One billiard ball rolls toward another. You expect the second ball to move after they meet. David Hume asks what supports that expectation. Is the result something you could discover just by thinking about the first ball, or does your expectation depend on earlier experience (Enquiry, §4.8–10)?
His question connects the way we learn from events with the confidence we place in our predictions. It also separates explaining how a belief forms from proving that its conclusion must be true.
A writer who revised his work
Hume was a Scottish philosopher, historian and essayist who lived from 1711 to 1776. Born in Edinburgh, he attended its university. His Treatise of Human Nature appeared in 1739–1740, with books on understanding, passions and morals (Qu and Radcliffe, §1; Treatise contents).
He later returned to related questions in shorter works. The book now called An Enquiry concerning Human Understanding first appeared in 1748 as Philosophical Essays concerning Human Understanding. It received its familiar title in 1758 (Peter Millican, edition notes). The Enquiry concerning the Principles of Morals appeared in 1751 (Qu and Radcliffe, §1).
The dates distinguish different books and versions. Hume’s later writing should not automatically be treated as an unchanged summary of everything in the Treatise.
Feeling something and thinking about it
Hume calls more lively perceptions impressions and less lively perceptions ideas. His examples include the difference between feeling an emotion and thinking about it later. Impressions include passions as well as sensations (Enquiry, §2.1–3).
He argues that the materials of thought come from inward and outward experience. Imagination can combine them: a golden mountain joins familiar materials into something new (§2.5–6).
He even considers an exception. Could someone imagine one missing shade of blue after seeing the neighboring shades? Hume thinks this may be possible, while treating it as too unusual to overturn his general principle (§2.8). That qualification belongs in the account; the book does not simply announce an exceptionless rule and move on.
Experience, proof and habit
Hume distinguishes relations of ideas, such as mathematical truths, from matters of fact. He argues that the opposite of a factual prediction can be imagined without a logical contradiction. A future event is therefore not established in the same way as a mathematical demonstration (Enquiry, §4.1–2).
He then asks why a pattern observed before should continue. Answering that experience has worked well in the past already assumes that past patterns can guide future expectations. Hume argues that using that answer as a proof goes in a circle (§4.18–19).
His explanation of our continuing expectations is custom or habit. Repeated encounters lead the mind to expect one event after another. In his account, this tendency makes experience useful in everyday action (§5.5–6). The argument questions the foundation of inference; it is not a request to stop learning from experience.
A self found through changing perceptions
In the Treatise, Hume reports finding particular perceptions when he looks inward, rather than an unchanging self apart from them. He describes a collection of perceptions continually changing (1.4.6.3–4).
But the 1740 Appendix adds a serious difficulty. He says he cannot satisfactorily explain the principles that unite successive perceptions in thought or consciousness (Appendix 20–21). This is Documented self-criticism. Presenting his account as a settled proof that persons do not exist would omit his own uncertainty.
Feeling, reasoning and choosing
Hume gives sentiment a central role in moral approval and disapproval. Yet he also says that reasoning can be necessary to understand the facts and relations involved before a moral judgment is reached (moral Enquiry, §1.9). Feeling and reasoning are not simply interchangeable, and his account does not make careful thought irrelevant.
On freedom, he defines liberty as being able to act or refrain from acting according to one’s will. He argues that this can coexist with regular connections between actions, motives and circumstances (first Enquiry, §8.23). This is his proposed reconciliation of liberty and necessity, rather than a claim that everyone accepted it.
An identifiable later response
In the preface to his Prolegomena, Immanuel Kant credits Hume with changing the direction of his philosophical inquiry. In the same passage, Kant says he did not simply follow Hume’s conclusions (Kant, preface). Influence here includes disagreement.
How we know
What survives: Published books, revised editions and an appendix recording Hume’s own difficulties.
Whose voices are missing: A philosopher’s examples do not represent every person’s experience.
What is uncertain: The identity argument includes unresolved problems acknowledged by its author.
Then vs. now
Hume’s texts distinguish an experience, an expectation, an argument and a feeling. Those distinctions let us examine his questions without turning his philosophy into a modern brain theory or a finished explanation of every belief.