David Hume

Key ideas

1. Impressions and ideas. Everything we think with comes from experience. Ideas are fainter copies of impressions, the lively perceptions of sense and feeling, and a word with no impression behind it has no clear meaning.
2. Relations of ideas and matters of fact. Every object of reasoning is either a relation of ideas, known by thought alone and certain, as in arithmetic, or a matter of fact, known only through experience. The division is often called Hume’s fork.
3. Causation and induction. We never observe a necessary connection between a cause and its effect, only one kind of event constantly followed by another. Our expectation that the future will resemble the past rests on custom, not on reasoning: the problem of induction.
4. The self. Looking inward, Hume finds no simple, enduring self, only perceptions, and concludes that a person is “nothing but a bundle or collection of different perceptions”.
5. Testimony and miracles. Our trust in testimony rests on experience of its agreement with the facts. Since a miracle runs against the uniform experience that establishes a law of nature, no testimony suffices to establish one unless its falsehood would be more miraculous than the event it reports.
6. Reason and the passions. Reason alone never moves us to act; it “is, and ought only to be the slave of the passions”. Morality rests on sentiment, and an “ought” cannot simply be drawn from an “is”.

David Hume was born in Edinburgh in 1711, studied at the university there, and was meant for the law. He found instead, as he wrote at the end of his life, that he could abide nothing but philosophy and general learning. The Treatise of Human Nature, written in France in his twenties and published in 1739–40, was ignored, and he recast its arguments in the shorter Enquiries of 1748 and 1751. He was refused professorships at Edinburgh and at Glasgow, in part for his reputation as a sceptic about religion, and won fame instead as an essayist and as the author of a History of England. He died in Edinburgh in 1776; the Dialogues Concerning Natural Religion appeared three years later.

The science of man

Hume’s ambition was to do for the mind what Newton had done for nature: to explain belief, emotion and action by a few principles of association, discovered by observation. The method is empiricist all the way down. If an idea cannot be traced to an impression, we should suspect that it has no content; and when he went looking for the impressions behind the ideas of necessary connection, of an enduring self and of substance, he found less than philosophers had assumed.

The results are mostly negative in form and constructive in effect. We cannot prove that the sun will rise tomorrow, but we cannot help expecting it, and the expectation is a natural and serviceable habit. We cannot find the self by introspection, but we can explain why we suppose there is one. Hume’s scepticism describes the limits of reason in order to show what does the work instead: custom, imagination and feeling.

Testimony, religion and morals

Hume’s essay on miracles applies the same principles to testimony. We believe witnesses because experience has taught us that what people report usually agrees with what happens, and that trust is calibrated, like every other inference from experience, against the rest of what we know. It was his critic Thomas Reid who argued that trust in testimony is instead a basic principle of the mind; the dispute between them still organises the epistemology of testimony.

In ethics Hume held that reason informs us about means and matters of fact but cannot by itself move us; moral distinctions are felt rather than inferred, and they rest on our capacity for sympathy with others. His observation that an “ought” cannot be drawn from an “is” without explanation remains one of the most cited sentences in moral philosophy.

Reading Hume

Begin with the first Enquiry, which is shorter and more polished than the Treatise: Sections IV and V on induction and custom, then Section X on miracles, which the library reproduces with commentary. The Dialogues reward reading as a whole; they are among the few philosophical dialogues in English that are also good dialogues.