William of Ockham (c. 1287–1347) was born in the southern English village of Ockham, joined the Franciscan order, and lectured at Oxford. In 1324 he was summoned to Avignon — the seat of the papacy — to face investigation on charges of heresy; the proceedings dragged on for four years and in 1328 he fled to Germany, spending the rest of his life under the protection of the emperor Louis IV, writing tracts against the papacy.

His best-known philosophical contribution is the principle popularly called 'Ockham's razor'. The actual formulation is 'pluralitas non est ponenda sine necessitate' — plurality should not be posited without necessity. The familiar short form 'entia non sunt multiplicanda praeter necessitatem' was attributed to him only in later centuries. The principle, however, is the genuine core of his thought: the best explanation of a phenomenon assumes as few entities and as few hidden properties as it can manage.

That simple principle had devastating consequences for medieval metaphysics. Ockham rejected the claim, accepted by Aquinas, that universals exist in things (universalia in re): only individuals exist, and 'humanity' or 'goodness' are concepts in the mind (nominalism). In epistemology he gave priority to observation (intuitive cognition), pushing inference to second place. In the next centuries this approach was read as a precursor to the empiricism of Bacon and Hume. Behind the Reformation theologians' opposition to Aquinas lay Ockham's earlier dismantling of his realism. From Aquinas to Ockham, scholastic thought reached its limits and quietly opened the door to modernity.