Hume: The Science of Man and the Limits of Reason

Where We Are

Locke emptied the mind of innate ideas but kept the world, the self, and God; Berkeley dropped matter but kept God and the self (Lesson 25). David Hume takes the empiricist principle — every idea must trace back to an experience — and applies it with a consistency neither predecessor dared, following it past every comfort. The result is the most unsettling philosophy in the British tradition: by the time Hume is finished, the necessary connection between cause and effect, the rational justification of science, the continuing self, and the credibility of miracles have all quietly dissolved. He did it, by all accounts, as the most serene and good-humored man of his age — which only makes the demolition harder to shrug off.

The Historical Moment

David Hume (1711–1776) was the leading mind of the Scottish Enlightenment, that astonishing flowering of thought in eighteenth-century Edinburgh and Glasgow. He wrote his masterpiece, A Treatise of Human Nature, before he was thirty; it "fell dead-born from the press," as he ruefully put it, so he recast its ideas in the more accessible Enquiries (1748, 1751). His reputation for irreligion cost him professorships at both Edinburgh and Glasgow, so he made his living as a librarian, a diplomat, and a wildly successful historian — his History of England was the bestseller that paid his bills. In Paris he was lionized by the philosophes as le bon David, "the good David," genial and rotund and beloved. His project was a "science of man": to do for the human mind what Newton had done for the heavens, finding the few simple principles that govern how we think.

Honesty requires complicating the genial portrait at once. In a footnote to his essay "Of National Characters," Hume wrote that he was "apt to suspect the negroes to be naturally inferior to the whites," doubting any non-white civilization had achieved anything of note. It is a repugnant passage, and not a slip the course will bury where Hume buried it: it sits in direct contradiction to his own moral philosophy of universal human sympathy, it lent the prestige of a great philosopher to the racial theories that justified slavery and empire, and it is part of the reckoning (Lesson 53) with how the Enlightenment proclaimed universal reason while drawing its circle of full humanity to exclude most of the world. A thinker can be both genuinely great and genuinely complicit, and saying both is the only honest course.

Impressions and Ideas

Hume begins by sorting the contents of the mind. All our "perceptions" come in two kinds: impressions — the lively originals, our sensations and feelings — and ideas — the fainter copies of them that we use in thinking and remembering. The fundamental law of his system is that every simple idea is a copy of a prior impression: you can form no genuine idea that was not first delivered by experience. (Hume honestly flags one counterexample of his own — a particular shade of blue you have never seen, which you might imagine from neighboring shades — then sets it aside as a curiosity.) This gives him a weapon of terrifying simplicity, a test to expose empty words masquerading as thoughts:

When we entertain, therefore, any suspicion that a philosophical term is employed without any meaning or idea (as is but too frequent), 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.

— David Hume, An Enquiry Concerning Human Understanding §2, 1748

Point this test at the metaphysician's prized concepts — substance, the soul, necessary connection — and demand the impression each is copied from. Where none can be produced, the word is hollow. Much of traditional philosophy, Hume implies, is about to fail the test.

Causation and the Problem of Induction

Now Hume aims the weapon at the single idea on which all of science rests: cause and effect. We believe the world is bound together by necessary connections — that the cue ball makes the other move, that fire must burn. But look for the impression of this "necessary connection," Hume says, and you will not find it. Watch one billiard ball strike another as many times as you like: you see the first move, then the second move; you see them touch; you never see any power, any necessity, passing between them. All that experience ever delivers is constant conjunction — events of type A have always been followed by events of type B. The "must" is not out there in the world. So where does our irresistible conviction come from?

Custom, then, is the great guide of human life. It is that principle alone which renders our experience useful to us, and makes us expect, for the future, a similar train of events with those which have appeared in the past.

— David Hume, Enquiry §5

Having seen B follow A a thousand times, the mind simply forms a habit of expecting B when A appears, and this felt expectation is the only original of our idea of necessity — a feeling in us, projected onto the world. This is the famous problem of induction. Why are we entitled to assume the future will resemble the past — that bread will nourish tomorrow as it did today? Not by logic, for there is no contradiction in supposing tomorrow differs. And not by experience, for to argue "the future has resembled the past before, so it will again" simply assumes the very principle it sets out to prove. Our most basic reasoning about the world — the engine of the entire scientific revolution (Lesson 20) — rests not on reason at all but on custom. (Six centuries earlier, the theologian al-Ghazali had made almost exactly this point: we see fire and burning together, but never the necessity binding them, Lesson 14. Hume, who could not have read him, rediscovered the argument and made it the cornerstone of modern epistemology.)

The Bundle Self

If the test dissolves causation, it makes short work of the self. Descartes had found in the cogito a thinking substance; Locke had at least kept a continuing consciousness (Lesson 25). Hume applies his rule — from what impression is the idea of the self derived? — and comes up empty-handed:

For my part, when I enter most intimately into what I call myself, I always stumble on some particular perception or other, of heat or cold, light or shade, love or hatred, pain or pleasure. I never can catch myself at any time without a perception, and never can observe any thing but the perception ... [men] are nothing but a bundle or collection of different perceptions, which succeed each other with an inconceivable rapidity, and are in a perpetual flux and movement.

— David Hume, A Treatise of Human Nature I.iv.6, 1739

Introspect as hard as you like, and you never encounter a self that has the perceptions — only the perceptions themselves, streaming past. The mind is "a kind of theatre" where perceptions appear — except, Hume adds, there is no theater and no audience, only the play. The enduring, unified self is a fiction the imagination spins out of the resemblance and succession of these fleeting states. (Readers have long noticed how close this comes to the Buddhist doctrine of anattā, no-self — Lesson 32 will find the first serious Western engagement with Indian thought in Schopenhauer.) The bundle self runs straight to Parfit (Lesson 52), who will argue that, this being so, personal identity matters far less than we think.

Reason, Passion, and the Is/Ought Gap

Hume's account of action and morality is just as subversive. We flatter ourselves that reason governs conduct — that we work out what is right and the will obeys. Hume denies it. Reason discovers facts and traces means to ends, but it never, by itself, moves us to act; only desire, feeling, passion can do that. Hence his most provocative sentence:

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.

— David Hume, Treatise II.iii.3

Reason is the navigator, but passion is the engine. From this it follows that morality cannot be founded on reason alone. We do not deduce that cruelty is wrong; we feel a sentiment of disapproval when we contemplate it. Morality is rooted in a moral sense — in sympathy, our capacity to feel with others — a view Hume shared with his teacher Francis Hutcheson and developed alongside his closest friend, the economist Adam Smith, whose Theory of Moral Sentiments grounds ethics in the judgments of an "impartial spectator" within. And Hume caps the argument with a logical observation that founded an entire field. Surveying the moral writers, he notices them slide, without warning, from statements about what is the case to pronouncements about what ought to be:

[F]or as this ought, or ought not, expresses some new relation or affirmation, 'tis necessary that it should be observed and explained; and ... that a reason should be given, for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it.

— David Hume, Treatise III.i.1

You cannot validly derive an ought from an is: no pile of facts about the world, by itself, entails a conclusion about what we should do. This "is/ought gap" (later nicknamed Hume's guillotine) still divides metaethics (Lesson 52) and stands as a permanent challenge to anyone who claims to read morality straight off nature or science.

Miracles, Religion, and the Common-Sense Rejoinder

Hume turned the same cool empiricism on religion, and here it cut deepest. In the essay "Of Miracles," he frames a principle of evidence and a devastating application: since a miracle is by definition a violation of a law of nature, and a law of nature is established by the most uniform experience humanity possesses, the evidence against any miracle is always maximal. So testimony could only establish one if the testimony's falsehood would be even more astonishing than the miracle — which, given how often people lie, err, and crave wonders, it never is.

A wise man, therefore, proportions his belief to the evidence ... no testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous than the fact which it endeavours to establish.

— David Hume, Enquiry §10

In the Dialogues Concerning Natural Religion, published only after his death (he knew their danger), he turned to the era's favorite proof of God, the argument from design — the world is like a vast machine, so it must have a designer. Through his skeptical mouthpiece Philo, Hume picks the analogy apart: the universe is not much like a watch; even if it had a cause, the evidence cannot tell us that cause is single, perfect, or good; and the world's suffering counts against a benevolent author. He left the design argument mortally wounded, eighty years before Darwin (Lesson 36) would supply the alternative it lacked. Not everyone accepted the demolition. Hume's countryman Thomas Reid led the common-sense school in a counterattack on the whole "way of ideas": if Locke's and Hume's starting assumptions lead to the absurd conclusions that we cannot know the external world, causation, or our own continued existence, then — Reid argued — so much the worse for the assumptions. We are entitled to trust the testimony of our senses, our memory, and our expectation of nature's regularity as first principles of common sense, which no philosophy has the authority to overturn. That rejoinder runs forward to G. E. Moore (Lesson 37) holding up his hand.

Legacy: Who Listened

Hume is the hinge of modern philosophy, because he set the problem the giants who followed had to solve. Immanuel Kant said that remembering Hume on causation "interrupted my dogmatic slumber" and gave his thought "a quite new direction" — the whole of Module 7 is Kant's attempt to rescue knowledge from Hume's skepticism without denying its force. The problem of induction became the central problem in the philosophy of science, driving Popper's falsificationism and Kuhn's response (Lesson 40). The bundle self feeds Parfit and the modern debate over personal identity (Lesson 52). The is/ought gap and the sentimentalist account of ethics shaped emotivism (Lesson 39) and the entire map of metaethics (Lesson 52), while Adam Smith's moral psychology and economics grew into disciplines of their own. "Of Miracles" and the Dialogues remain the reference points of the philosophy of religion, and Hume's naturalism — explaining belief by human nature rather than justifying it by reason — anticipates Quine (Lesson 41). Even his racism had issue, lending false authority to the scientific racism the next centuries built. He met his own death, in 1776, with a cheerful composure that astonished his visitors, declining the consolations of religion he had spent his life questioning — Adam Smith wrote that his friend approached "as near to the idea of a perfectly wise and virtuous man, as perhaps the nature of human frailty will permit." The Enlightenment had no purer embodiment of its motto — proportion your belief to the evidence — and no more dangerous one.

Reflection Questions

1. Hume argues we have no rational justification for expecting the future to resemble the past — only habit. Yet we cannot live or do science without that expectation. Does this show that reason is overrated, that habit is more rational than it looks, or that there is a gap in our knowledge we simply have to live with?
2. When you "enter most intimately into yourself," do you find a self, or only Hume's stream of perceptions? If there is no enduring self underneath the experiences, what holds your life together — and does anything important depend on the answer?
3. Hume insists no "ought" can be derived from an "is" — that facts alone never entail values. If he is right, where do our values come from, and can any of them be more than expressions of feeling? If he is wrong, show how a fact could, by itself, oblige you.

Self-Check Quiz

Further Reading

• Primary: David Hume, An Enquiry Concerning Human Understanding, edited by Peter Millican (Oxford World's Classics, 2007) — the most accessible entry; Dialogues Concerning Natural Religion, edited by Dorothy Coleman (Cambridge University Press, 2007); and, for the bold, A Treatise of Human Nature.
• Secondary (introductory): A. J. Ayer, Hume: A Very Short Introduction (Oxford University Press, 2000).
• Deeper: Barry Stroud, Hume (Routledge, 1977); Don Garrett, Cognition and Commitment in Hume's Philosophy (Oxford University Press, 1997); and on the racism, Aaron Garrett and Silvia Sebastiani, "David Hume on Race," in The Oxford Handbook of Philosophy and Race (2017).
• Free online: Stanford Encyclopedia of Philosophy, "David Hume" and "Hume on Religion"; the Enquiry and Treatise at Project Gutenberg.