Man is the measure of all things: of things which are, how they are, and of things which are not, how they are not. - Protagoras

Socrates: So you believe that each man’s opinion is as good as anyone else’s.

Protagoras: That’s correct.

Socrates: How do you make a living?

Protagoras: I am a teacher.

Socrates: I find this very puzzling. You admit you earn money teaching, but I cannot imagine what you could possibly teach anyone. After all, you admit that each person’s opinion is as good as anyone else’s. This means that what your students believe is as good as anything you could possibly teach them. Once they learn that each person is the measure of all things, what possible reason would they have to pay you for any further lessons? How can you possibly teach them anything once they learn that their opinions are as true as yours?

Protagoras: Truth is relative. It is only a matter of opinion.

Socrates: You mean that truth is mere subjective opinion?

Protagoras: Exactly. What is true for you is true for you, and what is true for me, is true for me. Truth is subjective.

Socrates: Do you really mean that? That my opinion is true by virtue of its being my opinion?

Protagoras: Indeed I do.

Socrates: My opinion is: Truth is absolute, not opinion, and that you, Mr. Protagoras, are absolutely in error. Since this is my opinion, then you must grant that it is true according to your philosophy.

Protagoras: You are quite correct, Socrates.