Protagoras of Abdera was the most prominent of the Sophists, the professional teachers who taught rhetoric and civic skill for a fee in fifth-century Athens. He is said to have been the first to charge for instruction, and to have made a great deal of money doing it.

Man is the measure

His most famous sentence survives because Plato quoted it in order to attack it: man is the measure of all things, of those that are that they are, and of those that are not that they are not.

Read strictly, it makes truth relative to the perceiver. If the wind feels cold to you and warm to me, it is cold to you and warm to me, and there is no further fact about the wind. Plato's objection in the Theaetetus is that the position undercuts itself — if every belief is true for whoever holds it, then the belief that Protagoras is wrong is also true, for whoever holds it.

On the gods

A second fragment records that he could not know whether the gods exist or not, since the question is obscure and life is short. The remark is one of the earliest recorded statements of agnosticism, and tradition holds it got his books burned and him exiled, though the story may be later invention.

Nomos and physis

Protagoras is associated with the distinction between nomos, convention, and physis, nature — the claim that laws and customs are human arrangements rather than features of the world. It is the foundation of later argument about the social contract and about whether justice is discovered or made.

What survives

Almost nothing. A handful of sentences, quoted by people arguing against him. Everything we have about the most influential teacher of his generation comes from the prosecution, which is worth remembering when his position sounds easily refuted.