-490 – -420
Protagoras of Abdera (c. 490–420 BC) was the most celebrated of the ancient Greek Sophists and a pioneering figure in epistemology and relativism. He is best known for the doctrine 'Man is the measure of all things,' asserting that truth and knowledge are relative to the individual perceiver. His professional teaching of rhetoric, his agnosticism about the gods, and his influence on Plato's epistemological dialogues secured his place as a central and controversial thinker in fifth-century Athenian intellectual life.
Formulated the homo mensura principle ('Man is the measure of all things'), the foundational statement of ancient relativism
Pioneered the professional teaching of rhetoric and argumentation for fees, establishing the Sophist model of education
Developed antilogic — the doctrine that contradictory propositions can both be defended
Articulated agnosticism about the gods in 'On the Gods', one of the earliest surviving expressions of religious skepticism
Served as a critical foil in Plato's Theaetetus, shaping the classical debate on false belief and the nature of knowledge