Hume on Cause & Experience

1. Scope and evidence

Most of this card reports Established matters: Hume's life and texts, the content of the Copy Principle, the empiricist meaning-test, and the structure of his causal analysis (constant conjunction, custom/habit, two definitions, billiard balls) are all directly grounded in his own texts and uncontested by scholars. The claim that Hume "awoke" Kant is Shared/Traditional — Kant said so himself, but exactly what in Hume did the awakening is debated. The deepest interpretive question — whether Hume eliminated objective causal necessity or merely declared it unknowable — is Disputed and genuinely unresolved: the "New Hume" controversy is a live scholarly question, and this card deliberately presents both sides rather than declaring a winner, in keeping with the site's "accuracy over patriotism" standard.

2. Hume's life and key texts

David Hume was born in Edinburgh in 1711 and educated at the University of Edinburgh, which he entered around the age of twelve and left in his mid-teens. Pressed towards the family profession of law, he found in himself "an insurmountable aversion to everything but the pursuits of philosophy and general learning." After a brief and unsuccessful trial of commerce in Bristol, he moved to France in 1734, settling for most of three years at La Flèche in Anjou — home of the Jesuit college where Descartes had been educated — where, still in his twenties, he wrote A Treatise of Human Nature.

The Treatise appeared anonymously in London: Books 1 ("Of the Understanding") and 2 ("Of the Passions") in 1739, and Book 3 ("Of Morals") in 1740. Its ambition was enormous — to introduce "the experimental method of reasoning into moral subjects" and thereby found a complete "science of man" on Newtonian lines. Its reception was, by Hume's own account, crushing. The book's run of a thousand copies was not supplemented by a second edition until 1817, and — according to figures cited from the Norton & Norton Clarendon edition — as few as three hundred copies may have been sold by the mid-1740s. In his short autobiography My Own Life, written in the year of his death, Hume recalled: "Never literary attempt was more unfortunate than my Treatise of Human Nature. It fell dead-born from the press, without reaching such distinction, as even to excite a murmur among the zealots." Modern scholars note he somewhat exaggerated the total neglect: Book 1 did in fact draw an anonymous, "somewhat abusive" review in the History of the Works of the Learned (Nov/Dec 1739) — attributed by recent scholarship (Coventry, Mazza and Mori, Hume Studies, 2024) to William Warburton — which nonetheless conceded that the work showed "a soaring genius."

Convinced that the failure lay "more from the manner than the matter," Hume recast the core of Book 1 in a more accessible, essayistic form as An Enquiry Concerning Human Understanding (1748). He later disowned the Treatise as juvenile and asked to be judged by the Enquiries — a request posterity has largely ignored, since the Treatise is now regarded as one of the most important works in Western philosophy. Hume achieved fame in his own lifetime chiefly as an essayist and as the author of a best-selling History of England, not as the metaphysician we remember. He never held an academic post: he was passed over for the Chair of Moral Philosophy at Edinburgh (1744–45) and, six years later, for the Chair of Logic at Glasgow (1751–52), largely because of accusations of irreligion drawn from the Treatise. The only teaching-like position he ever held was as tutor, in 1745–46, to the Marquess of Annandale near St Albans — a post that, as the Stanford Encyclopedia of Philosophy records, he accepted "only to find that the young Marquess was insane and his estate manager dishonest." Hume died in Edinburgh in 1776, admired by friends including Adam Smith for the equanimity with which he faced death.

3. The empiricist theory of ideas and the Copy Principle

Hume, following Locke and Berkeley in the tradition of British empiricism, rejected the rationalist doctrine of innate ideas: the mind holds no knowledge stamped on it before experience. Everything the mind contains — what Hume calls its "perceptions" — enters through experience. He divides all perceptions into two classes distinguished by their "force and vivacity":

Impressions — our livelier, more forceful perceptions: sensations (heat, cold, colour, pain) and the "impressions of reflection" (passions and emotions such as desire, aversion, hope and fear). Ideas — the fainter copies of impressions that we use in thinking, remembering and imagining. An idea is, in Hume's phrase, a "faint image" of an impression.

He cross-cuts this with a distinction between simple and complex perceptions. A simple impression (a particular shade of blue) cannot be broken down further; a complex impression (a whole landscape) is built from simpler ones. This yields Hume's central positive doctrine, the Copy Principle: every simple idea is derived from, and exactly resembles (bar its lesser vivacity), a corresponding prior simple impression. We can freely recombine ideas in imagination — hence we can think of a golden mountain, or a virtuous horse, without ever having met one — but the raw materials are always ultimately traceable to impressions. As Hume put it, our creative powers amount to no more than "the faculty of compounding, transposing, augmenting, or diminishing the materials afforded us by the senses and experience."

Hume himself flagged one famous apparent counterexample, the "missing shade of blue": a person who had seen every shade of blue but one could, he concedes, form the idea of the missing shade without a corresponding impression. He treats this as a harmless exception that does not overturn the principle as an empirical generalisation.

The Copy Principle's real philosophical bite lies in its use in reverse as a test of meaning. If a philosophical term is suspected of being empty — "employ'd without any meaning or idea (as is but too frequent)," as he writes in the Enquiry — "we need but enquire, from what impression is that supposed idea derived? And if it be impossible to assign any, this will serve to confirm our suspicion." Hume wields this test destructively against traditional metaphysics. He challenges the idea of substance (finding no impression from which a "substance" over and above its sensible qualities could derive) and, most strikingly, the idea of a simple, persisting self: "when I enter most intimately into what I call myself, I always stumble on some particular perception or other... I never can catch myself at any time without a perception." Finding no constant, invariable impression of a self, he concludes that the mind is better described as a "bundle" of perceptions than as a simple enduring substance. The same test, applied to causation, produces his most celebrated result.

4. The theory of causation: constant conjunction, custom, and the two definitions

Causation matters supremely for Hume because it is the only relation that lets the mind reason beyond what is immediately present to the senses and memory — all our factual reasoning about the unobserved runs through cause and effect. So he asks his signature question: from what impression is the idea of causation, and specifically the idea of necessary connexion or power, derived?

Examining any single instance of one event producing another, Hume finds we can observe only three things: the two events are contiguous in space and time; the cause is prior to the effect; and — across many instances — events of the first type are constantly conjoined with events of the second. What we never observe, in any single case, is the supposed tie that binds them. "When we look about us towards external objects, and consider the operation of causes, we are never able, in a single instance, to discover any power or necessary connexion; any quality, which binds the effect to the cause, and renders the one an infallible consequence of the other. We only find, that the one does actually, in fact, follow the other." Events, famously, "seem conjoined, but never connected."

The billiard-ball example is Hume's own preferred illustration. Seeing one ball roll towards another, then the second move on impact, we perceive one motion followed by another — but nothing in the first, however minutely scrutinised, contains or reveals the second. "Motion in the second billiard-ball is a quite distinct event from motion in the first; nor is there anything in the one to suggest the smallest hint of the other." A person seeing the collision for the very first time, Hume argues, could not predict the outcome; "a hundred different events might as well follow from the cause." Only repetition changes anything — and, crucially, it changes something in us, not in the balls.

So where does the potent idea of necessary connection come from? From the mind itself. After repeated experience of constant conjunction, "the mind is carried by habit, upon the appearance of one event, to expect its usual attendant." That felt, customary transition of the imagination — the internal impression of being determined to pass from cause to expected effect — is the missing impression. "This connexion, therefore, which we feel in the mind, this customary transition of the imagination from one object to its usual attendant, is the sentiment or impression from which we form the idea of power or necessary connexion." Necessity, on this account, "is something, that exists in the mind, not in objects"; the mind then "spreads itself on external objects," projecting its own habitual expectation onto the world and mistaking it for a feature of nature. Custom or habit, not reason, is thus "the great guide of human life."

This is why Hume offers what are traditionally read as two definitions of cause, each capturing one side of what he has found. In the Enquiry (Section VII) he defines a cause as:

1. (The "philosophical" / regularity definition) "an object, followed by another, and where all the objects, similar to the first, are followed by objects similar to the second" — adding the counterfactual gloss, "Or in other words, where, if the first object had not been, the second never had existed." This captures the external facts: priority, contiguity, constant conjunction. 2. (The "natural" / psychological definition) "an object followed by another, and whose appearance always conveys the thought to that other." This captures the internal fact: the mind's customary inference from cause to effect.

(The Treatise, Book 1, Part 3, gives parallel definitions in terms of the "determination of the mind.") Hume himself concedes the definitions are drawn "from circumstances foreign to the cause," admitting he cannot supply the deeper connecting principle we hoped for — a candour that is itself at the heart of the interpretive dispute below. Scholars argue over whether the two definitions are equivalent (Don Garrett, for instance, has argued they can be made to coincide if both are read consistently), whether one has primacy, and whether they are "definitions" at all or characterisations of the circumstances of causal judgement.

5. The "New Hume" debate: does Hume deny real causal powers, or only our knowledge of them?

Here lies the card's genuinely contested territory. Everyone agrees Hume denied that we can perceive or form a clear idea of necessary connection. What is disputed is his deeper metaphysical commitment.

The traditional "regularity" / positivist reading. On the long-dominant interpretation, Hume did not merely deny our knowledge of objective necessity — he denied there is any such thing (or that the very notion is coherent), reducing causation in the objects to nothing more than regular succession. This reading leans on Hume's most deflationary statements, above all: "we have no idea of connexion or power at all, and that these words are absolutely without any meaning, when employed either in philosophical reasonings, or common life" (Enquiry VII), together with his insistence in the Treatise that necessity "is something, that exists in the mind, not in objects." On this view Hume is the founding "regularity theorist": causation is constant conjunction plus the projected habit of expectation, full stop. Twentieth-century logical positivists embraced this Hume enthusiastically.

The "New Hume" / sceptical realist reading. Beginning with John Wright's The Sceptical Realism of David Hume (1983), Edward Craig's The Mind of God and the Works of Man (1987) and — most influentially — Galen Strawson's The Secret Connexion: Causation, Realism, and David Hume (1989), a rival camp argues that Hume was a sceptical realist: he took it for granted that there are real, objective causal powers or "necessary connexions" in nature, but insisted their nature is hidden from us, lying beyond any impression-derived idea. Strawson contends "that the regularity theory of causation is indefensible, and that Hume never adopted it in any case"; Hume, he says, "did not claim that causation in the natural world is just a matter of regular succession," which would be a "dogmatic metaphysical claim" wholly contrary to his scepticism — he "rightly took it for granted that there was more to causation than regularity of succession, claiming only that regularity of succession was all that we could ever know of causation." This camp leans on Hume's many references to nature's hidden machinery — that nature "conceals from us those powers and principles on which the influence of those objects entirely depends"; that "the power or force, which actuates the whole machine, is entirely concealed from us"; that experience never instructs us "in the secret connexion, which binds [events] together" (all Enquiry). Peter Kail (Projection and Realism in Hume's Philosophy, 2007) develops a sophisticated version.

A third position: quasi-realist projectivism. Simon Blackburn ("Hume and Thick Connexions," 1990) argues Hume is "neither a Positivist nor a Sceptical Realist, but rather a not-so-sceptical anti-realist." On this reading, modelled on Hume's ethics, the mind undergoes a real (but non-representing) change on observing regularities and then projects it, so that causal talk becomes legitimately quasi-objective without representing any "thick" connexion in the objects. Helen Beebee (Hume on Causation, 2006) and Angela Coventry have developed projectivist readings.

Current standing. The debate is genuinely unresolved. What can be said accurately is that the sceptical realist ("New Hume") position specifically lost ground over the 2000s and 2010s under criticism from Kenneth Winkler (whose 1991 paper "The New Hume" gave the debate its name) and Peter Millican, to the point where several leading scholars declared the crude regularity reading dead and projectivism the likely victor — Blackburn asserting "Hume is now agreed by all serious interpreters not to be a reductionist about causation," and Beebee charging the regularity reading with "an almost total lack of evidence in its favour." Yet consensus has not arrived. In a major 2024 paper, "Hume as Regularity Theorist—After All! Completing a Counter-Revolution" (Hume Studies 49.1, pp. 101–162), Millican argues that these "obituaries for the regularity interpretation... were premature and unjustified" and defends a refined objectivist regularity reading; meanwhile the projectivist reading was itself under fresh attack in 2024. The Stanford Encyclopedia of Philosophy still frames the matter as open, contrasting "Old Humeans" (Winkler, Millican) with "New Humeans" (Wright, Strawson, Kail) without declaring a winner. In short: sceptical realism has waned, but the deeper question of Hume's metaphysics of causation remains a live, contested problem — exactly the kind of point where precision matters more than a tidy verdict.

6. The connection to the problem of induction (see Card #6)

Hume's causal analysis is inseparable from his celebrated sceptical argument about induction — the question of whether, and how, we are justified in expecting the unobserved (especially the future) to resemble the observed. Because our only warrant for moving from "these billiard balls have always behaved so" to "they will behave so next time" is past constant conjunction, and because any attempt to justify that move seems to presuppose the very uniformity it needs to establish, Hume concludes that the inference is grounded in custom rather than reason. This card deliberately stops at the threshold: the Problem of Induction receives its own full, dedicated treatment on Card #6, which sets out the argument in detail. Here it is enough to note that Hume's account of causal belief and his inductive scepticism are two faces of one insight about the limits of experience.

7. The Kant connection

Hume's causal scepticism had perhaps its single most consequential effect on Immanuel Kant. In the Preface to his Prolegomena to Any Future Metaphysics (1783), Kant made his famous confession (4:260): "I freely admit that it was the remembrance of David Hume that was the very thing that many years ago first interrupted my dogmatic slumber and gave a completely different direction to my researches in the field of speculative philosophy." Kant accepted Hume's point that causal necessity cannot be extracted from experience by observation, but rejected Hume's purely psychological, habit-based solution. His answer was to treat causality as an a priori "category" of the understanding — a concept the mind necessarily brings to and imposes on experience, making objective experience of an ordered world of events possible in the first place. Kant develops this in the "Second Analogy of Experience" in the Critique of Pure Reason, arguing that the causal principle is a synthetic a priori truth rather than either an analytic triviality or a mere empirical generalisation. (Precisely which Humean "objection" awoke Kant — the analysis of causation specifically, or a broader attack on rationalist metaphysics — is itself debated among Kant scholars; that is beyond this card's scope.) The point to retain is the shape of the exchange: where Hume grounds causal necessity in a subjective habit, Kant relocates it in the necessary structure of any possible experience.

8. Legacy and influence on later philosophy of science

Hume's analysis founded the modern philosophical study of causation. His reduction of observable causation to regular succession is the direct ancestor of the regularity theory of causation, refined by John Stuart Mill in A System of Logic (1843) and carried into the twentieth century. His empiricist meaning-test — "from what impression is that idea derived?" — is a clear ancestor of the verification principle of the logical positivists; the Vienna Circle's preference for the label "logical empiricism" signalled their debt to Hume, and A. J. Ayer's Language, Truth and Logic (1936) is a self-consciously Humean manifesto. "Humean" remains a live term of art: contemporary debates about laws of nature, counterfactual and probabilistic theories of causation, and scientific realism are still structured around the question of whether the world contains genuine necessary connections or only, as the "Humean" holds, a vast mosaic of contingent regularities. David Lewis's influential counterfactual analysis of causation took its cue directly from the counterfactual gloss in Hume's first definition. Whatever the correct reading of Hume the man, "Humeanism" names one of the permanent options in metaphysics.