Sensitism: Sensitism: The source of knowledge is perceptual perception, and the experience feeling is the standard of truth. Such as the ancient Greek school of Cyrene and Epicurus. Relativism: Relativism holds that all knowledge is relative, because the objects in knowledge are in a specific external environment and a specific relationship between subject and object, and the objects are relative to other specific things and to a specific cognitive subject, so relativism holds that knowledge is relative. Such as the Greek school of the wise. Skepticism: Skepticism: Similar to relativism, it holds that all special finite beings are not true, truth is unknowable, and we can only have subjective opinions but can't acquire knowledge about truth. Doubt all arbitrary and defenceless statements about the truth. Therefore, we should not make any positive or negative judgments, but should suspend all judgments. Such as Greek pirism. The connection of three kinds of thoughts can be said that these three kinds of thoughts have their internal connections … Because things in the sense are special changes, they are relative, so we can doubt their truth. Hume is a typical example of the development from empiricism to skepticism. It is worth noting that skepticism is definitely not a negative theory. The original meaning of the word "doubt" in Greek is actually the meaning of inquiry, which means that if we can't find a fixed basis for knowledge, we can't give a judgment easily, but we should keep reservations and keep exploring it. Skepticism should be regarded as the most terrible and admirable opponent of philosophy, because it constantly urges us to find a solid foundation for knowledge, which is still a difficult problem in the field of epistemology.