Pmtagoras (about 490 BC-42 BC1year) was born in Aberdra, a remote city-state, and was a fellow countryman of democritus. He worked as a teacher in Athens for 40 years, and was later expelled because he was accused of blasphemy. According to Plato, he was the first person to call himself a wise man. Protagoras thinks that the sages who taught skills in past dynasties, such as orpheus, hesiod and Homer, are actually wise men, but not in the name of wise men. He claimed to follow them, but added, "I openly admit that I am a smart person and educate the people, which is the opposite of them." He is famous for his eloquence, has trained a large number of students, and has written many books, including On God, On Truth, Law of Contradiction and so on. Protagoras enjoyed a high reputation in ancient society. In the Ptolemy statue unearthed in Memphis, Egypt, protagoras was juxtaposed with Thales, Heraclitus, Plato and others.

Protagoras has a famous saying: "Man is the measure of everything." People often quote it to express all kinds of bad ideas. If "man" is understood as a human being opposed to everything in the world, then this sentence expresses the anthropocentric view. If "person" is understood here as an individual relative to others, then this sentence expresses a solipsistic view. There are also different understandings of "scale": do people measure everything according to their own desires or their own knowledge? If it is the latter, then, do people judge things by feeling or reason? Socrates understood that "man is the measure of all things" as "things are what they look like to me". If this is in line with protagoras's original intention, it is a sense of truth. Plato explained protagoras's words: "Each of us is a measure of existence or non-existence. Everything in the world is different from another person just because it exists for one person. In his view, it is different from what exists for another person. " According to this understanding, relativism boils down knowledge to feeling and equates feeling with personal feeling. Since everyone can only judge things according to their own feelings, since people feel different, they must not make the same common judgment on the existence and essence of things. Moreover, everyone has their own scales and standards, and there is no universal standard to measure the pros and cons of these different understandings. In this way, the conclusion of relativity is naturally deduced. Socrates retorted that if this relativism principle is carried out more thoroughly, there is no reason to deny that animals such as pigs, dogs and monkeys are also the yardstick of everything, because they have feelings like people. What is the reason for saying that human feelings are superior to animals? "Man is the yardstick of all things" then came to its opposite.

Gorgias

Gorky Jarls (about 4808- 370 BC) put forward three propositions in a book called On Non-existence or on Nature:

First, nothing exists; Second, if something exists, people can't know it; Third, even if you know, you can't tell others.

His motive for making these remarks is probably not to express an extremely nihilistic and thoroughly skeptical view, otherwise he should first explain why these views can exist and how he grasped and expressed them. His motivation seems to be to test the relativism principle that "everything has two opposite statements". According to parmenides, there are things, and nothing does not exist; Things that don't exist can neither be recognized nor spoken. Gorgias did the opposite, put forward and demonstrated the opposite viewpoint, so as to show people that since all truths that are regarded as absolutely unchangeable have opposite opinions, can other truths be an exception?

Socrates

Sures (469 BC-399 BC) was born in a middle-class family in Athens. He inherited a sculpture studio from his father. His mother is a midwife. Socrates is ugly, short, stumbling and has a special personality. He was a brave and tenacious soldier in the Peloponnesian War. He usually lives like a wise man and takes it as his duty to educate young people. His dialogue teaching method also has the demeanor of a wise man, but he is essentially different from the wise man: he never charges tuition fees, and he is more opposed to sophistry and specious rhetoric. He claims to be a man who loves wisdom but has no wisdom, rather than a wise man. Although there is only one word difference between "people who love wisdom" and "people with wisdom", their meanings are fundamentally different. The former refers to the philosopher who pursues the certainty of truth, while the latter refers to the wise man who makes money by showing off his knowledge. For Socrates, philosophy is not a purely speculative private issue, but his civic responsibility to the polis. In this sense, he claimed to be a sacred gadfly who criticized the shortcomings of the times.

Socrates set an immortal example for later philosophers with his practice and personality. He doesn't have any works, and his thoughts are expressed in conversations with others. Today, people understand his life and thoughts mainly through the works of his two students-Bonofini and Plato. Historian Xenophanes recorded Socrates' words and deeds in four books: Home Economics, Defense, Banquet Collection and Memoirs. Plato's dialogues are mostly centered on Socrates, but it is generally believed that only his early dialogues basically reflect Socrates' thoughts, among which four dialogues of Socrates during the trial, such as Complaint, Clifton, Yusef Frodo and Lax, are reliable materials.