Protagoras.

The full text is as follows: Human beings are the measure of all things, the measure of how beings exist, and the measure of non-existences.

Protagoras (approximately 490 or 480 BC ~ 420 or 410 BC) In the 5th century BC, the democratic politics of Greek city-states was highly developed. Protagoras was the main representative of the Sophists. He advocated that "man is the measure of all things." Protagoras was born between 490 and 480 BC and lived about 70 years old. He was born in the city of Abdera. He came to Athens, the center of the Greek slave-owning democracy at that time, many times. He became close friends with the democratic politician Pericles. He once formulated a code for the Athenian colony of Turi in southern Italy. He traveled around various places throughout his life, taking on apprentices and imparting knowledge of rhetoric and debate. He was the most respected "wise man" at that time.

Protagoras accepted Heraclitus’s thoughts on the flux of all things and believed that the sensory phenomenon of ever-changing things was real and that all things were constantly moving and changing. But he moved from this naive sensory theory to relativism and skepticism. It is asserted that everyone's feelings are reliable, and people make different judgments on everything based on their feelings, and there is no difference between true and false. Therefore, he put forward a famous proposition: "Man is the measure of all things." He believed that the existence of things is relative to human feelings. Things are how people feel. From this, he concluded that "knowledge is feeling" and advocated that knowledge can be obtained with the help of feeling. Based on this point of view, he raised doubts about traditional religious theology: "As for gods, I neither know whether they exist nor what they are like."

Protagoras’ ethical thought also has the characteristics of relativism. His famous proposition "Man is the measure of all things" emphasized the role and value of human beings and showed an individualistic ideological tendency, which later became a proposition of humanism. Protagoras believed that people obtain knowledge through their own feelings, and also use perceptual desires and the pursuit of self-interest as the standard of morality. Therefore, morality varies from person to person. He believes that morality is not mysterious. Just like skills, it is a manifestation of people's wisdom in engaging in private and public affairs. It can mediate family relationships and help people better participate in the affairs of the city-state. He pointed out that justice, wisdom, temperance, etc. are essential qualities for people, and respect and justice should be the principles between people. He opposed Socrates' innate morality theory, advocated that "virtue can be taught", and tried to prove that virtue cannot be born by nature or spontaneously. It is a character that can be taught and needs to be acquired through painstaking cultivation.