  The notion of dwelling as poetic habitation opens up a path to what Heidegger calls ‘the mystery’ (not to be confused with the kind of obscurantist mysticism discussed above). Even though the world always opens up as meaningful in a particular way to any individual human being as a result of the specific heritage into which he or she has been enculturated, there are of course a vast number of alternative fields of intelligibility ‘out there’ that would be available to each of us, if only we could gain access to them by becoming simultaneously embedded in different heritages. But Heidegger's account of human existence means that any such parallel embedding is ruled out, so the plenitude of alternative fields of intelligibility must remain a mystery to us. In Heidegger's later philosophy this mysterious region of Being emerges as a structure that, although not illuminated poetically in dwelling as a particular world-as-fourfold, nevertheless constitutes an essential aspect of dwelling in that it is ontologically co-present with any such world. Appropriation is necessarily a twofold event: as Dasein is thrown into an intelligible world, vast regions of Being are plunged into darkness. But that darkness is a necessary condition for there to be any intelligibility at all. As Heidegger puts it in The Question Concerning Technology (330), “[a]ll revealing belongs within a harboring and a concealing. But that which frees [entities for intelligibility]—the mystery—is concealed and always concealing itself…. Freedom [sense-making, the revealing of beings] is that which conceals in a way that opens to light, in whose clearing shimmers the veil that hides the essential occurrence of all truth and lets the veil appear as what veils”.