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, temperance, piety – and then patiently show that they could not. This is the Socratic method: 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.