Protagoras

Protagoras was the first to call himself a Sophist and to charge for teaching. He came from Abdera, Democritus' city, and for forty years travelled Greece teaching young men how to speak in the assembly and the courts, earning handsomely. Pericles asked him to write the laws for the new colony of Thurii. Plato named a dialogue after him and drew him as a gracious, confident, quick-witted old man.

His famous sentence, 'man is the measure of all things, of things that are that they are, and of things that are not that they are not', founded relativism. The same wind is cold to you and not to me, and both perceptions are true; there is no objective standard beyond individual experience. In On the Gods he said he could not know whether gods exist, the question being too obscure and life too short, a remark that reportedly got him expelled from Athens and his books burned.

He did not conclude that nothing matters. The Sophist's task, he said, is not to replace false with true but to replace a worse condition with a better one, as a physician uses drugs to improve a patient's sensations. What he taught he called the political art: how to be a good citizen. Much of Socrates' lifelong questioning was argument with him and his colleagues.