Hume followed empiricism — the view that knowledge comes from experience — further and more fearlessly than anyone before him, and the results were disquieting. If all our ideas ultimately trace back to what we perceive, he asked, then how much of our everyday confidence about the world can actually be justified? His answer, again and again, was: less than we think. Reason, he argued, is not the commanding faculty we imagine, but something closer to a servant of habit.
His most famous challenge is the problem of induction. We constantly assume that the future will resemble the past — it is the basis of all prediction and science. Yet Hume showed this assumption cannot be proven. You cannot prove it by logic, since a future unlike the past is not contradictory; and you cannot prove it by experience, since that would already assume the point. We believe it, he concluded, out of custom, not proof. Likewise with causation: when we see one billiard ball strike another, we never actually observe a necessary connection — only one event followed by another. Our sense of 'cause' is a habit of the mind, projected onto the world.
Is and oughtThe gap in every moral argument.
Hume left an equally sharp challenge to ethics. He noticed that moral arguments often slide, without warning, from statements about what is the case to statements about what ought to be done — as if the second followed automatically from the first. But it does not, he argued: you cannot logically derive an 'ought' from an 'is' alone. This 'is–ought gap' forces anyone making a moral claim to supply something more than facts, and it remains a central problem in moral philosophy. He also turned his skepticism on the self, arguing that when he looked inward he found only a shifting bundle of perceptions, never a fixed, unchanging 'I.'
For all his skepticism, Hume was no nihilist. He thought that although reason cannot ground these beliefs, nature and habit make everyday life perfectly workable — we go on expecting the sun to rise, and rightly so, even without proof. Genial, witty, and enormously influential, he shaped the Enlightenment and, by his own account, awakened later philosophers to problems they could not ignore. His willingness to follow reason to its limits, and then to acknowledge those limits honestly, makes him one of philosophy's most bracing figures.