hume's contributions to the critical phase of the causation debate are contained in Treatise 1.3.6 and Section 4 of the first Enquiry, appropriately titled “Sceptical doubts concerning the operations of the understanding”. The constructive phase in his Enquiry account is the following section, also appropriately titled “Sceptical solution of these doubts”, while the corresponding sections of the Treatise stretch from 1.3.7 through 1.3.10.

5.1 Causal Inference: Critical Phase

Causal inferences are the only way we can go beyond the evidence of our senses and memories. In making them, we suppose there is some connection between present facts and what we infer from them. But what is this connection? How is it established?

If the connection is established by an operation of reason or the understanding, it must concern either relations of ideas or matters of fact.

Hume argues that the connection can't involve relations of ideas. Effects are different events from their causes, so there is no contradiction in conceiving of a cause occurring, and its usual effect not occurring. Ordinary causal judgments are so familiar that we tend to overlook this; they seem immediate and intuitive. But suppose you were suddenly brought into the world as an adult, armed with the intellectual firepower of an Einstein. Could you, simply by examining an aspirin tablet, determine that it will relieve your headache?

When we reason a priori, we consider the idea of the object we regard as a cause independently of any observations we have made of it. It can't include the idea of any other distinct object, including the object we take to be its usual effect. But then it can't show us any “inseparable and inviolable connection”—any necessary connection—between those ideas. Trying to reason a priori from your idea of an aspirin, without including any information you have of its effects from your previous experience, yields only the simple ideas that compose your idea of its “sensible qualities”—its size, shape, weight, color, smell, and taste. It gives you no idea of what “secret powers” it might have to produce its usual effects. Hume concludes that a priori reasoning can't be the source of the connection between our ideas of a cause and its effect. Contrary to what the majority of his contemporaries and immediate predecessors thought, causal inferences do not concern relations of ideas.

Hume now moves to the only remaining possibility. If causal inferences don't involve a priori reasoning about relations of ideas, they must concern matters of fact and experience. When we've had many experiences of one kind of event constantly conjoined with another, we begin to think of them as cause and effect and infer the one from the other. But even after we've had many experiences of a cause conjoined with its effect, our inferences aren't determined by reason or any other operation of the understanding.

In the past, taking aspirin has relieved my headaches, so I believe that taking aspirin will relieve the headache I'm having now. But my inference is based on the aspirin's superficial sensible qualities, which have nothing to do with headache relief. Even if I assume that the aspirin has “secret powers” that are doing the heavy lifting in relieving my headache, they can't be the basis of my inference, since these “secret powers” are unknown.

Nonetheless, Hume observes, “we always presume, when we see like sensible qualities, that they have like secret powers, and expect that effects, similar to those we have experienced, will follow from them” (EHU 4.2.16/33). Since we neither intuit nor infer a priori that similar objects have similar secret powers, our presumption must be based in some way on our experience.

But our past experience only gives us information about objects as they were when we experienced them, and our present experience only tells us about objects we are experiencing now. Causal inferences, however, do not just record our past and present experiences. They extend or project what we have gathered from experience to other objects in the future. Since it is not necessarily true that an object with the same sensible qualities will have the same secret powers that past objects with those sensible qualities had, how do we project those experiences into the future, to other objects that may only appear similar to those we've previously experienced?

Hume thinks we can get a handle on this question by considering two clearly different propositions:

  • (1) I've found that headache relief has always followed my taking aspirin;

and

  • (2) Taking aspirin similar to the ones I've taken in the past will relieve my present headache.

There is no question that “the one proposition may be justly inferred from the other”, and that “it is always inferred”. But since their connection obviously isn't intuitive, Hume challenges us to produce the “chain of reasoning” that takes us from propositions like (1) to propositions like (2) (EHU 4.2.16/34).

(1) summarizes my past experience, while (2) predicts what will happen in the immediate future. The chain of reasoning I need must show me how my past experience is relevant to my future experience. I need some further proposition or propositions that will establish an appropriate link or connection between past and future, and take me from (1) to (2) using either demonstrative reasoning, concerning relations of ideas, or probable reasoning, concerning matters of fact.

Hume thinks it is evident that demonstrative reasoning can't bridge the gap between (1) and (2). However unlikely it may be, we can always intelligibly conceive of a change in the course of nature. Even though aspirin relieved my previous headaches, there's no contradiction in supposing that it won't relieve the one I'm having now, so the supposition of a change in the course of nature can't be proven false by any reasoning concerning relations of ideas.

That leaves probable reasoning. Hume argues that there is no probable reasoning that can provide a just inference from past to future. Any attempt to infer (2) from (1) by a probable inference will be viciously circular—it will involve supposing what we are trying to prove.

Hume spells out the circularity this way. Any reasoning that takes us from (1) to (2) must employ some connecting principle that connects the past with the future. Since one thing that keeps us from moving directly from past to future is the possibility that the course of nature might change, it seems plausible to think that the connecting principle we need will assure us that nature is uniform—that the course of nature won't change—something like the uniformity principle:

  • [UP]The future will be like the past.

Adopting [UP] will indeed allow us to go from (1) to (2). But before we can use it to establish that our causal inferences are determined by reason, we need to determine our basis for adopting it. [UP] is clearly not intuitive, nor is it demonstrable, as Hume has already pointed out, so only probable arguments could establish it. But to attempt to establish [UP] this way would be to try to establish probable arguments using probable arguments, which will eventually include [UP] itself.

Hume has exhausted the ways reason might establish a connection between cause and effect to show that our causal inferences are based on reason. He assures us that he offers his “sceptical doubts” not as a “discouragement, but rather an incitement … to attempt something more full and satisfactory”. Having cleared the way for his constructive account, Hume is ready to do just that.

5.2 Causal Inference: Constructive Phase

Hume calls his constructive account of causal inference a “sceptical solution” to the “sceptical doubts” he raised in the critical phase of his argument.

Since we're determined—caused —to make causal inferences, then if they aren't “determin'd by reason”, there must be “some principle of equal weight and authority” that leads us to make them. Hume maintains that this principle is custom or habit:

whenever the repetition of any particular act or operation produces a propensity to renew the same act or operation … we always say, that this propensity is the effect of Custom. (EHU 5.1.5/43)

It is therefore custom, not reason, which “determines the mind … to suppose the future conformable to the past” (Abstract 16). But even though we have located the principle, it is important to see that this isn't a new principle by which our minds operate. Custom and habit are general names for the principles of association.

Hume describes their operation as a causal process: custom or habit is the cause of the particular propensity you form after your repeated experiences of the constant conjunction of smoke and fire. Causation is the operative associative principle here, since it is the only one of those principles that can take us beyond our senses and memories.

Hume concludes that custom alone “makes us expect for the future, a similar train of events with those which have appeared in the past” (EHU 5.1.6/44). Custom thus turns out to be the source of the Uniformity Principle—the belief that the future will be like the past.

5.3 Belief

Causal inference leads us not only to conceive of the effect, but also to expect it. When I expect that the aspirin will relieve my headache, I'm not just abstractly considering the idea of headache relief, I believe that the aspirin will relieve it. What more is involved in believing that aspirin will relieve my headache than in merely conceiving that it will?

It can't be that beliefs have some additional idea—the idea of belief, perhaps—that conceptions lack. If there were some such idea, given our ability to freely combine ideas, we could, by simply willing, add that idea to any conception whatsoever, and believe anything we like.

Hume concludes that belief must be some sentiment or feeling aroused in us independently of our wills, which accompanies those ideas that constitute them. It is a particular way or manner of conceiving an idea that is generated by the circumstances in which we find ourselves.

If constant conjunctions were all that is involved, my thoughts about aspirin and headaches would only be hypothetical. For belief, one of the conjoined objects must be present to my senses or memories; I must be taking, or just have taken, the aspirin. In these circumstances, believing that my headache will soon be relieved is as unavoidable as feeling affection for a close friend, or anger when someone harms us. “All these operations are species of natural instincts, which no reasoning … is able either to produce or prevent” (EHU 5.1.8/46–47).

While Hume thinks that defining this sentiment may be impossible, we can describe belief, if only by analogy, although he was never completely satisfied with his attempts to do so. Belief is a livelier, firmer, more vivid, steady, and intense conception of an object. Hume intends these characterizations to go beyond merely recording intensity of feeling to capture how belief

renders realities … more present to us than fictions, causes them to weigh more in the thought, and gives them a superior influence on the passions and imagination. (EHU 5.2.12/49)

But how does an idea come to be conceived in such a manner that it constitutes a belief?

Hume's explanation is that as I become accustomed to aspirin's relieving my headaches, I develop a propensity—a tendency—to expect headache relief to follow taking aspirin. The propensity is due to the associative bond that my repeated experiences of taking aspirin and headache relief have formed. My present impressions of taking an aspirin are as forceful and vivid as anything could be, and some of their force and vivacity transfers across the associative path to the idea of headache relief, enlivening it with enough force and vivacity to give it the “strength and solidity” that constitutes belief.

Since I don't know how aspirin relieves headaches, it is fortunate that there is “a kind of pre-established harmony between the course of nature and the succession of our ideas” that teaches me to take aspirin when I have a headache. Custom, Hume maintains, in language that anticipates and influenced Darwin,

is that principle by which this correspondence has been effected; so necessary to the subsistence of our species, and the regulation of our conduct, in every circumstance of human life. (EHU 5.2.21/55)

It is far better, Hume concludes, to rely on “the ordinary wisdom of nature”, which ensures that we form beliefs “by some instinct or mechanical tendency”, rather than trusting it to “the fallacious deductions of our reason” (EHU 5.2.22/55).

In keeping with his project of providing a naturalistic account of how our minds work, Hume has given empirical explanations of our propensity to make causal inferences, and the way those inferences lead to belief.

6. The Idea of Necessary Connection

The early modern causation debate revolved around a family of “nearly synonymous” key ideas, the most prominent of which were the ideas of power and necessary connection. For Hume, “there are no ideas, which occur in metaphysics, more obscure and uncertain”. He showcases the critical and constructive uses of his account of definition as he attempts “to fix … the precise meaning of these terms”, in order to “remove some part of that obscurity, which is so much complained of in this species of philosophy” (EHU 7.1.3/61–62).

6.1 Necessary Connection: Critical Phase

To get clear about the idea of power or necessary connection, we need to determine the impressions that are its source. Hume identifies three possible sources in the work of his predecessors: Locke thought we get our idea of power secondarily from external impressions of the interactions of physical objects, and primarily from internal impressions of our ability to move our bodies and to consider ideas. Malebranche argued that what we take to be causes of the motion of bodies or mental activity aren't causes at all. They are only occasions for God, the sole source of necessary connection, to act in the world. Hume rejects all three possibilities.

He argues that external impressions of the interactions of bodies can't give rise to our idea of power. When we see that the motion of one billiard ball follows another, we're only observing their conjunction, never their connection.

Attending to internal impressions of the operations of our minds doesn't help. Although voluntary bodily movements follow our willing that those movements occur, this is a matter of fact I learn through experience, not from some internal impression of my will's power. When I decide to type, my fingers move over the keyboard. When I decide to stop, they stop, but I have no idea how this happens. Were I aware of the power of my will to move my fingers, I'd know both how it worked and its limits.

Our ability to control our thoughts doesn't give us an impression of power, either. We don't have a clue about how we call up our ideas. Our command over them is limited and varies from time to time. We learn about these limitations and variations only through experience, but the mechanisms by which they operate are unknown and incomprehensible to us. If I decide to think about Istanbul, my idea of that city comes to mind, but I experience only the succession of my decision followed by the idea's appearance, never the power itself.

When ordinary people can't determine an event's cause, they attribute it to some “invisible intelligent principle”. Malebranche and other occasionalists do the same, except they apply it across the boards. True causes aren't powers in the physical world or in human minds. The only true cause is God's willing that certain objects should always be conjoined with certain others.

Anyone aware of our minds' narrow limits should realize that Malebranche's theory takes us into “fairyland”—it goes so far beyond our experience that we have no way of intelligibly assessing it. It also capitalizes on how little we know about the interactions of bodies, but since our idea of God is based on extrapolations from our faculties, our ignorance should also apply to him.

6.2 Necessary Connection: Constructive Phase

Since we've canvassed the leading contenders for the source of our idea of necessary connection and found them wanting, it might seem as if we have no such idea, but that would be too hasty. In our discussion of causal inference, we saw that when we find that one kind of event is constantly conjoined with another, we begin to expect the one to occur when the other does. We suppose there's some connection between them, and don't hesitate to call the first, the cause, and the second, the effect. We also saw that there's nothing different in the repetition of constantly conjoined cases from the exactly similar single case, except that after we've experienced their constant conjunction, habit determines us to expect the effect when the cause occurs.

Hume concludes that it is just this felt determination of the mind—our awareness of this customary transition from one associated object to another—that is the source of our idea of necessary connection. When we say that one object is necessarily connected to another, we really mean that they have acquired an associative connection in our thought that gives rise to this inference.

Having located the missing ingredient, Hume is ready to offer a definition of cause. In fact, he gives us two. The first,

A cause is an object, followed by another, where all the objects similar to the first are followed by objects similar to the second,

gives the relevant external impressions, while the second,

A cause is an object followed by another, and whose appearance always conveys the thought to the other,

captures the internal impression—our awareness of being determined by custom to move from cause to effect. Both are definitions on Hume's account, but his “just definition” of our idea of cause is the conjunction of the two (EHU 7.2.29/76–77). Only together do they capture all the relevant impressions involved.

Hume locates the source of the idea of necessary connection in us, not in the objects themselves or even in our ideas of those objects we regard as causes and effects. In doing so, he completely changes the course of the causation debate, reversing what everyone else thought about the idea of necessary connection. Subsequent discussions of causation must confront the challenges Hume poses for traditional, more metaphysical, ways of looking at our idea of causation.

Hume's treatment of our idea of causation is his flagship illustration of how his method works and the revolutionary results it can achieve. He goes on to apply both his method, and its concrete results, to other prominent debates in the modern period, including probable inference, testimony for miracles, free will, and intelligent design.

7. Moral Philosophy

Hume's explanation of morality is an important part of his efforts to reform philosophy. He takes his primary task to be an investigation into the origin of the basic moral ideas, which he assumes are the ideas of moral goodness and badness. As with the idea of cause and necessary connection, he wants to explain moral ideas as economically as possible in terms of their “simplest and fewest causes”. Determining their causes will determine what their content is—what we mean by them. His secondary concern is to establish what character traits and motives are morally good and bad.

Hume follows his sentimentalist predecessor, Francis Hutcheson (1694–1746), in building his moral theory around the idea of a spectator who approves or disapproves of people's character traits and motives. The sentiments of approval and disapproval are the source of our moral ideas of goodness and badness. To evaluate a character trait as morally good is to evaluate it as virtuous; to evaluate it as morally bad is to evaluate it as vicious.

As he did in the causation debate, Hume steps into an ongoing debate about ethics, often called the British Moralists debate, which began in the mid-seventeenth century and continued until the end of the eighteenth. He uses the same method here as he did in that debate: there is a critical phase in which he argues against his opponents, and a constructive phase in which he develops his version of sentimentalism. Hume has two sets of opponents: the self-love theorists and the moral rationalists. He became the most famous proponent of sentimentalism.

Thomas Hobbes' (1588–1679) brilliant but shocking attempt to derive moral and political obligation from motives of self-interest initiated the British Moralists debate. Hobbes, as his contemporaries understood him, characterizes us as naturally self-centered and power-hungry, concerned above all with our own preservation. In the state of nature, a pre-moral and pre-legal condition, we seek to preserve ourselves by trying to dominate others. Since we are equally powerful, this results in a state of “war of all against all” in which life is “nasty, brutish, and short”. The way out is to make a compact with one another. We agree to hand over our power and freedom to a sovereign, who makes the laws necessary for us to live together peacefully and has the power to enforce them. While acting morally requires that we comply with the laws the sovereign establishes, the basis of morality is self-interest.

Bernard Mandeville's (1670–1733) The Fable of the Bees served to reinforce this reading of Hobbes during the early 18th century. According to Mandeville, human beings are naturally selfish, headstrong, and unruly. Some clever politicians, recognizing that we would be better off living together in a civilized society, took up the task of domesticating us. Realizing that we are proud creatures, highly susceptible to flattery, they were able to dupe many of us to live up to the ideal of virtue—conquering our selfish passions and helping others—by dispensing praise and blame. Moral concepts are just tools clever politicians used to tame us.

Two kinds of moral theories developed in reaction first to Hobbes and then to Mandeville—rationalism and sentimentalism. The rationalists oppose Hobbes' claim that there is no right or wrong in the state of nature, that rightness or wrongness is determined by the sovereign's will, and that morality requires sanctions to motivate us. The sentimentalists object to Hobbes' and Mandeville's “selfish” conceptions of human nature and morality. By the mid–eighteenth century, rationalists and sentimentalists were arguing not only against Hobbes and Mandeville, but also with each other.

Hume opposes both selfish and rationalist accounts of morality, but he criticizes them in different works. In the Treatise, Hume assumes that Hobbes' theory is no longer a viable option, so that there are only two possibilities to consider. Either moral concepts spring from reason, in which case rationalism is correct, or from sentiment, in which case sentimentalism is correct. If one falls, the other stands. In the second Enquiry, Hume continues to oppose moral rationalism, but his arguments against them appear in an appendix. More importantly, he drops the assumption he made in the Treatise and takes the selfish theories of Hobbes and Mandeville as his primary target. Once again, he thinks there are only two possibilities. Either our approval is based in self-interest or it has a disinterested basis. The refutation of one is proof of the other.

7.1 Moral Rationalism: Critical Phase in the Treatise

Hume thinks that “systems and hypotheses” have also “perverted our natural understanding” of morality. The views of the moral rationalists—Samuel Clarke (1675–1729), Locke and William Wollaston (1660–1724)—are prominent among them. One distinctive, but unhealthy, aspect of modern moral philosophy, Hume believes, is that it allies itself with religion and thus sees itself as serving the interests of “popular superstition”. Clarke's theory and those of the other rationalists epitomize this tendency.

Clarke, Hume's central rationalist opponent, appeals to reason to explain almost every aspect of morality. He believes that there are demonstrable moral relations of fitness and unfitness that we discover a priori by means of reason alone. Gratitude, for example, is a fitting or suitable response to kindness, while ingratitude is an unfitting or unsuitable response. He believes that the rational intuition that an action is fitting has the power both to obligate us and to move us. To act morally is to act rationally.

Hume's most famous and important objection to moral rationalism is two-pronged. In Treatise 2.3.3, “Of the influencing motives of the will”, he rejects the rationalist ideal of the good person as someone whose passions and actions are governed by reason. In T 3.1.1, he uses these arguments to show that moral ideas do not spring from reason alone.

In the first prong of his objection, Hume begins by remarking that nothing is more common than for philosophers, as well as ordinary people, to talk about the “combat” between reason and passion. They say we ought to be governed by reason rather than passion, and if our passions are not in line with reason's commands, we ought to restrain them or bring them into conformity with reason. Hume counters that “reason alone can never be a motive to any action of the will” and that by itself it can never oppose a passion in the direction of the will.

His first argument rests on his empiricist conception of reason. As we saw in his account of causation, demonstrative reasoning consists in comparing ideas to find relations among them, while probable reasoning concerns matters of fact. He considers mathematical reasoning from the relation of ideas category and causal reasoning from the category of matters of fact. He asks us to look at instances of actions where these two types of reasoning are relevant and says that when we do, we will see that reason alone couldn't have moved us.

No one thinks that mathematical reasoning by itself is capable of moving us. Suppose you want to stay out of debt. This may move you to calculate how much money comes in and how much goes out, but mathematical reasoning by itself does not move us to do anything. Mathematical reasoning, when it bears on action, is always used in connection with achieving some purpose and thus in connection with causal reasoning.

Hume, however, argues that when causal reasoning figures in the production of action, it always presupposes an existing desire or want. On his view, reasoning is a process that moves you from one idea to another. If reasoning is to have motivational force, one of the ideas must be tied to some desire or affection. As he says,

It can never in the least concern us to know, that such objects are causes, and such others effects, if both the causes and effects are indifferent to us. Where the objects themselves do not affect us, their connexion can never give them any influence; and ‘tis plain, that as reason is nothing but the discovery of this connexion, it cannot be by its means that the objects are able to affect us (T 2.3.3.3/414).

Noticing a causal connection between exercise and losing weight will not move you to exercise, unless you want to lose weight.

It immediately follows that reason alone cannot oppose a passion in the direction of the will. To oppose a passion, reason must be able to give rise to a motive by itself, since only a motive can oppose another motive, but he has just shown that reason by itself is unable to do this.

Having exposed reason's pretensions to rule, Hume inverts the rationalist's ideal of the good person, and concludes that “Reason is, and ought only to be the slave of the passions and can never pretend to any other office than to serve and obey them” (T 2.3.3.4/415).

The second prong of Hume's objection, the argument from motivation, is directed primarily against Clarke and concerns the source of our moral concepts: either they spring from reason or from sentiment. Couching this debate in terms of his own version of the theory of ideas, he reminds us that to engage in any sort of mental activity is to have a perception before the mind, so “to approve of one character, to condemn another, are only so many different perceptions” (T 3.1.1.2/456). Since there are only two types of perception—ideas and impressions—the question between rationalism and sentimentalism is

Whether 'tis by means of our ideas or impressions we distinguish betwixt vice and virtue, and pronounce an action blameable or praise-worthy? (T 3.1.1.3/456)

The argument from motivation has only two premises. The first is that moral ideas have pervasive practical effects. Experience shows that we are often motivated to perform an action because we think it is obligatory or to refrain because we think it is unjust. We try to cultivate the virtues in ourselves and are proud when we succeed and ashamed when we fail. If morality did not have these effects on our passions and actions, moral rules and precepts would be pointless, as would our efforts to be virtuous. Thus “morals excite passions, and produce or prevent actions” (T 3.1.1.6/457).

The second premise is that by itself reason is incapable of exciting passions or producing and preventing actions, which Hume supports with the arguments we just looked at about the influencing motives of the will. The argument from motivation, then, is that if moral concepts are capable of exciting passions and producing or preventing actions, but reason alone is incapable of doing these things, then moral concepts can't spring from reason alone.

Reason for Hume is essentially passive and inert: it is incapable by itself of giving rise to new motives or new ideas. Although he thinks the argument from motivation is decisive, in T 3.1.1, he offers a battery of additional arguments, which are intended to show that moral concepts do not arise from reason alone.

Hume takes the defeat of rationalism to entail that moral concepts spring from sentiment. Of course, he was not the first to claim that moral ideas arise from sentiment. Hutcheson claimed that we possess, in addition to our external senses, a special moral sense that disposes us to respond to benevolence with the distinctive feelings of approbation. Hume, however, rejects the idea that the moral sentiments arise from a sense that is an “original quality” and part of our “primary constitution”.

He first argues that there are many different types of virtue, not all of which are types of benevolence—respecting people's property rights, keeping promises, courageousness, and industriousness—as Hutcheson maintained. If we agree with Hume, but keep Hutcheson's idea of a moral sense, we would have to believe that we have many different “original” senses, which dispose us to approve of the variety of different virtues separately. But he complains that this is not only highly implausible, but also contrary to the

usual maxims, by which nature is conducted, where a few principles produce all the variety we observe in the universe. (T 3.1.2.6/473)

Instead of multiplying senses, we should look for a few general principles to explain our approval of the different virtues.

The real problem, however, is that Hutcheson just claims—hypothesizes—that we possess a unique, original moral sense. If asked why we have a moral sense, his reply is that God implanted it in us. Although in his critical phase Hume freely borrows many of Hutcheson's arguments to criticize moral rationalism, his rejection of a God-given moral sense puts him on a radically different path from Hutcheson in his constructive phase. One way of understanding Hume's project is to see it as an attempt to naturalize Hutcheson's moral sense theory. He aims to provide a wholly naturalistic and economical explanation of how we come to experience the moral sentiments that also explains why we approve of the different virtues. In the course of explaining the moral sentiments, Hutcheson's idea of an original moral sense disappears from Hume's account of morality.

7.2 Sentimentalism: Constructive Phase

In Treatise 3.3.1, Hume turns to his constructive task of providing a naturalistic explanation of the moral sentiments. He refers to them as feelings of approval or disapproval, praise or blame, esteem or contempt. Approval is a kind of pleasant or agreeable feeling; disapproval a kind of painful or disagreeable feeling. In several key passages, he describes the moral sentiments as calm forms of love and hatred. When we evaluate our own character traits, pride and humility replace love and hatred.

Hume's project is “to discover the true origin of morals, and of that love or hatred, which arises” (T 3.3.1/575) when we contemplate our own or other people's character traits and motives. He traces the moral sentiments to sympathy. Sympathy is a psychological mechanism that explains how we come to feel what others are feeling. It is not itself a feeling or sentiment and so should not be confused with feelings of compassion or pity. Hume appeals to sympathy to explain a wide range of phenomena: our interest in history and current affairs, our ability to enjoy literature, movies, and novels, as well as our sociability. It is central to his explanations of our passions, our sense of beauty, and our sense of what is morally good and bad.

Sympathy is a process that moves me from my idea of what someone is feeling to actually experiencing the feeling. There are four steps to this process. I first arrive at the idea of what someone is feeling in any of the usual ways. I next become aware of the resemblances between us, so we are linked by that principle of association. While we resemble every human being to some extent, we also resemble some individuals more than others—for instance, those who share our language or culture or are the same age and sex as we are. The associative principles of contiguity and causality also relate individuals who are located closely to us in time or space or who are family members or teachers. According to Hume, we are able to sympathize more easily and strongly with individuals with whom we have strong associative ties. The stronger the associative relations, the stronger our sympathetic responses. Hume then claims—controversially— that we always have a vivid awareness of ourselves. Finally, he reminds us that the principles of association not only relate two perceptions, but they also transmit force and vivacity from one perception to another.

Suppose my friend recently suffered a devastating loss and I realize she is feeling sad. The associative principles transmit force and vivacity from my vivid awareness of myself to my idea of my friend's sadness. Since for Hume the difference between impressions and ideas is that impressions are more lively and vivacious than ideas, if an idea of a passion is sufficiently enlivened, it becomes the very passion itself. I now feel sad too, but not quite as strongly as my friend.

The way Hume uses the idea that the associative principles transmit force and vivacity in his explanation of sympathy is parallel to the way he uses it in his explanation of causal inference. In the case of causal inference, if we have an impression of an effect (smoke), the associative principles give rise not only to the idea of its cause (fire), but they also transmit some of the impression's force and vivacity to the idea of its cause, so that we come to believe that fire is the cause of the smoke. A belief is an idea that is so lively that it is like an impression, and influences us in the way impressions do. Similarly, my lively awareness of myself enlivens by association my idea of my friend's sadness. But the result in the case of sympathy is even stronger: when an idea of a passion is sufficiently enlivened, it becomes the very passion itself.

One advantage Hume's explanation of the moral sentiments in terms of sympathy has over Hutcheson's claim that we possess a God-given moral sense is that it enables him to provide a unified theory of the mind. He explains the moral sentiments by appealing to sympathy, which, in turn, he explains in terms of the same associative principles he invoked to explain causal beliefs. Without sympathy, and the associative principles that explain it, we would be unimaginatively different than we are—creatures without causal or moral ideas.

Hume develops his account of moral evaluation further in response to two objections to his claim that the moral sentiments arise from sympathy. The first is the “sympathy is variable” objection. Sympathy enables us to enter into the feelings of anyone, even strangers, because we resemble everyone to some extent. But it is an essential feature of his account of the natural and spontaneous operation of sympathy that our ability to respond sympathetically to others varies with variations in the associative relations. I am able to sympathize more easily and strongly with someone who resembles me or is related to me by contiguity or causation. The objection is that the moral sentiments can't be based in sympathy because the loves and hatreds that result from the natural and spontaneous workings of sympathy vary, but our moral approval doesn't vary. The second objection is that “virtue in rags” still evokes our approval. Sympathy works by looking at the actual effects of a person's character traits, but sometimes misfortune or lack of opportunity may prevent an individual from exercising her good character traits, yet we still admire them.

Hume argues that moral love and hatred spring from sympathy, but only when we regulate our sympathetic reactions by taking up what he calls “the general point of view”. There are two regulatory features to the general point of view. The first is that we survey a person's character from the perspective of the person and his usual associates—friends, family, neighbors, and co-workers. We sympathize with the person and the people with whom that person regularly interacts and judge character traits in terms of whether they are good or bad for these people. Second, we regulate sympathy further by relying on general rules that specify the general effects and tendencies of character traits rather than sympathizing with their actual effects.

By putting together these two regulatory features, we arrive at Hume's idea of the general point of view, which defines a perspective from which we may survey a person's character traits that we share with everyone. When we occupy the general point of view, we sympathize with the person herself and her usual associates, and come to admire the person for traits that are normally good for everyone. The general point of view is, for Hume, the moral perspective. We do not experience the moral sentiments unless we have already taken up the general point of view. The moral sentiments and the concepts to which they give rise are products of taking up that standpoint.

Hume offers the claim that we admire four sorts of character traits—those that are useful or immediately agreeable to the agent or to others—as an empirical hypothesis. While he provides support for it in his discussion of the individual virtues, he also uses his fourfold classification to undermine Christian conceptions of morality. He makes pride a virtue and humility a vice. He throws out the “monkish” virtues—celibacy, fasting, and penance—on the grounds that they are not pleasant or useful to anyone. He also rejects the distinction between virtues and natural talents, which legislators, “divines” and modern moralists defend by claiming that the moral virtues are voluntary, whereas natural talents aren't. Their goal is to reform us—or at least our outward behavior—making us better, when understood in Christian terms. They accordingly restrict the domain of the moral to actions that proceed from character traits because they believe only they can be modified, shaped, and controlled by sanctions, while talents can't. Hume, however, rejects the distinction along with the dubious function these reformers assign to morality.

Hume identifies both what has value and what makes things valuable with features of our psychology. Our first-order sentiments, passions and affections, as well as actions expressive of them, are what have moral value. Our second-order reflective sentiments about our own or other people's sentiments, passions and affections are what give them value. On his view, morality is entirely a product of human nature.

In the “Conclusion” of the second Enquiry, Hume summarizes his explanation of morality with a definition of virtue or merit:

every quality of mind, which is useful or agreeable to the personhimself or to others, communicates a pleasure to the spectator, engages his esteem, and is admitted under the honourable denomination of virtue or merit. (EPM 9.1.12/277)

This is a precise parallel of his two definitions of cause in the first Enquiry. Both sets of definitions pick out features of events, and both record a spectator's response to those events.

7.3 Self-Interest Theories: Critical Phase in the Enquiry

Hume's second Enquiry is a sustained and systematic attack on the “selfish” or “self-love” theories of Hobbes and Mandeville. He follows Hutcheson in thinking that they assign two distinct roles to self-interest in their accounts of morality: first, moral approval and disapproval are based in a concern for our own interest and, second, the motive of which we ultimately approve is self-interest. Although many people during this period understood Hobbes' theory through Mandeville's lens, Hume believes it is important to distinguish them. As he sees it, Mandeville's theory is superficial and easily dismissed. Hobbes is his main opponent.

Hume's rejection of Hobbes' selfish account of approval and disapproval begins in Section II and ends in Part I of the “Conclusion” of the Enquiry. Like Hutcheson, he mistakenly supposes that Hobbes was offering a rival theory of approval and disapproval. We approve of people's character traits when they benefit us and disapprove of them when they harm us. Hume looks at each of the four types of virtue and argues that in each case, our approval does not spring from a concern for our own happiness, but rather from sympathy.

In Section II, Hume argues that one reason we approve of benevolence, humanity and public spiritedness is that they are useful to others and to society. In Sections III and IV, he argues that the sole ground for approving of justice and political allegiance is that they are useful to society. In Section V, he asks: But useful for whom? Since it is obvious that it has to be “for some body's interest”, the question is “Whose interest then?” He assumes there are only two possibilities: approval and disapproval spring either from sentiments that are interested or from a disinterested source.

According to Hume, Hobbes' “deduction of morals from self-love” begins with our realization that we cannot subsist alone. A social order provides security, peace and mutual protection, conditions that allow us to promote our own interests better than if we lived alone. Our own good is thus bound up with the maintenance of society. Although Hume agrees with Hobbes up to this point, he rejects his explanation that we approve of justice, benevolence and humanity because they promote our own happiness.

Hume is confident that “the voice of nature and experience” will show that Hobbes' theory, understood in this way, is mistaken. Borrowing many of Hutcheson's arguments, he points out that if approval and disapproval were based on thoughts about the possible advantages and disadvantages of people's characters and actions to us, we would never feel approval and disapproval of people from “very distant ages and remote countries”, since they cannot possibly affect us. We would never admire the good deeds of our enemies or rivals, since they are hurtful to us. We would also never approve or disapprove of characters portrayed in novels or movies, since they are not real people and cannot possibly help or harm us. We approve of character traits and actions that are useful not because they benefit us, but because we sympathize with the benefits they bestow on others or society.

Hume next examines the remaining three types of character traits—those that are useful to the agent (industriousness, good judgment), agreeable to the agent (cheerfulness) or agreeable to others (politeness, decency). For example, why do we approve of industriousness and good judgment, character traits that are primarily advantageous to the possessor? In most cases they are of absolutely no benefit to us and, in cases of rivalry, they counteract our own interest. We approve of these character traits not because they are beneficial to us, but because we sympathize with the benefits they confer on others. Hume takes this as further evidence against Hobbes' explanation in terms of self-interest and in support of his sympathy-based account.

In Part I of the “Conclusion”, Hume complains that Hobbes' self-love theory is unable to explain two important features of our moral sentiments: we tend to approve of the same sorts of character traits and we are able to morally evaluate anyone, at any time or place. If our approval and disapproval were based on thoughts about our own benefits and harms, the moral sentiments would vary from person to person and for the same person over time. We wouldn't have moral feelings about most people, since most people don't affect us. The moral sentiments spring from our capacity to respond sympathetically to others.

Hume is equally adamant that any explanation of the motives that prompt us to virtuous actions in terms of self-interest is mistaken. He opposes them in Appendix II of the Enquiry, which was originally part of Section II, “Of Benevolence”. He follows Hutcheson in thinking that the issue is whether the various benevolent affections are genuine or arise from self-interest. Once again he distinguishes Mandeville's from Hobbes' explanations of benevolence and takes Hobbes to be his main opponent. On Hume's reading of Hobbes, while we approve of kindness, friendship, and other benevolent affections, any desire to benefit others really derives from self-interest, although we may not always be conscious of its influence on those desires.

Hume offers two arguments against this selfish view. He first asks us to consider cases in which people are motivated by a genuine concern for others, even when such concern could not possibly benefit them and might even harm them. We grieve when a friend dies, even if the friend needed our help and patronage. How could our grief be based in self-interest? Parents regularly sacrifice their own interests for the sake of their children. Non-human animals care about members of their own species and us. Is their concern a “deduction” of self-interest? He concludes that these and “a thousand other instances… are marks of a general benevolence in human nature, where no interest binds us” (EPM App 2.11/300).

David Hume: Causation

David Hume (1711-1776) is one of the British Empiricists of the Early Modern period, along with John Locke and George Berkeley. Although the three advocate similar empirical standards for knowledge,  that is, that there are no innate ideas and that all knowledge comes from experience, Hume is known for applying this standard rigorously to causation and necessity. Instead of taking the notion of causation for granted, Hume challenges us to consider what experience allows us to know about cause and effect.

Hume shows that experience does not tell us much. Of two events, A and B, we say that A causes B when the two always occur together, that is, are constantly conjoined. Whenever we find A, we also find B, and we have a certainty that this conjunction will continue to happen. Once we realize that “A must bring about B” is tantamount merely to “Due to their constant conjunction, we are psychologically certain that B will follow A”, then we are left with a very weak notion of necessity. This tenuous grasp on causal efficacy helps give rise to the Problem of Induction--that we are not reasonably justified in making any inductive inference about the world. Among Hume scholars it is a matter of debate how seriously Hume means us to take this conclusion and whether causation consists wholly in constant conjunction.

This article examines the empirical foundations that lead Hume to his account of causation before detailing his definitions of causation and how he uses these key insights to generate the Problem of Induction. After explicating these two main components of Hume’s notion of causation, three families of interpretation will be explored: the causal reductionist, who takes Hume’s definitions of causation as definitive; the causal skeptic, who takes Hume’s problem of induction as unsolved; and the causal realist, who introduces additional interpretive tools to avoid these conclusions and maintains that Hume has some robust notion of causation.

Table of Contents

  1. Causation's Place in Hume's Taxonomy
  2. Necessary Connections and Hume's Two Definitions
  3. The Problem of Induction
  4. Causal Reductionism
  5. Causal Skepticism
  6. Causal Realism
  7. References and Further Reading
    1. A Note on Hume's Works
    2. Hume's Works on Causation
    3. Works in the History of Philosophy
    4. Contemporary Metaphysics of Causation

1. Causation's Place in Hume's Taxonomy

Hume’s most important contributions to the philosophy of causation are found in A Treatise of Human Nature, and An Enquiry concerning Human Understanding, the latter generally viewed as a partial recasting of the former. Both works start with Hume’s central empirical axiom known as the Copy Principle. Loosely, it states that all constituents of our thoughts come from experience. By learning Hume’s vocabulary, this can be restated more precisely. Hume calls the contents of the mind perceptions, which he divides into impressions and ideas. Though Hume himself is not strict about maintaining a concise distinction between the two, we may think of impressions as having their genesis in the senses, whereas ideas are products of the intellect. Impressions, which are either of sensation or reflection (memory), are more vivid than ideas. Hume’s Copy Principle therefore states that all our ideas are products of impressions.

At first glance, the Copy Principle may seem too rigid. To use Hume’s example, we can have an idea of a golden mountain without ever having seen one. But to proffer such examples as counter to the Copy Principle is to ignore the activities of the mind. The mind may combine ideas by relating them in certain ways. If we have the idea of gold and the idea of a mountain, we can combine them to arrive at the idea of a golden mountain. The Copy Principle only demands that, at bottom, the simplest constituent ideas that we relate come from impressions. This means that any complex idea can eventually be traced back to genesis constituent impressions.

In the Treatise, Hume identifies two ways that the mind associates ideas, via natural relations and via philosophical relations. Natural relations have a connecting principle such that the imagination naturally leads us from one idea to another. The three natural relations are resemblance, contiguity, and cause and effect. Of these, Hume tells us that causation is the most prevalent. But cause and effect is also one of the philosophical relations, where the relata have no connecting principle, instead being artificially juxtaposed by the mind. Of the philosophical relations, some, such as resemblance and contrariety, can give us certitude. Some cannot. Cause and effect is one of the three philosophical relations that afford us less than certain knowledge, the other two being identity and situation. But of these, causation is crucial. It alone allows us to go beyond what is immediately present to the senses and, along with perception and memory, is responsible for all our knowledge of the world. Hume therefore recognizes cause and effect as both a philosophical relation and a natural relation, at least in the Treatise, the only work where he draws this distinction.

The relation of cause and effect is pivotal in reasoning, which Hume defines as the discovery of relations between objects of comparison. But note that when Hume says “objects”, at least in the context of reasoning, he is referring to the objects of the mind, that is, ideas and impressions, since Hume adheres to the Early Modern “way of ideas”, the belief that sensation is a mental event and therefore all objects of perception are mental. But causation itself must be a relation rather than a quality of an object, as there is no one property common to all causes or to all effects. By so placing causation within Hume’s system, we arrive at a first approximation of cause and effect. Causation is a relation between objects that we employ in our reasoning in order to yield less than demonstrative knowledge of the world beyond our immediate impressions. However, this is only the beginning of Hume’s insight.

2. Necessary Connections and Hume's Two Definitions

In both the Treatise and the Enquiry, we find Hume’s Fork, his bifurcation of all possible objects of knowledge into relations of ideas and matters of fact. Hume gives several differentiae distinguishing the two, but the principal distinction is that the denial of a true relation of ideas implies a contradiction. Relations of ideas can also be known independently of experience. Matters of fact, however, can be denied coherently, and they cannot be known independently of experience. Although Immanuel Kant later seems to miss this point, arguing for a middle ground that he thinks Hume missed, the two categories must be exclusive and exhaustive. A true statement must be one or the other, but not both, since its negation must either imply a contradiction or not. There is no middle ground. Yet given these definitions, it seems clear that reasoning concerning causation always invokes matters of fact. For Hume, the denial of a statement whose truth condition is grounded in causality is not inconceivable (and hence, not impossible; Hume holds that conceivability implies possibility). For instance, a horror movie may show the conceivability of decapitation not causing the cessation of animation in a human body. But if the denial of a causal statement is still conceivable, then its truth must be a matter of fact, and must therefore be in some way dependent upon experience. Though for Hume, this is true by definition for all matters of fact, he also appeals to our own experience to convey the point. Hume challenges us to consider any one event and meditate on it; for instance, a billiard ball striking another. He holds that no matter how clever we are, the only way we can infer if and how the second billiard ball will move is via past experience. There is nothing in the cause that will ever imply the effect in an experiential vacuum. And here it is important to remember that, in addition to cause and effect, the mind naturally associates ideas via resemblance and contiguity. Hume does not hold that, having never seen a game of billiards before, we cannot know what the effect of the collision will be. Rather, we can use resemblance, for instance, to infer an analogous case from our past experiences of transferred momentum, deflection, and so forth. We are still relying on previous impressions to predict the effect and therefore do not violate the Copy Principle. We simply use resemblance to form an analogous prediction. And we can charitably make such resemblances as broad as we want. Thus, objections like: Under a Humean account, the toddler who burned his hand would not fear the flame after only one such occurrence because he has not experienced a constant conjunction, are unfair to Hume, as the toddler would have had thousands of experiences of the principle that like causes like, and could thus employ resemblance to reach the conclusion to fear the flame.

If Hume is right that our awareness of causation (or power, force, efficacy, necessity, and so forth - he holds all such terms to be equivalent) is a product of experience, we must ask what this awareness consists in. What is meant when some event is judged as cause and effect?  Strictly speaking, for Hume, our only external impression of causation is a mere constant conjunction of phenomena, that B always follows A, and Hume sometimes seems to imply that this is all that causation amounts to. (And this notion of causation as constant conjunction is required for Hume to generate the Problem of induction discussed below.)  Nevertheless, ‘causation’ carries a stronger connotation than this, for constant conjunction can be accidental and therefore doesn’t get us the necessary connection that gives the relation of cause and effect its predictive ability. We may therefore now say that, on Hume’s account, to invoke causality is to invoke a constant conjunction of relata whose conjunction carries with it a necessary connection.

Hume points out that this second component of causation is far from clear. What is this necessity that is implied by causation?  Clearly it is not a logical modality, as there are possible worlds in which the standard laws of causation do not obtain. It might be tempting to state that the necessity involved in causation is therefore a physical or metaphysical necessity. However, Hume considers such elucidations unhelpful, as they tell us nothing about the original impressions involved. At best, they merely amount to the assertion that causation follows causal laws. But invoking this common type of necessity is trivial or circular when it is this very efficacy that Hume is attempting to discover.

We must therefore follow a different route in considering what our impression of necessity amounts to. As causation, at base, involves only matters of fact, Hume once again challenges us to consider what we can know of the constituent impressions of causation. Once more, all we can come up with is an experienced constant conjunction. Of the common understanding of causality, Hume points out that we never have an impression of efficacy. Because of this, our notion of causal law seems to be a mere presentiment that the constant conjunction will continue to be constant, some certainty that this mysterious union will persist. Hume argues that we cannot conceive of any other connection between cause and effect, because there simply is no other impression to which our idea may be traced. This certitude is all that remains.

For Hume, the necessary connection invoked by causation is nothing more than this certainty. Hume’s Copy Principle demands that an idea must have come from an impression, but we have no impression of efficacy in the event itself. Instead, the impression of efficacy is one produced in the mind. As we experience enough cases of a particular constant conjunction, our minds begin to pass a natural determination from cause to effect, adding a little more “oomph” to the prediction of the effect every time, a growing certitude that the effect will follow again. It is the internal impression of this “oomph” that gives rise to our idea of necessity, the mere feeling of certainty that the conjunction will stay constant. Ergo, the idea of necessity that supplements constant conjunction is a psychological projection. We cannot help but think that the event will unfurl in this way.

Having approached Hume’s account of causality by this route, we are now in a position to see where Hume’s two definitions of causation given in the Treatise come from. (He gives similar but not identical definitions in the Enquiry.) He defines “cause” in the following two ways:

(D1)      An object precedent and contiguous to another, and where all the objects resembling the former are placed in like relations of precedency and contiguity to those objects that resemble the latter.

(D2)      An object precedent and contiguous to another, and so united with it, that the idea of the one determined the mind to form the idea of the other, and the impression of the one to form a more lively idea of the other. (T 1.3.14.31; SBN 170)

There are reams of literature addressing whether these two definitions are the same and, if not, to which of them Hume gives primacy. J.A. Robinson is perhaps the staunchest proponent of the position that the two are nonequivalent, arguing that there is an nonequivalence in meaning and that they fail to capture the same extension. Two objects can be constantly conjoined without our mind determining that one causes the other, and it seems possible that we can be determined that one object causes another without their being constantly conjoined. But if the definitions fail in this way, then it is problematic that Hume maintains that both are adequate definitions of causation. Some scholars have argued for ways of squaring the two definitions (Don Garrett, for instance, argues that the two are equivalent if they are both read objectively or both read subjectively), while others have given reason to think that seeking to fit or eliminate definitions may be a misguided project.

One alternative to fitting the definitions lies in the possibility that they are doing two separate things, and it might therefore be inappropriate to reduce one to the other or claim that one is more significant than the other. There are several interpretations that allow us to meaningfully maintain the distinction (and therefore the nonequivalence) between the two definitions unproblematically. For instance, D1 can be seen as tracing the external impressions (that is, the constant conjunction) requisite for our idea of causation while D2 traces the internal impressions, both of which are important to Hume in providing a complete account. As Hume says, the definitions are “presenting a different view of the same object.” (T 1.3.14.31; SBN 170)  Supporting this, Harold Noonan holds that D1 is “what is going on in the world” and that D2 is “what goes on in the mind of the observer” and therefore, “the problem of nonequivalent definitions poses no real problem for understanding Hume.” (Noonan 1999: 150-151)  Simon Blackburn provides a similar interpretation that the definitions are doing two different things, externally and internally. However, Blackburn has the first as giving the “contribution of the world” and the latter giving the “functional difference in the mind that apprehends the regularity.” (Blackburn 2007: 107)  However, this is not the only way to grant an nonequivalence without establishing the primacy of one over the other.

Another method is to cash out the two definitions in terms of the types of relation. Some scholars have emphasized that, according to Hume’s claim in the Treatise, D1 is defining the philosophical relation of cause and effect while D2 defines the natural relation. Walter Ott argues that, if this is right, then the lack of equivalence is not a problem, as philosophical and natural relations would not be expected to capture the same extension. (Ott 2009: 239)  This way of dismissing the nonequivalence of the two definitions becomes more problematic, however, when we realize that Hume does not make the distinction between natural and philosophical relations in the Enquiry, yet provides approximately the same two definitions. If the definitions were meant to separately track the philosophical and natural relations, we might expect Hume to have explained that distinction in the Enquiry rather than dropping it while still maintaining two definitions. Perhaps for this reason, Jonathan Bennett suggests that it is best to forget Hume’s comment of this correspondence. (Bennett 1971: 398)

Though this treatment of literature considering the definitions as meaningfully nonequivalent has been brief, it does serve to show that the definitions need not be forced together. In fact, later in the Treatise, Hume states that necessity is defined by both, either as the constant conjunction or as the mental inference, that they are two different senses of necessity, and Hume, at various points, identifies both as the essence of connection or power. Whether or not Robinson is right in thinking Hume is mistaken in holding this position, Hume himself does not seem to believe one definition is superior to the other, or that they are nonequivalent.

Beyond Hume’s own usage, there is a second worry lingering. Attempting to establish primacy between the definitions implies that they are somehow the bottom line for Hume on causation. But Hume is at pains to point out that the definitions are inadequate. In discussing the “narrow limits of human reason and capacity,” Hume asks,

And what stronger instance can be produced of the surprizing ignorance and weakness of the understanding than [the analysis of causation]?...so imperfect are the ideas we form concerning it, that it is impossible to give any just definition of cause, except what is drawn from something extraneous and foreign to it….But though both these definitions be drawn from circumstances foreign to cause, we cannot remedy this inconvenience, or attain any more perfect definition…. (EHU 7.29; SBN 77, emphasis his)

The tone this passage conveys is one of resigned dissatisfaction. Although Hume does the best that can be expected on the subject, he is dissatisfied, but this dissatisfaction is inevitable. This is because, as Hume maintains in Part VII of the Enquiry, a definiens is nothing but an enumeration of the constituent simple ideas in the definiendum. However, Hume has just given us reason to think that we have no such satisfactory constituent ideas, hence the “inconvenience” requiring us to appeal to the “extraneous.”  This is not to say that the definitions are incorrect. Note that he still applies the appellation “just” to them despite their appeal to the extraneous, and in the Treatise, he calls them “precise.”  Rather, they are unsatisfying. It is an inconvenience that they appeal to something foreign, something we should like to remedy. Unfortunately, such a remedy is impossible, so the definitions, while as precise as they can be, still leave us wanting something further. But if this is right, then Hume should be able to endorse both D1 and D2 as vital components of causation without implying that he endorses either (or both) as necessary and sufficient for causation. For these reasons, Hume’s discussion leading up to the two definitions should be taken as primary in his account of causation rather than the definitions themselves.

3. The Problem of Induction

The second of Hume’s influential causal arguments is known as the problem of induction, a skeptical argument that utilizes Hume’s insights about experience limiting our causal knowledge to constant conjunction. Though Hume gives a quick version of the Problem in the middle of his discussion of causation in the Treatise (T 1.3.6), it is laid out most clearly in Section IV of the Enquiry. An influential argument, the Problem’s skeptical conclusions have had a drastic impact on the field of epistemology. It should be noted, however, that not everyone agrees about what exactly the Problem consists in. Briefly, the typified version of the Problem as arguing for inductive skepticism can be described as follows:

Recall that proper reasoning involves only relations of ideas and matters of fact. Again, the key differentia distinguishing the two categories of knowledge is that asserting the negation of a true relation of ideas is to assert a contradiction, but this is not the case with genuine matters of fact. But in Section IV, Hume only pursues the justification for matters of fact, of which there are two categories:

(A)           Reports of direct experience, both past and present

(B)           Claims about states of affairs not directly observed

Matters of fact of category (A) would include sensory experience and memory, against which Hume never raises doubts, contra René Descartes. For Hume, (B) would include both predictions and the laws of nature upon which predictions rest. We cannot claim direct experience of predictions or of general laws, but knowledge of them must still be classified as matters of fact, since both they and their negations remain conceivable. In considering the foundations for predictions, however, we must remember that, for Hume, only the relation of cause and effect gives us predictive power, as it alone allows us to go beyond memory and the senses. All such predictions must therefore involve causality and must therefore be of category (B). But what justifies them?

It seems to be the laws governing cause and effect that provide support for predictions, as human reason tries to reduce particular natural phenomena “…to a greater simplicity, and to resolve the many particular effects into a few general causes….” (EHU 4.12; SBN 30)  But this simply sets back the question, for we must now wonder what justifies these “general causes.”  One possible answer is that they are justified a priori as relations of ideas. Hume rejects this solution for two reasons:  First, as shown above, we cannot meditate purely on the idea of a cause and deduce the corresponding effect and, more importantly, to assert the negation of any causal law is not to assert a contradiction.

Here we should pause to note that the generation of the Problem of Induction seems to essentially involve Hume’s insights about necessary connection (and hence our treating it first). Since the Problem of Induction demands that causal connections cannot be known a priori, and that our access is only to constant conjunction, the Problem seems to require the most crucial components of his account of necessity. It is therefore an oddity that, in the Enquiry, Hume waits until Section VII to explicate an account of necessity already utilized in the Problem of Section IV. In the Treatise, however, a version of the Problem appears after Hume’s insights about experience limiting causation to constant conjunction but before the explication of the projectivist necessity and his presenting of the two definitions. It is therefore not entirely clear how Hume views the relationship between his account of necessity and the Problem. Stathis Psillos, for instance, views Hume’s inductive skepticism as a corollary to his account of necessary connection. (Psillos 2002: 31)  However, Peter Millican rightly points out that the Problem can still be construed so as to challenge most non-reductive causal theories as well. (Millican 2002: 141)  Kenneth Clatterbaugh goes further, arguing that Hume’s reductive account of causation and the skepticism the Problem raises can be parsed out so they are entirely separable. (Clatterbaugh 1999: 186)  D.M. Armstrong disagrees, arguing that “…if laws of nature are nothing but Humean uniformities, then inductive scepticism is inevitable.” (Armstrong 1999: 52)

Whether the Problem of induction is in fact separable from Hume’s account of necessary connection, he himself connects the two by arguing that “…the knowledge of this relation is not, in any instance, attained by reasonings a priori; but arises entirely from experience, when we find that any particular objects are constantly conjoined with each other.” (EHU 4.6; SBN 27)  Here, Hume invokes the account of causation explicated above to show that the necessity supporting (B) is grounded in our observation of constant conjunction. This is to say that (B) is grounded in (A). But again, (A) by itself gives us no predictive power. We have thus merely pushed the question back one more step and must now ask with Hume, “What is the foundation of all conclusions from experience?” (EHU 4.14; SBN 32, emphasis his)

The answer to this question seems to be inductive reasoning. We use direct observation to draw conclusions about unobserved states of affairs. But this is just to once more assert that (B) is grounded in (A). The more interesting question therefore becomes how we do this. What lets us reason from (A) to (B)?  The only apparent answer is the assumption of some version of the Principle of the Uniformity of Nature (PUN), the doctrine that nature is always uniform, so unobserved instances of phenomena will resemble the observed. This is called an assumption since we have not, as yet, established that we are justified in holding such a principle. Once more, it cannot be known a priori, as we assert no contradiction by maintaining its falsity. A sporadic, random universe is perfectly conceivable. Therefore, knowledge of the PUN must be a matter of fact. But the principle is predictive and not directly observed. This means that the PUN is an instance of (B), but we were invoking the PUN as the grounds for moving from beliefs of type (A) to beliefs of type (B), thus creating a vicious circle when attempting to justify type (B) matters of fact. We use knowledge of (B) as a justification for our knowledge of (B). The bottom line for Hume’s Problem of induction seems to be that there is no clear way to rationally justify any causal reasoning (and therefore no inductive inference) whatsoever. We have no ground that allows us to move from (A) to (B), to move beyond sensation and memory, so any matter of fact knowledge beyond these becomes suspect.

Louis Loeb calls this reconstruction of Hume targeting the justification of causal inference-based reasoning the “traditional interpretation” (Loeb 2008: 108), and Hume’s conclusion that causal inferences have “no just foundation” (T 1.3.6.10; SBN 91) lends support to this interpretation. Under this reconstruction, the epistemic circularity revealed by Hume’s Problem of Induction seems detrimental to knowledge. However, there are philosophers (Max Black, R. B. Braithwaite, Charles Peirce, and Brian Skyrms, for instance) that, while agreeing that Hume targets the justification of inductive inference, insist that this particular justificatory circle is not vicious or that it is unproblematic for various reasons. As discussed below, Hume may be one such philosopher. Alternatively, there are those that think that Hume claims too much in insisting that inductive arguments fail to lend probability to their conclusions. D. C. Stove maintains that, while Hume argues that inductive inference never adds probability to its conclusion, Hume’s premises actually only support “inductive fallibilism”, a much weaker position that induction can never attain certainty (that is, that the inferences are never valid). Hume illicitly adds that no invalid argument can still be reasonable. (Stove 1973: 48)

But not all are in agreement that Hume’s intended target is the justification of causal or inductive inference. Tom Beauchamp and Alexander Rosenberg agree that Hume’s argument implies inductive fallibilism, but hold that this position is adopted intentionally as a critique of the deductivist rationalism of Hume’s time. (Beauchamp and Rosenberg 1981: 44)  Annette Baier defends a similar account, focusing on Hume’s use of “reason” in the argument, which she insists should be used only in the narrow sense of Hume’s “demonstrative sciences”. (Baier 1991: 60)  More recently, Don Garret has argued that Hume’s negative conclusion is one of cognitive psychology, that we do not adopt induction based on doxastically sufficient argumentation. Induction is simply not supported by argument, good or bad. Instead, it is an instinctive mechanism that we share with animals. (Garrett 1997: 92, 94)  Similarly, David Owen holds that Hume’s Problem of induction is not an argument against the reasonableness of inductive inference, but, “Rather Hume is arguing that reason cannot explain how we come to have beliefs in the unobserved on the basis of past experience.” (Owen 1999: 6)  We see that there are a variety of interpretations of Hume’s Problem of induction and, as we will see below, how we interpret the Problem will inform how we interpret his ultimate causal position.

4. Causal Reductionism

Having described these two important components of his account of causation, let us consider how Hume’s position on causation is variously interpreted, starting with causal reductionism. The family of reductionist theories, often read out of Hume’s account of necessity outlined above, maintain that causation, power, necessity, and so forth, as something that exists between external objects rather than in the observer, is constituted entirely by regular succession. In the external world, causation simply is the regularity of constant conjunction. In fact, the defender of this brand of regularity theory of causation is generally labeled a “Humean” about causation. However, since this interpretation, as Hume’s own historical position, remains in contention, the appellation will be avoided here.

Because of the variant opinions of how we should view the relationship between the two definitions proffered by Hume, we find two divergent types of reduction of Humean causation. First, there are reductionists that insist Hume reduces causation to nothing beyond constant conjunction, that is, the reduction is to a simple naïve regularity theory of causation, and therefore the mental projection of D2 plays no part. The motivation for this interpretation seems to be an emphasis on Hume’s D1, either by saying that it is the only definition that Hume genuinely endorses, or that D2 somehow collapses into D1 or that D2 does not represent a genuine ontological reduction, and is therefore not relevant to the metaphysics of causation. Robinson, for instance, claims that D2 is explanatory in nature, and is merely part of an empiricist psychological theory. (Robinson 1962)

This focus on D1 is regarded as deeply problematic by some Hume scholars (Francis Dauer, H.O. Mounce, and Fred Wilson, for instance), because it seems to be an incomplete account of Hume’s discussion of necessary connection presented above. A reductive emphasis on D1 as definitive ignores not only D2 as a definition but also ignores all of the argument leading up to it. This is to disregard the discussion through which Hume accounts for the necessity of causation, a component which he describes as “of much greater importance” than the contiguity and succession of D1. (T 1.3.2.11; SBN 77)  In short, a reduction to D1 ignores the mental determination component. However, this practice may not be as uncharitable as it appears, as many scholars see the first definition as the only component of his account relevant to metaphysics. For instance, D.M. Armstrong, after describing both components, simply announces his intention to set aside the mental component as irrelevant to the metaphysics of causation. (Armstrong 1983: 4)  J. L. Mackie similarly stresses that, “It is about causation so far as we know about it in objects that Hume has the firmest and most fully argued views,” (Mackie 1980: 21) and it is for this reason that he focuses on D1.

However, not everyone agrees that D2 can or should be dropped so easily from Hume’s system. In addition to its accounting for the necessity of causation mentioned above, recall that Hume makes frequent reference to both definitions as accurate or just, and at one point even refers to D2 as constituting the essence of causation. Therefore, whether or not the projectivism of D2 actually is relevant to the metaphysics of causation, a strong case can be made that Hume thinks it is so, and therefore an accurate historical interpretation needs to include D2 in order to capture Hume’s intentions. (Below, the assumption that Hume is even doing metaphysics will also be challenged.)  The more common Humean reduction, then, adds a projectivist twist by somehow reducing causation to constant conjunction plus the internal impression of necessity. (See, for instance, Beauchamp and Rosenberg 1981: 11, Goodman 1983: 60, Mounce 1999: 42, Noonan 1999: 140-145, Ott 2009: 224 or Wilson 1997: 16)  Of course while this second type of reductionist agrees that the projectivist component should be included, there is less agreement as to how, precisely, it is supposed to fit into Hume’s overall causal picture. Largely for this reason, we have a host of reductionist interpretations rather than a single version. The unifying thread of the reductionist interpretations is that causation, as it exists in the object, is constituted by regularity.

But given the Humean account of causation outlined above, it is not difficult to see how Hume’s writings give rise to such reductionist positions. After all, both D1 and D2 seem reductive in nature. If, as is often the case, we take definitions to represent the necessary and sufficient conditions of the definiendum, then both the definitions are reductive notions of causation. D1 reduces causation to proximity, continuity, and constant conjunction, and D2 similarly reduces causation to proximity, continuity, and the internal mental determination that moves the first object or idea to the second. Even considering Hume’s alternate account of definitions, where a definition is an enumeration of the constituent ideas of the definiendum, this does not change the two definitions’ reductive nature. Given that Hume’s discussions of causation culminate in these two definitions, combined with the fact that the conception of causation they provide is used in Hume’s later philosophical arguments of the Treatise, the definitions play a crucial role in understanding his account of causation. Therefore, the various forms of causal reductionism can constitute reasonable interpretations of Hume. By putting the two definitions at center state, Hume can plausibly be read as emphasizing that our only notion of causation is constant conjunction with certitude that it will continue. Nevertheless, reductionism is not the only way to interpret Hume’s theory of causation.

5. Causal Skepticism

One way to interpret the reasoning behind assigning Hume the position of causal skepticism is by assigning similar import to the passages emphasized by the reductionists, but interpreting the claims epistemically rather than ontologically. In other words, rather than interpreting Hume’s insights about the tenuousness of our idea of causation as representing an ontological reduction of what causation is, Humean causal skepticism can instead be viewed as his clearly demarcating the limits of our knowledge in this area and then tracing out the ramifications of this limiting. (Below, we will see that the causal realists also take Hume’s account of necessity as epistemic rather than ontological.)  If Hume’s account is intended to be epistemic, then the Problem of induction can be seen as taking Hume’s insights about our impressions of necessity to an extreme but reasonable conclusion. If it is true that constant conjunction (with or without the added component of mental determination) represents the totality of the content we can assign to our concept of causation, then we lose any claim to robust metaphysical necessity. But once this is lost, we also sacrifice our only rational grounding of causal inference. Our experience of constant conjunction only provides a projectivist necessity, but a projectivist necessity does not provide any obvious form of accurate predictive power. Hence, if we limit causation to the content provided by the two definitions, we cannot use this weak necessity to justify the PUN and therefore cannot ground predictions. We are therefore left in a position of inductive skepticism which denies knowledge beyond memory and what is present to the senses. By limiting causation to constant conjunction, we are incapable of grounding causal inference; hence Humean inductive skepticism.

In this way, the causal skeptic interpretation takes the “traditional interpretation” of the Problem of induction seriously and definitively, defending that Hume never solved it. Since we never directly experience power, all causal claims certainly appear susceptible to the Problem of Induction. The attempted justification of causal inference would lead to the vicious regress explained above in lieu of finding a proper grounding. The supporters of Humean causal skepticism can then be seen as ascribing to him what seems to be a reasonable position,  which is, the conclusion that we have no knowledge of such causal claims, as they would necessarily lack proper justification. The family of interpretations that have Hume’s ultimate position as that of a causal skeptic therefore maintain that we have no knowledge of inductive causal claims, as they would necessarily lack proper justification. We can never claim knowledge of category (B)  D. M. Armstrong reads Hume this way, seeing Hume’s reductivist account of necessity and its implications for laws of nature as ultimately leading him to skepticism. (Armstrong 1983: 53)  Other Hume scholars that defend a skeptical interpretation of causation include Martin Bell, (Rupert and Richman 2007: 129) and Michael Levine, who maintains that Hume’s causal skepticism ultimately undermines his own Enquiry argument against miracles.

There are, however, some difficulties with this interpretation. First, it relies on assigning the “traditional interpretation” to the Problem of induction though, as discussed above, this is not the only account. Secondly, reading the conclusion of the Problem of Induction in this way is difficult to square with the rest of Hume’s corpus. For instance, the Copy Principle, fundamental to his work, has causal implications, and Hume relies on inductive inference as early as T 1.1.1.8; SBN 4. Hume consistently relies on analogical reasoning in the Dialogues Concerning Natural Religion even after Philo grants that the necessity of causation is provided by custom, and the experimental method used to support the “science of man” so vital to Hume’s Treatise clearly demands the reliability of causal inference. Hume’s causal skepticism would therefore seem to undermine his own philosophy. Of course, if this is the correct way to read the Problem of Induction, then so much the worse for Hume.

A more serious challenge for the skeptical interpretation of Hume is that it ignores the proceeding Part of the Enquiry, in which Hume immediately provides what he calls a “solution” to the Problem of Induction. Hume states that, even though they are not supported by reason, causal inferences are “essential to the subsistence of all creatures,” and that:

It is more comfortable to the ordinary wisdom of nature to secure so necessary an act of the mind, by some instinct or mechanical tendency, which may be infallible in its operations, may discover itself at the first appearance of life and thought, and may be independent of all the laboured deductions of the understanding. As nature has taught us the use of our limbs, without giving us the knowledge of the muscles and nerves by which they are actuated; so she has implanted in us an instinct, which carries forward the thought in a correspondent course to that which she has established among external objects; though we are ignorant of those powers and forces, on which this course and succession of objects totally depends. (EHU 5.22; SBN 55)

Here, Hume seems to have causal inference supported by instinct rather than reason. The causal skeptic will interpret this as descriptive rather than normative, but others are not so sure. It is not clear that Hume views this instinctual tendency as doxastically inappropriate in any way. Therefore, another interpretation of this “solution” is that Hume thinks we can be justified in making causal inferences. However, it is not reason that justifies us, but rather instinct (and reason, in fact, is a subspecies of instinct for Hume, implying that at least some instinctual faculties are fit for doxastic assent). This will be discussed more fully below.

6. Causal Realism

Against the positions of causal reductionism and causal skepticism is the New Hume tradition. It started with Norman Kemp Smith’s The Philosophy of David Hume, and defends the view that Hume is a causal realist, a position that entails the denial of both causal reductionism and causal skepticism by maintaining that the truth value of causal statements is not reducible to non-causal states of affairs and that they are in principle, knowable. (Tooley 1987: 246-47)  The case for Humean causal realism is the least intuitive, given the explications above, and will therefore require the most explanation. However, the position can be rendered more plausible with the introduction of three interpretive tools whose proper utilization seems required for making a convincing realist interpretation. Of these, two are distinctions which realist interpretations insist that Hume respects in a crucial way but that non-realist interpretations often deny. The last is some mechanism by which to overcome the skeptical challenges Hume himself raises.

The first distinction is between ontological and epistemic causal claims. Strawson points out that we can distinguish:

(O)  Causation as it is in the objects, and

(E)  Causation so far as we know about it in the objects.

He maintains, “…Hume’s Regularity theory of causation is only a theory about (E), not about (O).” (Strawson 1989: 10)  Whether or not we agree that Hume limits his theory to the latter, the distinction itself is not difficult to grasp. It simply separates what we can know from what is the case. The realist interpretation then applies this to Hume’s account of necessary connection, holding that it is not Hume’s telling us what causation is, but only what we can know of it. Hume’s account is then merely epistemic and not intended to have decisive ontological implications. This undercuts the reductionist interpretation. Simply because Hume says that this is what we can know of causation, it does not follow that Hume therefore believes that this is all that causation amounts to. In fact, such an interpretation might better explain Hume’s dissatisfaction over the definitions. If Hume were a reductionist, then the definitions should be correct or complete and there would not be the reservations discussed above.

Further, given Hume’s skeptical attitude toward speculative metaphysics, it seems unlikely that he would commit the Epistemic Fallacy and allow the inference from “x is all we can know of y” to “x constitutes the real, mind-independent essence of y,”  as some (though not all) reductionist accounts would require. In fact, Hume must reject this inference, since he does not believe a resemblance thesis between perceptions and external objects can ever be philosophically established. He makes this denial explicit in Part XII of the Enquiry.

The epistemic interpretation of the distinction can be made more compelling by remembering what Hume is up to in the third Part of Book One of the Treatise. Here, as in many other areas of his writings, he is doing his standard empiricist investigation. Since we have some notion of causation, necessary connection, and so forth, his Copy Principle demands that this idea must be traceable to impressions. Hume’s account of causation should therefore be viewed an attempt to trace these genesis impressions and to thereby reveal the true content of the idea they comprise. Thus, it is the idea of causation that interests Hume. In fact, the title of Section 1.3.2 is “Of probability; and of the idea of cause and effect”. He announces, “To begin regularly, we must consider the idea of causation, and see from what origin it is deriv’d.” (T 1.3.2.4; SBN 74, his emphasis )  Hume therefore seems to be doing epistemology rather than metaphysics. (Mounce 1999: 32 takes this as indicative of a purely epistemic project.)

Although this employment of the distinction may proffer a potential reply to the causal reductionist, there is still a difficulty lurking. While it may be true that Hume is trying to explicate the content of the idea of causation by tracing its constituent impressions, this does not guarantee that there is a coherent idea, especially when Hume makes occasional claims that we have no idea of power, and so forth. The challenge seems to amount to this:  Even if the previous distinction is correct, and Hume is talking about what we can know but not necessarily what is, the causal realist holds that substantive causal connections exist beyond constant conjunction. This is to posit a far stronger claim than merely having an idea of causation. The realist Hume says that there is causation beyond constant conjunction, thereby attributing him a positive ontological commitment, whereas his own skeptical arguments against speculative metaphysics rejecting parity between ideas and objects should, at best, only imply agnosticism about the existence of robust causal powers. (It is for this reason that Martin Bell and Paul Russell reject the realist interpretation.)  There therefore seems to be a tension between accepting Hume’s account of necessary connection as purely epistemic and attributing to Hume the existence of an entity beyond what we can know by investigating our impressions.

Put another way, Hume’s Copy Principle requires that our ideas derive their content from constitutive impressions. However, if the previous distinction is correct, then Hume has already exhaustively explicated the impressions that give content to our idea of causation.  This is the very same content that leads to the two definitions. It seems that Hume has to commit himself to the position that there is no clear idea of causation beyond the proffered reduction. But if this is true, and Hume is not a reductionist, what is he positing?  It is here that the causal realist will appeal to the other two interpretive tools, viz. a second distinction and a belief mechanism, the former allowing us to make sense of the positive claim and the latter providing justification for it.

The realists claim that the second distinction is explicit in Hume’s writing. This is the distinction between “conceiving” or “imagining” and merely “supposing”. The general proposal is that we can and do have two different levels of clarity when contemplating a particular notion. We can either have a Cartesian clear and distinct idea, or we can have a supposition, that is, a vague, incomplete, or “relative” notion. The suggestion is this:  Simple ideas are clear and distinct (though not as vivid as their corresponding impressions) and can be combined via the various relations. Groups compiled by relating these simple ideas form mental objects. In some cases, they combine in a coherent way, forming clear and distinct complex ideas, while in other cases, the fit is not so great, either because we do not see how the constituent ideas relate, or there is something missing from our conception. These suppositions do not attain the status of complex ideas in and of themselves, and remain an amalgamation of simple ideas that lack unity. The claim would then be that we can conceive distinct ideas, but only suppose incomplete notions.

Something like this distinction has historical precedence. In the Fifth Replies, Descartes distinguishes between some form of understanding and a complete conception. Berkeley also distinguishes between an “idea” and a mere “notion” in the third Dialogue and the second edition of the Principles. Perhaps most telling, Locke uses terminology identical to Hume’s in regard to substance, claiming we have “…no other idea of it at all, but only a Supposition….” (Essay, II.xxiii.2, emphasis his)  Such a supposition is “an obscure and relative Idea.” (Essay, II.xxiii.3)

The realist employment of this second distinction is two-fold. First, the realist interpretation will hold that claims in which Hume states that we have no idea of power, and so forth, are claims about conceiving of causation. They only claim that we have no clear and distinct idea of power, or that what is clearly and distinctly conceived is merely constant conjunction. But a more robust account of causation is not automatically ruled out simply because our notion is not distinct. In this way, the distinction may blunt the passages where Hume seems pessimistic about the content of our idea of causation.

The second step of the causal realist interpretation will be to then insist that we can at least suppose (in the technical sense) a genuine cause, even if the notion is opaque, that is, to insist that mere suppositions are fit for doxastic assent. There doesn’t seem to be anything terribly problematic in believing in something of which we have an unclear representation. To return to the Fifth Replies, Descartes holds that we can believe in the existence and coherence of an infinite being with such vague ideas, implying that a clear and distinct idea is not necessary for belief. Hume denies clear and distinct content beyond constant conjunction, but it is not obvious that he denies all content beyond constant conjunction.

This second distinction is not introduced without controversy. Briefly, against the distinction, Kenneth Winkler offers an alternative suggestion that Hume’s talk of secret connections is actually a reference to further regularities that are simply beyond current human observation (such as the microscopic or subatomic), while ultimately interpreting Hume as an agnostic about robust causation. (Winkler 1991: 552-556)  John Wright argues that this is to ignore Hume’s reasons for his professed ignorance in the hidden, that is, our inability to make causal inferences a priori. (Wright 1983: 92)  Alternatively, Blackburn, a self-proclaimed “quasi-realist”, argues that the terminology of the distinction is too infrequent to bear the philosophical weight that the realist reading would require. (Blackburn 2007: 101-102)  P.J.E. Kail resists this by pointing out that Hume’s overall attitude strongly suggests that he “assumes the existence of material objects,” and that Hume clearly employs the distinction and its terminology in at least one place: T 1.4.2.56; SBN 217-218. (Kail, 2007: 60) There, Hume describes a case in which philosophers develop a notion impossible to clearly and distinctly perceive, that somehow there are properties of objects independent of any perception. We simply cannot conceive such an idea, but it certainly remains possible to entertain or suppose this conjecture. Clatterbaugh takes an even stronger position than Blackburn, positing that for Hume to talk of efficacious secret powers would be literally to talk nonsense, and would force us to disregard Hume’s own epistemic framework, (Clatterbaugh 1999: 204) while Ott similarly argues that the inability to give content to causal terms means Hume cannot meaningfully affirm or deny causation. (Ott 2009: 198)

Even granting that Hume not only acknowledges this second distinction but genuinely believes that we can suppose a metaphysically robust notion of causal necessity, the realist still has this difficulty. How can Hume avoid the anti-realist criticism of Winkler, Ott, and Clatterbaugh that his own epistemic criteria demand that he remain agnostic about causation beyond constant conjunction?  In other words, given the skeptical challenges Hume levels throughout his writings, why think that such a seemingly ardent skeptic would not merely admit the possibility of believing in a supposition, instead of insisting that this is, in fact, the nature of reality?  The realist seems to require some Humean device that would imply that this position is epistemically tenable, that our notion of causation can reasonably go beyond the content identified by the arguments leading to the two definitions of causation and provide a robust notion that can defeat the Problem of Induction.

This is where the realists (and non-realists) seem most divided in their interpretations of Hume. Generally, the appeal is to Hume’s texts suggesting he embraces some sort of non-rational mechanism by which such beliefs are formed and/or justified, such as his purported solution to the Problem of Induction. This picture has been parsed out in terms of doxastic naturalism, transcendental arguments, psychological necessity, instinct, and even some form of proper function. However, what the interpretations all have in common is that humans arrive at certain mediate beliefs via some method quite distinct from the faculty of reason.

Let us now consider the impact that adopting these naturally formed beliefs would have on Hume’s causal theory. The function is two-fold. First, it provides some sort of justification for why it might be plausible for Hume to deem mere suppositions fit for belief. The other role is to answer the skeptical challenges raised by the “traditional interpretation” of the Problem of Induction. It would provide a way to justify causal beliefs despite the fact that said beliefs appear to be without rational grounds. It accomplishes the latter by emphasizing what the argument concludes, namely that inductive reasoning is groundless, that there is no rational basis for inductive inference. As Hume says, “Reason can never show us the connexion of one object with another….” (T 1.3.6.12; SBN 92, emphasis mine)  In granting such a mechanism, we grant Hume the epistemic propriety of affirming something reason cannot establish. Further, it smoothes over worries about consistency arising from the fact that Hume seemingly undercuts all rational belief in causation, but then merrily shrugs off the Problem and continues to invoke causal reasoning throughout his writings.

In the realist framework outlined above, doxastic naturalism is a necessary component for a consistent realist picture. Kemp Smith argues for something stronger, that this non-rational mechanism itself implies causal realism. After engaging the non-rational belief mechanism responsible for our belief in body, he goes on to argue, “Belief in causal action is, Hume argues, equally natural and indispensable; and he freely recognizes the existence of ‘secret’ causes, acting independently of experience.” (Kemp Smith 2005: 88)  He connects these causal beliefs to the unknown causes that Hume tells us are “original qualities in human nature.” (T 1.1.4.6; SBN 13)  Kemp Smith therefore holds that Humean doxastic naturalism is sufficient for Humean causal realism. The reductionist, however, will rightly point out that this move is entirely too fast. Even granting that Hume has a non-rational mechanism at work and that we arrive at causal beliefs via this mechanism does not imply that Hume himself believes in robust causal powers, or that it is appropriate to do so. However, combining Humean non-rational justification with the two distinctions mentioned above at least seems to form a consistent alternative to the reductionist and skeptical interpretations. Just which of these three is right, however, remains contentious.

7. References and Further Reading

a. A Note on Hume's Works

Hume wrote all of his philosophical works in English, so there is no concern about the accuracy of English translation. For the casual reader, any edition of his work should be sufficient. However, Oxford University Press produced the definitive Clarendon Edition of most of his works. For the serious scholar, these are a must have, as they contain copious helpful notes about Hume’s changes in editions, and so forth. The general editor of the series is Tom L. Beauchamp.

When referencing Hume’s works, however, there are standard editions of the Treatise and his Enquiries originally edited by L.A. Selby-Bigge and later revised by P.H. Nidditch. Hence, citations will often be given with an SBN page number. But Hume also numerated his own works to varying degrees. The Treatise is divided into three Books, each with Parts, Sections, and paragraphs. Hence, four numbers can give a precise location of a passage. Hume’s two definitions of cause are found at T 1.3.14.31; SBN 170, that is, in the Treatise, Book One, Part Three, Section Fourteen, paragraph thirty-one. This paragraph can be found on page 170 of the Selby-Bigge Nidditch editions. Hume’s shorter works, such as the Enquiry Concerning Human Understanding, are not as thoroughly outlined.  Instead, the Enquiry is only divided into Sections, only some of which have Parts. Hence, we also find Hume’s definitions at EHU 7.29; SBN 76-77, or Part Seven of the Enquiry, paragraph twenty-nine, pages 76 and 77 of the Selby-Bigge Nidditch editions.

b. Hume's Works on Causation

  • Hume, David. A Treatise of Human Nature. Clarendon Press, Oxford, U.K., 2007, edited by David Fate Norton and Mary J. Norton.
  • Hume, David. An Enquiry Concerning Human Understanding. Clarendon Press, Oxford, U.K., 2000, edited by Tom L. Beauchamp.

c. Works in the History of Philosophy

  • Ayers, Michael. “Natures and Laws from Descartes to Hume”, in The Philosophical Canon in the Seventeenth and Eighteenth Centuries: Essays in Honour of John W. Yolton, edited by G.A.J. Rogers and S. Tomaselli, University of Rochester Press, Rochester, New York, 1996.
    • This article argues that there are two main traditions of efficacy in the Early Modern period, that objects have natures or that they follow laws imposed by God. This bifurcation then informs how Hume argues, as he must engage the former.
  • Baier, Annette C. A Progress of Sentiments- Reflections on Hume’s Treatise. Harvard University Press, Cambridge Massachusetts, 1991.
    • Baier argues for a nuanced reading of the Treatise, that we can only understand it with the addition of the passions, and so forth, of the later Books.
  • Beauchamp, Tom L. and Rosenberg, Alexander. Hume and the Problem of Causation. Oxford University Press, New York, New York, 1981.
    • This is an important but technical explication and defense of the Humean causal reductionist position, both as a historical reading and as a contemporary approach to causation. The authors argue directly against the skeptical position, instead insisting that the Problem of induction targets only Hume’s rationalist predecessors.
  • Beebee, Helen. Hume on Causation. Routledge University Press, New York, New York, 2006.
    • Beebee rejects the standard interpretations of Hume’s causation before proffering her own, which is grounded in human nature and his theory of mind. Her critiques of the standard Humean views are helpful and clear.
  • Bennett, Jonathan. Learning from Six Philosophers. (two volumes)  Oxford University Press, Oxford, U.K., 2001.
    • These two volumes constitute a solid introduction to the major figures of the Modern period. Volume One discusses Descartes, Spinoza, and Leibniz, and Volume Two is an updated recasting of his Locke, Berkeley, Hume- Central Themes.
  • Bennett, Jonathan. Locke, Berkeley, Hume- Central Themes. Oxford University Press, Glasgow, U.K., 1971.
    • This is an excellent overview of the main doctrines of the British empiricists.
  • Blackburn, Simon. “Hume and Thick Connexions”, as reprinted in Read, Rupert and Richman, Kenneth A. (Editors). The New Hume Debate (Revised Edition). Routledge, New York, New York, 2007, pages 100-112.
    • This is the second, updated version of an important investigation into the realism/reductionism debate. He ultimately adopts a “quasi-realist” position that is weaker than the realist definition given above.
  • Buckle, Stephen. Hume’s Enlightenment Tract- The Unity and Purpose of An Enquiry Concerning Human Understanding. Clarendon Press, Oxford, Oxford U.K., 2001.
    • This book examines the Enquiry, distancing it from the standard reading of a recasting of the Treatise. Instead, Buckle argues that the work stands alone as a cohesive whole.
  • Clatterbaugh, Kenneth. The Causation Debate in Modern Philosophy, 1637-1739. Routledge, New York, New York, 1999.
    • This book traces the various causal positions of the Early Modern period, both rationalist and empiricist.
  • Costa, Michael J. “Hume and Causal Realism”. Australasian Journal of Philosophy, 67: 2, pages 172-190.
    • Costa gives his take on the realism debate by clarifying several notions that are often run together. Like Blackburn, he ultimately defends a view somewhere between reductionism and realism.
  • Craig, Edward. “The Idea of Necessary Connexion” in Reading Hume on Human Understanding, edited by Peter Millican, Oxford University Press, New York, New York, 2002, pages 211-229.
    • This article is an updated and expanded defense of the Hume section of The Mind of God and the Works of Man.
  • Craig, Edward. The Mind of God and the Works of Man. Oxford University Press Clarendon, New York, New York, 1987.
    • A complex book that discusses the works of several philosophers in arguing for its central thesis, Craig’s work is one of the first to defend a causal realist interpretation of Hume.
  • Dauer, Francis Watanabe. “Hume on the Relation of Cause and Effect” in A Companion to Hume, edited by Radcliffe, Elizabeth S, Blackwell Publishing, Ltd, Malden, MA, 2008, pages 89-105.
    • Dauer takes a careful look at the text of the Treatise, followed by a critical discussion of the three most popular interpretations of the two definitions.
  • Fogelin, Robert J. Hume’s Skepticism in the Treatise of Human Nature. Routledge and Kegan Paul, London, U.K., 1985.
  • Garrett, Don. Cognition and Commitment in Hume’s Philosophy. Oxford University Press. New York, New York, 1997.
    • This is a great introduction to some of the central issues of Hume’s work. Garrett surveys the various positions on each of ten contentious issues in Hume scholarship before giving his own take. Among other things, he argues for a novel way to square the two definitions of cause.
  • Howson, Colin. Hume’s Problem: Induction and the Justification of Belief. Oxford University Press, Oxford, U.K., 2000.
    • This highly technical text first defends Hume’s skeptical induction against contemporary attempts at refutation, ultimately concluding that the difficulties in justifying induction are inherent. Nevertheless, given certain assumptions, induction becomes viable. He then goes on to provide a reliable Bayesian framework of a limited type.
  • Kail, P.J.E. Projection and Realism in Hume’s Philosophy. Oxford University Press, Oxford, U.K., 2007.
    • This book explores the projectivist strand of Hume’s thought, and how it helps clarify Hume’s position within the realism debate, presenting Hume’s causal account as a combination of projectivism and realism.
  • Kemp Smith, Norman. The Philosophy of David Hume. Palgrave MacMillan, New York, New York, 2005.
    • This is the work that started the New Hume debate. Palgrave MacMillan has released it in a new edition with an extended introduction describing the work’s importance and the status of the debate.
  • Livingston, Donald W. “Hume on Ultimate Causation.”  American Philosophical Quarterly, Volume 8, 1971, pages 63-70.
    • This is a concise argument for causal realism, which Livingston later expands into a book. Here, he defends the Humean skeptical realism that he considers necessary for other strands of Hume’s philosophy.
  • Livingston, Donald W. Hume’s Philosophy of Common Life. University of Chicago Press, Chicago, Illinois, 1984.
    • This is one of the standard explications of Humean causal realism. It stresses Hume’s position that philosophy should conform to and explain common beliefs rather than conflict with them.
  • Loeb, Louis E. “Inductive Inference in Hume’s Philosophy”, in A Companion to Hume, edited by Radcliffe, Elizabeth S, Blackwell Publishing, Ltd, Malden, MA, 2008, pages 106-125.
    • This is a contemporary analysis of the Problem of induction that ultimately rejects causal skepticism.
  • Loeb, Louis E. Stability and Justification in Hume’s Treatise, Oxford University Press, Oxford, U.K., 2002.
    • This well-argued work offers an interpretation of the Treatise building around Hume’s claim that the mind ultimately seeks stability in its beliefs. Linking justification with “settled beliefs” provides a positive rather than merely destructive epistemology.
  • McCracken, Charles J. Malebranche and British Philosophy. Clarendon Press, Oxford, U.K., 1983.
    • Among other things, McCracken shows how much of Hume’s insight into our knowledge of causal necessity can be traced back to the occasionalism of Malebranche.
  • Millican, Peter. “Hume, Causal Realism, and Causal Science”, Mind, Volume 118, Issue 471, July, 2009, pages 647-712.
    • After giving an overview of the recent debate, Millican argues that the New Hume debate should be settled via Hume’s logic, rather than language, and so forth. He largely rejects the realist interpretation, since the reductionist interpretation is required to carry later philosophical arguments that Hume gives.
  • Millican, Peter. “Hume’s Sceptical Doubts concerning Induction”, in Reading Hume on Human Understanding, edited by Peter Millican, Clarendon Press, Oxford, Oxford, U.K. 2002, pages 107-173.
    • This is a somewhat technical reconstruction of the Problem of Induction, as well as an exploration of its place within Hume’s philosophy and its ramifications.
  • Mounce, H.O. Hume’s Naturalism, Routledge, New York, New York, 1999.
    • This book is an extended development of Hume’s doxastic naturalism over his empiricism.
  • Noonan, Harold W. Routledge Philosophy Guidebook to Hume on Knowledge. Routledge, London, U.K., 1999.
    • Noonan gives an accessible introduction to Hume’s epistemology.
  • Ott, Walter. Causation and Laws of Nature in Early Modern Philosophy. Oxford University Press, Oxford, U.K., 2009.
    • This is an advanced survey of causation in the Early Modern period, covering both the rationalists and the empiricists.
  • Owen, David. Hume’s Reason. Oxford University Press, New York, New York, 1999.
    • This book is an extended treatment of Hume’s notion of reason and its impact on many of his important arguments.
  • Robinson, J. A. “Hume’s Two Definitions of ‘Cause’”. The Philosophical Quarterly, Volume 12, 1962.
    • This article is a concise argument for the difficulties inherent to squaring the two definitions.
  • Robinson, J. A. “Hume’s Two Definitions of ‘Cause’ Reconsidered”. The Philosophical Quarterly, Volume 15, 1965, as reprinted in Hume, A Collection of Critical Essays, edited by V. C. Chappell. University of Notre Dame Press, Notre Dame, Indiana, 1966.
    • This is an updated follow-up to his previous article.
  • Read, Rupert and Richman, Kenneth A. (editors). The New Hume Debate- Revised Edition. Routledge, New York, New York, 2007.
    • This compilation presents a balanced collection of the important works on both sides of the causal realism debate.
  • Stove, David. Probability and Hume’s Inductive Skepticism. Oxford University Press, Oxford, U.K., 1973.
    • Stove presents a math-heavy critique of Hume’s inductive skepticism by insisting that Hume claims too much. Instead of concluding that inductive inference adds nothing to the probability of a conclusion, his premises only imply inductive fallibilism, that is, that they never attain deductive certainty. While no inductive inference is valid, this does not imply that they cannot be reasonable.
  • Strawson, Galen. The Secret Connexion- Causation, Realism, and David Hume. Oxford University Press Clarendon, New York, New York, 1989.
    • This book is perhaps the most clear and complete explication of the New Hume doctrines.
  • Wilson, Fred. Hume’s Defense of Causal Inference. University of Toronto Press, Toronto Canada, 1997.
    • Wilson’s main goal is to defend an anti-skeptical interpretation of Hume’s causal inference, but the book is wide-ranging and rich in many areas of Hume scholarship.
  • Winkler, Kenneth P. “The New Hume”, The Philosophical Review, Volume 100, Number 4, October 1991, pages 541-579.
    • Winkler presents a clear and concise case against the realist interpretation.
  • Wright, John P. The Sceptical Realism of David Hume. University of Minnesota Press, Minneapolis, Minnesota, 1983.
    • This book is one of the standard explications of Humean causal realism. The interpretation is arrived at via a focus on Hume’s attention to human nature. The book also places Hume’s notion of knowledge within its historical context.

d. Contemporary Metaphysics of Causation

  • Armstrong, D. M. What is a Law of Nature? Cambridge University Press, New York, New York, 1983.
    • This book investigates the status of the laws of nature. He ultimately argues that laws are relations between universals or properties.
  • Goodman, Nelson. Fact, Fiction, and Forecast. Fourth Edition, Harvard University Press, Cambridge, Massachusetts, 1983.
    • Goodman explicates the Problem of induction and makes a more general form of the difficulty it raises.
  • Mackie, J. L. The Cement of the Universe- A Study of Causation. Oxford University Press Clarendon, New York, New York, 1980.
    • This work begins with Hume’s analysis of causation and then goes on to consider what we can know about causation as it exists in external objects. Though it is highly technical, it touches many issues important to contemporary metaphysics of causation.
  • Psillos, Stathis. Causation and Explanation. MCGill-Queen’s University Press, Montreal, Canada, 2002.
    • This book is an accessible survey of contemporary causality, linking many of the important issues and engaging the relevant literature.
  • Tooley, Michael. Causation–A Realist Approach. Clarendon Press, Oxford, U.K., 1987.
    • Tooley presents a contemporary defense of realism with efficacy as relations among universals. In doing so, he clarifies many notions and commitments of the various realist and anti-realist positions.

Author Information

C. M. Lorkowski
Email: lorkowcs@uc.edu
University of Cincinnati
U. S. A.

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