Protagoras

Protagoras of Abdera (c. 490–420 BCE) was the most famous of the Sophists, a travelling teacher of rhetoric and civic excellence whose dictum “man is the measure of all things” made each person’s perception the standard of truth.

Agnostic about the gods and bold in argument, he is the searching interlocutor of Plato’s dialogue that bears his name, where Socrates probes whether virtue can be taught.