c. 490-c. 421 B.C.

Greek sophist who said "man is the measure of all things," meaning all perceptions are true and only individuals can judge the quality of their own sensations. According to Protagoras, the phenomenal world is composed of contradictory qualities. When one experiences something as cool while another experiences it as warm, neither is wrong because both qualities are in the thing. The qualitative experience each has derives from their selective perception of qualities that coexist in matter.