Descartes epistemology is known as foundationalism.
In his Meditations, Descartes tries to discover certain, indubitable foundations for knowledge. He is searching for absolute certainty, and does this by subjecting everything to doubt. Through this he reaches the one thing he believes to be certain, his existence.
In Meditation One, Descartes describes his method of doubt. He subjects all of his beliefs to the strongest of doubts. He invokes the notion of an all-powerful, evil demon who could be deceiving him in the realm of sensory perception, in his very understanding of matter and even in the simplest cases of mathematics such as in the equation 2+3=5. The doubts may be obscure, but this is the strength of the method; the weakness of criteria for what makes a doubt reasonable means that almost anything can count as a doubt. Therefore whatever withstands doubt must be something that he considers absolutely certain.
In Meditation Two, Descartes finds the one indubitable principle that he has been seeking. He exists, at least when he thinks he exists. This view holds that Descartes asserts that he is thinking, he believes that ‘whatever thinks must exist’ and therefore that he logically concludes that he exists. Furthermore Descartes is convinced that he exists since there is a God deceiving him about his existence which could only be done if he did exist. “But there is a deceiver of supreme power and cunning who is deliberately and constantly deceiving me. In that case I too undoubtedly exist, if he is deceiving me; and let him deceive me as much as he can, he will never bring it about that I am nothing so long as I think that I am something.
So after considering everything very thoroughly, I must finally conclude that this proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind.” (p. 80). This leaves him with a problem. He can know his own existence, that he is a thinking thing and the contents of his consciousness, but how can any of this ever lead to any knowledge of anything outside of himself? The answer is that, by itself, it can’t.
Descartes establishes that the human mind is better known than the human body. He states that no belief based on sense-perception is free from doubt. Thus he cant be certain about the existence of his hands, head or body in general since they are all perceived through his senses. Descartes tries to show that we know bodies through reason and not through senses. To accomplish this he considers a physical piece of wax even though the evil demon might be deceiving him. At this point the piece of wax has a honey flavor, and it has the sent of flowers.
It has a color, and a distinct shape and size. It is hard and cold and if you rap on it will emit a sound. He then put the piece of wax next to a fire which melts the wax and in turn, changes its contingent qualities.
The wax no longer tastes of honey or smells of flowers. The original shape disappears and its size increases. It becomes a hot liquid you can hardly touch. And if you rap on it no sound is emitted.
It looks, tastes, smells, feels, and sounds completely different from the original piece of wax. Each of the sensory qualities have changed or been transformed, yet the same piece of wax remains. After you remove everything that does not belong to the wax, it is precisely something extended, flexible, and mutable. If you ignore the senses, the wax is still wax; but if you focus on the accidental qualities, the two pieces of wax have nothing in common. This means that you cannot look to the senses for truth about physical objects. The wax is capable of innumerable changes even though the imagination is not capable of relating them, therefore this insight is not achieved by the faculty of imagination.
Descartes concedes that he does not grasp what this wax is through the imagination, but rather perceives it through the mind alone. His imagination gives him finite pictures whereas the wax has infinite shapes. When he distinguishes the wax from its external forms there might be an error in his judgment, but it is definite that he cannot perceive it without a human mind. If he judges that the wax exists from the fact that he sees it, then from the same fact that he sees the wax, it is much more evident that he himself exists. It is possible that what he sees is not wax at all, but it is impossible that while he sees or thinks he sees, he who thinks is not something. If he thinks or senses or imagines, then he and the nature of his mind necessarily exist.
At the end of the Meditation Two, Descartes comes to the conclusion that nothing can be perceived more easily and more evidently than his own mind. He has discovered that even bodies are not accurately perceived by the senses or the faculty of imagination, and are only accurately being perceived by the intellect. He also realizes that they are not distinguished through being touched, smelled, or tasted, but by being understood alone. It is the ability of reason that gives the knowledge and lets the mind know the truths and essences of objects.