First principle foundationalist system of philosophy. He used the "method of doubt", now called Cartesian doubt, to systematically doubt everything he could possibly doubt, until he was left with what he saw as purely indubitable truths. Using these self-evident propositions as his axioms, or foundations, he went on to deduce his entire body of knowledge from them. The foundations are also called "a priori" truths. His most famous proposition is "Je pense, donc je suis." ("I think, therefore I am", or "Cogito ergo sum") Descartes describes the concept of a first principle in the following excerpt from the preface to the "Principles
Cogito, ergo sum 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 the proposition, "I am, I exist," is necessarily true whenever it is put forward by me or conceived in my mind. (AT VII 25; CSM II 16–17) There are three important notes to keep in mind here. First, he claims only the certainty of "his own" existence from the first-person point of view — he has not proved the existence