Socrates

Socrates wrote nothing. Everything we know of him comes from his students — chiefly Plato — and his enemies. He spent his days in the Athenian agora questioning anyone who claimed to know something: generals, politicians, poets, craftsmen. He would ask them to define a word — justice, courage, piety — and then patiently show that they could not. This is the Socratic method: not teaching, but the systematic destruction of false certainty.

His genius was to turn philosophy from cosmology toward ethics. The question shifted from "what is the world made of?" to "how should I live?" His one claim to knowledge was that he knew nothing — which, paradoxically, made him the wisest man in Athens.

In 399 BCE the city tried him for impiety and corrupting the young. He could have escaped; he chose to drink hemlock, arguing that to flee would be to betray the laws under which he had lived. His death is the founding martyrdom of philosophy.