Who Was David Hume? The Sceptic Who Took Apart Cause and Effect
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A Scottish writer published a book at twenty-eight that, in his own words, fell dead-born from the press, failed twice to obtain a university chair because he was suspected of atheism, made his living as a librarian, a tutor, a diplomat's secretary and finally as a bestselling historian, and died in 1776 cheerfully refusing to recant. He is now generally regarded as the most important philosopher ever to write in English, largely because of an argument about causation that nobody has satisfactorily answered in two hundred and fifty years.
A life outside the academy
Born in Edinburgh in 1711 to minor gentry, he went to university at about twelve, abandoned the law his family intended, and at eighteen had what he described as a new scene of thought that consumed him. He wrote his major work in France in his early twenties and published it in 1739 and 1740 to almost no response, a failure he later blamed on the manner rather than the matter, and spent years recasting the same arguments in shorter and more readable form. He was twice rejected for professorships, at Edinburgh and Glasgow, on grounds of irreligion. His fame in his lifetime came from a six-volume history of England that was a commercial success and remained a standard work for decades. He was a central figure of the Scottish Enlightenment and a close friend of Adam Smith, who wrote a public account of his death that caused more outrage than any of his books, because it described a man facing the end without religion and entirely without distress.
Where knowledge comes from
His starting point is a thoroughgoing empiricism: everything in the mind is either an impression, a direct sensation or feeling, or an idea, a fainter copy of an impression. Any purported concept must be traced back to the impression it derives from, and one that cannot be traced is meaningless, a test he applies mercilessly. He then divides all reasoning into two kinds, which later writers called Hume's fork:
- •Relations of ideas, meaning mathematics and logic, which are certain, knowable by thought alone, and true because of how the terms are defined, so that denying them is a contradiction
- •Matters of fact, meaning everything about the world, which are known only through experience, and whose denial is never contradictory, since it is perfectly conceivable that the sun will not rise tomorrow
- •Anything that is neither, he concludes in a famous closing passage, contains nothing but sophistry and illusion and should be committed to the flames
The problem of induction
From that division comes the argument he is most famous for. We believe that bread will nourish us and that the sun will rise because it always has, which is reasoning from observed cases to unobserved ones. What justifies it? Not logic, since no contradiction arises from supposing the future to differ from the past. Not experience, since the claim that the future will resemble the past is exactly what we are trying to justify, and using past experience to support it assumes what is in question. The reasoning is therefore circular, and no amount of further observation helps. His conclusion is not that we should stop expecting the sun to rise, which would be absurd, but that the expectation is produced by habit rather than by reason. Custom, he says, is the great guide of human life, and it is a psychological mechanism rather than a logical one. Karl Popper took the problem as the starting point for his own philosophy of science, and no reply commands general agreement.
Causation, the self and miracles
The same analysis dismantles cause and effect. Examine one billiard ball striking another and you observe the first move, the contact and the second move; you do not observe any necessary connection between them, and no impression corresponds to the power supposedly transferred. What we have is constant conjunction plus a felt expectation in our own minds, which we project onto the world. Applied to ourselves, the method is equally destructive: looking inward, he reports finding only a bundle of perceptions, never the self that is supposed to be having them. Applied to religion, it produces his argument about miracles, which is not that they cannot happen but that no testimony could ever make believing one more reasonable than believing the witness was mistaken or lying, since a miracle is by definition the least probable kind of event. His posthumous dialogues on natural religion demolish the argument that the order of the world proves a designer, and the phrase ought cannot be derived from is, the point that no set of factual statements entails a moral conclusion, comes from a short paragraph in his first book.
What he left
Kant said reading him interrupted his dogmatic slumber and set him the problem his entire critical philosophy was written to solve. The logical positivists of the twentieth century took his fork as their criterion of meaning. Philosophy of science begins its account of induction with him. Cognitive science has come around to his picture of a mind governed by association and habit rather than by reason, and his claim that reason is and ought only to be the slave of the passions, meaning that reason establishes facts and desire supplies ends, is close to the standard modern account of motivation. His moral philosophy, which grounds ethics in sentiment and in the approval of an impartial spectator, shaped Adam Smith directly. He described himself in a short autobiography written as he was dying as a man of mild dispositions and great moderation in all his passions, and the most striking thing about his work remains the gap between the destructiveness of the arguments and the equanimity of the person making them.
The takeaway
The Edinburgh writer who published his major work at twenty-eight to no response, and was twice refused a chair for irreligion, argued that all content in the mind traces to sensation, that reasoning splits into certain relations of ideas and uncertain matters of fact, and that our confidence in the future resembling the past cannot be justified by logic or by experience without circularity. Causation, on inspection, is constant conjunction plus a habit of expectation, the self is a bundle of perceptions, and no factual premise yields a moral conclusion.