Protagoras of Abdera in Thrace, most famous of the Sophists, was born not later than 490 BCE and probably died soon after 421 BCE. According to Plato, he was the first to declare himself a professional Sophist. He went from city to city in the Greek world, offering instruction in return for money, and he undertook above all to train young men in the art of politics. He was well known in Athens, where he enjoyed the friendship of Pericles—he produced a theoretical basis for Periclean democracy and was asked by Pericles to draft the constitution for the new colony of Thurii in 443 BCE. He made contributions to grammatical and rhetorical theory, and his views on religion provoked charges of impiety against him in the courts, which led to his exile from Athens at the end of his life and to the public burning of...