On Doubt and Death From Being and Time up through his later writings, Heidegger regards mortality as man's defining characteristic. In Being and Time, Heidegger argues that the core of the authentic behavior which reveals Being and individuates human existence lies in an authentic being-towards-death. Dasein is not the ground of its existence, but the ground of the "not". For Dasein individuates itself by choosing among its possibilities. Yet with every choice, it annuls all other possibilities, since it can select only one. Its power and capacity to be is mainly a power not to be. It is the ground of a nullity (Nichtigkeit). The not is a possibility rooted in Dasein's existential constitution which, far from being a negation of things, makes them possible: it allows them to show themselves as they are in themselves. In his later writings (for example, in "The Thing"), Heidegger uses the term 'mortals' to refer to man. The concept of "mortals" suggests that the meaning of things and the semantic field created around them is preserved only so long as the human being participates in the play of revelation as a mortal, finite being. It is death that allows man to give meaning to his existence and to his world. Human existence is not the ground (grund) of Being, but the abyss (abgrund) which creates meaning, which lets meaning arise through his existence. It follows that while human existence is not the ground of itself for Heidegger, like Dasein earlier, it is the ground of the not. The role of death in Heidegger's thinking is analogous to that of doubt in Descartes'. The Cartesian doubt is generally regarded, rightly, as a mechanism for the production of first principles. It is a means of pushing knowledge to its limits so as to discover what cannot be doubted. Those ideas which can survive even the strongest doubt can thus be considered unshakable foundations for philosophy. Doubt is thus part of the cognitive act, it is "essentially connected with the indubitable," (7) that is with certainty. But the basis of doubt for Descartes is man's finitude. In his third meditation, Descartes argues that if man had been able to produce the idea of an infinitely perfect being - that is, God - he would be perfect and all knowing himself and not suffer from doubt. But clearly, being finite, man cannot grasp God's infinite substance. The doubt that ultimately leads to certainty thus rests on human mortality, much as does Heidegger's meaning. In criticizing Descartes' view of man, Heiddeger does not question the cogito as such. As we saw above, Heidegger regards man as having an understanding of Being. His argument is that taking the ego cogito as a starting point leaves the sum indeterminate. In a lecture in 1925, Heidegger says: 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, but insofar as I am, I am moribundus. The MORIBUNDUS first gives the SUM its sense. (8) With this turn of the Cartesian formula, Heidegger is not trying to exorcise traditional philosophy from the Cartesian phantom. Rather, he is trying to conquer Cartesianism by completing the Cartesian inquiry on man. To be, to exist, is to be finite, that the possibility of death, which is ultimately realized, accompanies all of our acts, including the act of thinking. The meaning of sum, for Heidegger, is finitude. (1) Heidegger M., Poetry, Language and Thought, New York: Harper and Row, 1971, p. 18. (2) Poetry, Language and Thought, p. 41. (3) Destruktion is the term used in Heidegger's Being and Time (Oxford: Basil Blackwell, 1980); Abbau can be find Heidegger's Basic Problems of Phenomenology (Bloomington: Indiana University Press, 1982); Verbindung is discussed mainly in "The Principle of Identity," in Identity and Difference (New York: Harper and Row, 1969, pp. 23-41); for Uberwindung see Heidegger's Nietzsche. (4) Nietzsche, vol. 4 p. 97. See Aristotle's words: "that which is called a substance most strictly, primarily, and most of all, is that which is neither said of a subject nor in a subject, e.g., the individual man or the individual horse." (Aristotle's Categories, 2a 11-13). (5) Heidegger, Basic Problems of Phenomenology p.111. (6) Heidegger M., Discourse on Thinking New York: Harper and Row, 1966, p. 7. (7) Nietzsche, vol. 4, p. 106. (8) Heidegger, M. History of the Concept of Time, Bloomington: Indiana University Press, 1992, pp. 316-317.