Protagoras prōtăgˈərəs [key], c.490–c.421 b.c., Greek philosopher of Abdera, one of the more distinguished Sophists. He taught for a time in Athens, where he was a friend of Pericles and knew Socrates, but was forced to flee because of his professed agnosticism. Protagoras was the author of the famous saying, “Man is the measure of all things.” He held that each man is the standard of what is true to himself, that all truth is relative to the individual who holds it and can have no validity beyond him. Thus he denied the possibility of objective knowledge and refused to differentiate between sense and reason. None of his works have survived, but one of Plato's most famous dialogues bears his name.

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