Rene descartes (1596- 1650) is a famous French philosopher, physicist, physiologist and mathematician. Descartes not only founded analytic geometry, but also linked algebra and geometry, which were completely separated at that time, and answered "How to judge whether something is true?" A big step has been taken on this philosophical issue, which has made later philosophers relaxed a lot, because they no longer need to count sheep to confirm the truth. Descartes overthrew the ancient Greek scholars' answers to this question: spiritual temperament (because I told you), pathos (because it would be better) and logos (because it makes sense), but advocated solving problems with paper and pen from the beginning.

Descartes put forward the method of "universal doubt". In On Methods, he pointed out that all existing knowledge is unreliable, and we should be skeptical about them, including feelings, dreams and even mathematical concepts. In order to rebuild knowledge, we must find a solid and reliable foundation. He found that thought can doubt all objects, but not itself, that is, I can't doubt "I doubt" and "I" is the subject of this doubt, and he came to the conclusion that "I think, therefore I am".

The so-called "I think" and "I am" are juxtaposed and both point to the same "I" entity. "I think" is the essence, and "I am" is existence. The significance of this proposition lies in establishing that the self is an ideological entity, that is, the starting point of all cognition. Moreover, from the confirmation of the existence of "self" consciousness, it can be inferred that everything as self-evident as "I think" is a true concept. He said: "consciousness only knew its existence, but now it tries to expand its knowledge and finds that it has many ideas;" ..... it won't deceive itself on these ideas. " "I think we can establish a universal law, that is, what we think clearly and clearly is true." Therefore, Descartes believes that intuition is the primary way to discover self-evident truth.

The so-called intuition is not the appearance of feeling or false imagination, "but the idea of pure and focused mind, which is easy and unique, so that we will not have any doubt about what we understand;" ..... that is, the pure and dedicated mind is produced by the only light ",such as self-existence, thought, triangle bounded by three sides, circle on the plane, etc. These are all obtained through intuition. But the initial principle (first principle) is intuitive, while the distant inference needs to be deduced. Deduction means: "everything that must be deduced from what is known." "According to Descartes' point of view, deduction starts from the first principle of simplicity and self-evident, and adopts reasoning similar to geometric methods to draw complex propositions.

If we sum up Descartes' methodology in one sentence, we should start from doubting everything, base our knowledge on human's own reason, get the self-evident first principle from intuition, and deduce other propositions from it.