|Portrait of Descartes|
But this is a misrepresentation of what Descartes is trying to do in his 1637 work the Discourse on Method. He is not starting from the point of a “witch doctor,” and he certainly does not end up there after he has presented his arguments.
He is not doubting the existence of the external world (as Rand is alleging in her essay)—he is trying to develop a mode of enquiry that will save him from the error of skepticism. His doubt is not metaphysical; it’s epistemological. The aim of his philosophical project is to resolve his doubt concerning “his knowledge” of the existence of the external world. He decides that the right method to resolve this doubt is to begin with an utterly skeptical position and a profession of ignorance. This standard essentially rules out the objects of perception, as well as feelings and thoughts.
“Cogito, ergo sum,” (I think, therefore I am), which Rand misunderstood as Descartes’s attempt to “prove” his own existence, is a summary of the grounds on which he bases his valid knowledge claims. Adopting an axiomatic method of inquiry, he begins by assuming that one cannot claim with authority that one is an extended thing (res extensa), because the perception of physical characteristics and actions in the material realm can be a deception caused by a demonic force. It is possible for a human being to fail to distinguish between reality and a dreamed reality. As the empirical modes of verification are based on the observation of things which exist outside the mind, they are susceptible to the machinations of demonic forces.
But one’s thought is inseparably linked to the reality of one’s being or existence as an extended thing. In the realm of thought one cannot be deceived because an individual can be the subject of deception only if he is a thinking thing. This is the point of Descartes’s “cogito, ergo sum.” He is not trying to prove or deduce his existence or the reality of the external world, he already knows that he exists and the world exists; he is trying to prove the “knowledge” of his and the world's existence. What he is looking for is the necessary precondition for skepticism, which he finds in the reality of the mental space (res cogitans). The res cogitans is affirmed in the act of doubting it.