David Hume

Hume took Locke's empiricism and pushed it until it broke. If all knowledge comes from sense impressions, then what justifies our belief that the future will resemble the past? Strictly nothing — only custom, the mind's habit of expectation. Causation is not something we perceive; we perceive only that one thing follows another, and we invent the rest. The self is not an inner observer; introspect honestly and you find only a bundle of perceptions, never the perceiver.

These conclusions were so radical that Hume himself, the most cheerful of philosophers, said only common life and a game of backgammon could pull him out of them. He also formulated the is-ought problem: no description of how things are can by itself justify a claim about how they ought to be.

Kant said Hume "woke me from my dogmatic slumber." Without Hume there is no Kant, no positivism, no modern philosophy of science.