Protagoras is best known for his dictum: "Man is the measure of all things." He denied the existence of objective truth and values, replaced reality with appearance, and reduced truth to a matter of individual’s interpretation and perspective (perspectivism). Truth became thus relative to a group of people and individuals (relativism). Based upon the relativist view, as a Sophist he taught rhetorical skills to win arguments, thereby reducing philosophy from a quest for truth to mere skills of argumentation and persuasion. The shift of the locus of truth from the sphere of existence to language parallels an orientation of postmodernism. Both Socrates and Plato challenged his philosophy, and Plato named one of his dialogues after him.
Protagoras taught for nearly 40 years traveling Athens and surrounding cities, teaching the art of rhetoric and his philosophy to mostly wealthy Greek citizens. By the request of his friend Pericles, he drafted the laws of a new Greek colony Thurii. Protagoras wrote at least two books, Truth (or Refutatory Arguments or On Being) and On the Gods. His agnostic view of the gods presented in the latter caused his conviction on impiety and forced him to flee Athens, and his books were publicly burned. None of his works have survived except a few fragments. He was discussed in Plato’s dialogues, Protagoras and Theaetetus in particular, and Aristotle’s Metaphysics (IV. 4-5). In Aristophanes’ play, The Clouds, a teacher of rhetoric named Socrates was probably modeled after Protagoras or one of his followers.
Protagoras is best known for the dictum: "Man is the measure of all things: of things which are, that they are so, and of things which are not, that they are not" (DK. 80b1). He denied the existence of objective, eternal, and unchanging criteria of truth, values, good and evil.
Because the existence of unchanging truth, the common criteria to establish the objectivity of truth was denied; truth became merely what appeared to people living in diverse traditions and customs, and ultimately to individuals holding different beliefs and perspectives. ontologically, reality was replaced and reduced to appearance. Within this framework of thought, people can no longer argue about what is “real” since there is no objective reality, but can only argue what appears or looks real to each person.
Protagoras was probably the first theological agnostic. In On the Gods, he wrote, "Concerning the gods, I have no means of knowing whether they exist or not or of what sort they may be, because of the obscurity of the subject, and the brevity of human life" (DK 80b4). Protagoras neither denied nor affirmed the existence of gods but denied or was skeptical of the capacity of human beings to know gods, which is not atheism but agnosticism, a philosophical position in the theory of knowledge.
The Protagoras crater on the Moon was named in his honor.
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