"Man is the measure of all things."Born in Abdera, Protagoras became one of the travelling teachers known as the sophists, professing to teach the art of politics. He lived for a time in Sicily and was well known in Athens, becoming a friend of the statesman, Pericles, who asked him to write a constitution for the Athenian colony of Thurii. A story of his condemnation for impiety and flight from Athens seems to be refuted by Plato's testimony in the Meno that he was much admired and enjoyed a good reputation until his death and afterwards. He and his ideas figure in a number of Plato's dialogues.("Truth")
Protagoras was most famous in antiquity for his agnosticism concerning the existence of the gods. The meaning of his opening statement of one of his books, that man is the measure of all things, is discussed in Plato's dialogue, Theatatus . This is based on the thesis that all sensory appearances and beliefs are true for the person experiencing them but not objectively true. His rejection of universal truths does not mean that he advocated that each man should act according to his own beliefs and opinions, ignoring the interests of others. In the Protagoras , Plato represents him as maintaining that humans need to develop social institutions to survive in a hostile world and that the basic social virtues of justice and self-control must generally be observed if institutions are to survive.