Protagoras 490 BC – 420 BC

Protagoras of Abdera was a Greek thinker traditionally counted as the first of the Sophists. He traveled widely as a teacher of rhetoric and civic virtue, charging substantial fees, and spent much time in Athens, where he was a friend of Pericles. His best-known doctrine, that man is the measure of all things, has been read since Plato as a thoroughgoing relativism about truth and value. He also professed agnosticism about the gods, a stance for which his works were reportedly burned in Athens.

Protagoras was born around 490 BC at Abdera in Thrace, the city that also produced Democritus. The most famous of the professional teachers of arete known as Sophists, he traveled extensively across the Greek world over a career of more than forty years, and is reported by Plato to have been the first to charge a fee for instruction in rhetoric and political excellence. He visited Athens repeatedly and enjoyed the friendship of Pericles, who entrusted him in 443 BC with the drafting of the laws of the new Panhellenic colony of Thurii.

His writings, which Diogenes Laertius lists at length, are lost beyond a few sentences and the testimony of Plato — above all the dialogues Protagoras and Theaetetus — and Aristotle. The most famous fragment is the opening of his treatise On Truth: 'Of all things the measure is man, of the things that are that they are, of the things that are not that they are not.' From his book On the Gods comes the equally celebrated agnostic opening: 'Concerning the gods, I am not in a position to know.'

Protagoras's relativism about appearance and his claim that on every question two opposing arguments can be made provoked Plato's lifelong argument with sophistry and shaped the entire Greek discussion of knowledge and objectivity. Tradition reports that he was tried for impiety in Athens and that, fleeing to Sicily, he was drowned in a shipwreck around 420 BC.