believed in an independent nonmaterial soul inhabiting and finding expression in a mechanicallynonmaterial soul inhabiting and finding expression in a mechanically operated body.operated body. • The reality of the body needed no proof, the reality of the soul did.The reality of the body needed no proof, the reality of the soul did. • Descartes used his aphorism as proof:Descartes used his aphorism as proof: cogito ergo sumcogito ergo sum, "I Think,, "I Think, therefore I am."therefore I am." • We cannot doubt the existence of our own self, because we cannotWe cannot doubt the existence of our own self, because we cannot doubt it unless there is a self to do the doubting.doubt it unless there is a self to do the doubting.