(c. 490—420 bc)
Of Abdera (c.490–420 bc), the most celebrated of the sophists. He travelled widely throughout the Greek world, including several visits to Athens, where he was associated with Pericles. He was invited to write the constitution for the Athenian colony of Thurii. The ancient tradition of his condemnation for impiety and flight from Athens is refuted by Plato's evidence (Meno 91e) that he enjoyed a universally high reputation till his death and afterwards. See intolerance, intellectual and religious. He was famous in antiquity for agnosticism concerning the existence and nature of the gods, and for the doctrine that ‘The human being is the measure of all things’, i.e. the thesis that all sensory appearances and all beliefs are true for the person whose appearance or belief they are; on the most plausible construal that doctrine attempts to eliminate objectivity and truth altogether. It was attacked by Democritus and Plato (in Theaetetus) on the ground that it is self‐refuting; if all beliefs are true, then the belief that it is not the case that all beliefs are true is itself true. Protagoras also held that on every subject, there were two opposed arguments, and he wrote two books of ‘Opposed Arguments’. In Protagoras Plato represents him as maintaining a fairly conservative form of social morality, based on a version of social contract theory; humans need to develop social institutions to survive in a hostile world, and the basic social virtues, justice and self‐control, must be generally observed if those institutions are to flourish.