Heidegger’s destruktion of Cartesian metaphysics stands as one of his most arguably revolutionary and monumental contributions to philosophical thinking in Being and Time.
Not only did Heidegger’s critique of Descarte’s epistemology implicate the latter’s divorce of the subject from the world, but also its implications within which the Cartesian subject as self-enclosed and distinct from its world gave rise to generations of philosophical “pseudo-problems,” most notably the problem of the external world and the problem of other minds.
Accordingly, the very problems taken to be the central focus of modern philosophy (vis-a-vis epistemology) are derived on account of the fact that they have taken-for-granted certain assumptions, beginning with the mis-conception of the human-being as first existing and then finding its world.
For Heidegger, human being is being-in-the-world and being-with-others; these are the very structures of human existence itself. The problem of Cartesianism is the problem of the positing of the human being as the ontological center, and thus leads invariably to subjectivism.
For Descrates, doubt pushes knowledge to its outermost limits, and thus is the means by which first principles are determined: whatever can survive Cartesian “methodological doubt” thus serves as the foundational principles of philosophical thinking. Thus, when Descartes applies his thoroughgoing doubt to all, he inevitably comes to the conclusion that he, the doubter, must in fact exist — and this is the unquestionable grounds upon which philosophy in the Cartesian (metaphysical) tradition takes as its starting point. Thus, the very act of doubting inevitably proves that the “I” for whom does the doubting — it follows that the “I” must exist (or at least think).
Thus, Cartesianism makes the move from “I think” to “I exist.” But this doesn’t really seem to accord with the phenomena (and Heidegger goes on to show that this is indeed the case) by which the individual “I” must exist in the first place in order to “think.” It is from here that Cartesian subjectivism mis-construes human being (the “thinking substance” as self-evident grounding.
At the same time, Heidegger’s existential analytic of Dasein’s radical finitude in its authentic being-towards-death does indeed seem to manifest a “destructive retrieval” and creative repetition of Cartesian doubt. But whereas Descarte’s “doubt” was a cognitive method by which all assumptions were put to the wall of doubt in order to see if they could measure up, Heidegger’s thinking as regards Dasein and its mortality. If we look closely, we can begin to see how Heidegger’s retrieval of Descarte’s epistemological doubt provides the basis for an existential-ontological doubt as understood in “death.”
In Being and Time, Heidegger employs an existential analysis of “death,” by which he takes to signify not merely Dasein’s “ontic” death – that is, the event by which life ceases; but rather, as the ultimate impossibility of all Dasein’s possibilities. Death, then, undercuts all Dasein’s inauthentic certitude with respect to its average-everyday existence.
Death, and the accompanying mood of anxiety that precedes an authentic stance on one’s own mortality and radical finitude, dis-closes Dasein as not the ground of its existence, but rather the ground of the “not.” In coming to grips with its ownmost possibility of death, its only certainty, Dasein realizes its possibility of individuating itself by choosing among its possibilities; yet, such decisions always involve a nullification of all other possibilities, since Dasein is incapable of exercising such infinite possibility. Therefore, death opens up Dasein as the ground of a “nullity,” the “not” as possibility rooted in the existential structure of Dasein allowing its possibilities to show themselves as they are in themselves.
It is in the mood of anxiety (Angst) that Dasein is presented with the possibility of coming to terms with its own mortality and finitude. Anxiety, contra fear, is a feeling of free-floating uncanny-ness. It is not something that Dasein can overcome, for it is a part of Dasein’s structure.
Heidegger describes the feeling of anxiety with the German word “unheimlich,” which translated means “un-home-like,” or a “not-being-at-home.” Authentic selfhood can only come about in Dasein’s confrontation and acceptance of its death as its “ownmost” possibility (that which can belong only to that individual Dasein) and as its “uttermost” possibility (the possibility that nothing succeeds death).
The relationship between “death” and “existential doubt” can be understood in two distinct manners.
First, the possibility of Dasein’s death as its ownmost possibility undercuts the myriad certainties and platitudes that make up its average-everyday being-in-the-world. When one experiences the grips of anxiety, one is confronted with the radical uncertainty of human existence. Existential doubt, as opposed to epistemological or cognitive doubt, permeates through existence itself: casting its shadow over all of Dasein’s existence and de-limiting its possibilities through its ownmost impossibilities.
Secondly, just as Cartesian doubt is the means by which the modern philosopher is to determine the unshakable foundations of philosophical thought, so too does death/existential doubt provides Dasein with the possibility of giving meaning to its existence and its world by cutting-through the “idle talk” and “curiosity” of its inauthentic mode of being-in-the-world — opening up (dis-closing) the abyss (abgrund) of Dasein’s existence which lets meaning arise through its existence.
“To be be” is to be finite; and to be “finite” is to be cast in (existential) doubt. Only mortals are besieged by doubt; by the exhausting and inescapable uncertainties of the abyss of human existence.