What is Science (5). Rationalism
Descartes is using the “method of doubt” in order to find the “first element of knowledge” the unshakable first principle. Since, according to methodic doubt, all my thoughts might be mistaken, there is only one element left, the thought itself; I think therefore I am.
“I have convinced myself that there is absolutely nothing in the world, no sky, no earth, no minds, no bodies. Does it now follow that I too do not exist? No: if I convinced myself of something then I certainly existed. 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.”
So according to Descartes thought is the “first element of knowledge”³ is the basis, the axiom e that we will start building from. The belief that we have internal knowledge independent from experience, that we can use to build new knowledge is what makes Descartes a rationalist;
Rationalism is a method or a theory “in which the criterion of the truth is not sensory but intellectual and deductive”
According to the Stanford Encyclopedia of Philosophy, to be a rationalist is to adopt at least one of three claims;
The Intuition/Deduction Thesis: Some propositions in a particular subject area, S, are knowable by us by intuition alone; still others are knowable by being deduced from intuited propositions.
The Innate Knowledge Thesis: We have knowledge of some truths in a particular subject area, S, as part of our rational nature.
The Innate Concept Thesis: We have some of the concepts we employ in a particular subject area, S, as part of our rational nature.
We can generate different versions of the Intuition/Deduction thesis by substituting different subject areas for the variable ‘S’. Some rationalists take mathematics to be knowable by intuition and deduction. Some place ethical truths in this category. Some include metaphysical claims, such as that God exists, we have free will, and our mind and body are distinct substances. The more propositions rationalists include within the range of intuition and deduction, and the more controversial the truth of those propositions, the more radical their rationalism.
³ Some believe that the unshakable knowledge that is hidden behind “I think therefore I am” is only the existence of God. (http://plato.stanford.edu/entries/descartes-epistemology/#4.2)