Rationalism senses. For instance, his famous dictum, "cogito ergo sum" or "I think, therefore I am", is a conclusion reached "a priori" i.e., prior to any kind of experience on the matter. The simple meaning is that doubting one's existence, in and of itself, proves that an "I" exists to do the thinking. In other words, doubting one's own doubting is absurd. This was, for Descartes, an irrefutable principle upon which to ground all forms of other knowledge. Descartes posited a metaphysical dualism, distinguishing between the substances of the human body (""res extensa"") and the mind or soul (""res cogitans""). This
Cogito, ergo sum in the former case, and an objective thinker in the latter. He argues, first, that it is impossible to make sense of "there is thinking" without relativizing it to "something." However, this something cannot be Cartesian egos, because it is impossible to differentiate objectively between things just on the basis of the pure content of consciousness. The obvious problem is that, through introspection, or our experience of consciousness, we have no way of moving to conclude the existence of any third-personal fact, to conceive of which would require something above and beyond just the purely subjective contents of the mind.