  Because the mystery is unintelligible, it is the nothing (no-thing). It is nonetheless a positive ontological phenomenon—a necessary feature of the essential unfolding of Being. This vision of the nothing, as developed in Heidegger's What is Metaphysics?, his 1929 inaugural lecture as Professor of Philosophy at Freiburg, famously attracts the philosophical disdain of the logical positivist Carnap. Carnap judged Heidegger's lecture to turn on a series of unverifiable statements, and thus to be a paradigm case of metaphysical nonsense (Carnap 1932/1959; for a nice account and analysis of the disagreement between Heidegger and Carnap, see Critchley 2001). But placing Carnap's positivist critique to one side, the idea of the nothing allows Heidegger to rethink our relationship with death in relation to poetic habitation. In Being and Time, Being-towards-death is conceived as a relation to the possibility of one's own non-existence. This gives us a sense in which Dasein has an internal structural relation to the nothing. That internal structural relation remains crucial to the later philosophy, but now ‘the nothing’ is to be heard explicitly as ‘the mystery’, a kind of ‘dark matter’ of intelligibility that must remain concealed in the unfolding of Being through which beings are unconcealed. This necessary concealment is “the essential belongingness of the not to being as such” (Contributions 160: 199). In Being-towards-death, this “essential belongingness” is “sheltered” and “comes to light with a singular keenness” (Contributions 160: 199). This is because (echoing a point made earlier) the concealing-unconcealing structure of Being is ultimately to be traced to Dasein's essential finitude. Sheehan (2001) puts it like this: “[o]ur finitude makes all ‘as’-taking… possible by requiring us to understand things not immediately and ontically… but indirectly and ontologically (= imperfectly), through their being”. In Being-towards-death, the human finitude that grounds the mystery, the plenitude of possible worlds in which I am not, is highlighted. As mortals, then, our internal relation to death links us to the mystery (see The Thing). So dwelling (as poetic habitation) involves not only embeddedness in the fourfold, but also, as part of a unitary ontological structure, a necessary relationship with the mystery. (As mentioned earlier (2.2.7), it is arguable that the sense of the nothing as unactualized possibilities of Being is already at work in Being and Time (see Vallega-Neu 2003, 21). Indeed, Heidegger's explicit remarks on Being-towards-death in the Contributions (sections 160–2) suggest that it is. But even if that is so, the idea undoubtedly finds its fullest expression in the later work.)