He thinks that reason is more reliable than sensory feeling. He gave an example: when we dream, we think we are in a real world, but in fact this is just an illusion. He found four laws from logic, geometry and algebra:
Never admit that anything is true, and what I have no doubt about is taken as truth;
Every problem must be divided into several simple parts to deal with;
Thought must be from simple to complex;
We should make a thorough inspection from time to time to ensure that nothing is missed.
Descartes applied this method not only to philosophical thinking, but also to geometry, and founded analytic geometry.
Therefore, Descartes first thought that doubt is the starting point, and the knowledge of sensory perception can be doubted, so we can't trust our senses. So he won't say "I see therefore I am" or "I listen therefore I am". From this, he realized a truth: what we can't doubt is "our doubt."
Meaning: What we can't doubt is the "doubt itself" when we "doubt" this matter. Only in this way can we be sure that our "suspicion" is true, not a fake. Anyone who takes it for granted or takes it for granted is puzzled, so he introduces the famous philosophical proposition-"cogito ergosum".
Descartes regards this as the most basic starting point in metaphysics, from which it is concluded that "I" must be something independent of body and thinking. Descartes also tried to prove the existence of God from this starting point.
Descartes thinks that we all have the concept of perfect entity. Because we can't get a perfect concept from an imperfect entity, we must have a perfect entity-God-to get this concept.
Starting from these two points, Descartes continued to infer that since the perfect thing (God) exists, then we can be sure that the previous demon hypothesis cannot be established, because a perfect thing cannot allow such a demon to deceive people, so we can determine that "this world really exists" through constant doubts, and the mathematical logic after proof should be correct.
In the real world, there are many characteristics that can be rationally perceived, that is, their mathematical characteristics (such as length, width and height). When our reason can clearly recognize a thing, then it must not be illusory, but it must be what we know.
Although Descartes proved the existence of the real world, he believed that there are two different entities in the universe, namely, thinking (mind) and the external world (matter), both of which come from God, and God exists independently. He believes that only man has a soul, and man is a binary being, who can think and occupy space. Animals belong only to the material world.