This essay aims to clarify Heidegger’s perspectives in criticizing the certainty of Descartes’ cogito ergo sum, and, from these perspectives, to explain Heidegger’s idea of the certainty of Being based on death in Sein und Zeit (1927).
Heidegger interpreted the certainty of Descartes’ ‘I am’ as a mere semblance in his lecture course published as Prolegomena zur Geschichte des Zeitbegriffs (1925), because the most certain eventuality for Dasein is its own death. Only by understanding its death can Dasein find the proper certainty of its own Being, not by thinking. In this way, Heidegger destroyed and reconstructed the certainty of ‘I am’.
However, Heidegger had already critically interpreted the certainty of ‘I am’ in his lecture course published as Einführung in die phänomenologische Forschung (1923–1924), without any discussion of death. In this course, Heidegger remarked that a mathematical standard governs thinking or doubt in Descartes’ philosophy, and this standard became canonical for the distinction between certainty and uncertainty. In Heidegger's view, Descartes arrived at the certainty of ‘I am’ on the basis of that standard, without thinking deeply about this self-assertion's meaning. Thus, Heidegger judged that Descartes’ certainty is insufficient for establishing the certainty of Dasein who can say ‘I am’. Moreover, Heidegger indicated that the proposition cogito ergo sum conceals the Being of Dasein. In short, Heidegger criticized certainty in Descartes from two perspectives: 1) objecting to a mathematicallybased certainty and 2) concealing the Being of Dasein who says ‘I am’.
Heidegger therefore returned to the Being of Dasein who says ‘I am’ , and disclosed its certainty from Dasein’s Being as such. He arrived at the idea of death, which is appropriate and certain for Dasein’s Being. The critical reading of Descartes in Heidegger’s early thought is necessary to establish the true argument for the certainty of Dasein in Sein und Zeit.