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 As a critic of Cartesian subjectivity, Heidegger sought to ground human subjectivity in death as that certainty which individualizes and authenticates our being. As he wrote in 1927: "This certainty, that "I myself am in that I will die," is the basic certainty of Dasein itself. It is a genuine statement of Dasein, while "cogito sum" is only the semblance of such a statement. If such pointed formulations mean anything at all, then the appropriate statement pertaining to Dasein in its being would have to be "sum moribundus" [I am in dying], "moribundus" not as someone gravely ill or wounded,