The famous philosophical proposition of protagoras, a wise man in ancient Greece in the 5th century BC. It was first seen in Plato's dialogue Tyatedes: "Man is the measure of all things, the measure of the existence of things that exist, and the measure of the non-existence of things that do not exist." It means that the existence of things is relative to people. How people feel, how things are; The feeling of the same thing varies from person to person and from time to time. These different feelings are not true or false. This view was criticized by Socrates, Plato and Aristotle.

Aristotle criticized him in the fifth chapter of the fourth volume of Metaphysics: According to protagoras, if there were no animals, there would be no world. Later, Pyrrho pushed protagoras's view to extreme skepticism. /kloc-before the 9th century, most thinkers regarded this proposition of protagoras as sophistry. It was not until G.W.F Hegel affirmed that this proposition reflected the initiative of thinking from the perspective of cognitive historical development.

This proposition represents protagoras's wrong views on relativism and skepticism, and has subjective idealism, but it touches on the relationship between subjective and objective, indicating the progress of human understanding.