There are two phrases in Heidegger’s Sein und Zeit that provide a clue to what is going on in that book: Dasein ist geworfener Entwurf and Dasein existiert faktisch (Dasein is thrown projection and Dasein exists factically). I begin by trying to show how an interpretation of these phrases can help clarify Heidegger’s philosophical claim about what it means to be human. I then try and explain why it is that, in a couple of important passages in Sein und Zeit, Heidegger describes thrown projection as an enigma (ein Rätsel). After considering the meaning and etymology of the word ‘enigma’, I trace its usage in Sein und Zeit, and try and show how and why the relations between Heidegger’s central conceptual pairings – state-of-mind (Befindlichkeit) and understanding (Verstehen), thrownness and projection, facticity and existentiality – are described by Heidegger as enigmatic. My thesis is that at the heart of Sein und Zeit, that is, at the heart of the central claim of the Dasein-analytic as to the temporal character of thrown-projective being-in-the-world, there lies an enigmatic apriori. That is to say, there is something resiliently opaque at the basis of the constitution of Dasein’s being-in-the-world which both resists phenomenological description and which, I shall claim, is that in virtue of which the phenomenologist describes. In the more critical part of the paper, I try and show precisely how this notion of the enigmatic apriori changes the basic experience of understanding Sein und Zeit. I explore this in relation to three examples from Division II: death, conscience and temporality. I try and read Heidegger’s analyses of each of these concepts against the grain in order to bring into view much more resilient notions of facticity and thrownness that place in doubt the move to existentiality, projection and authenticity. The perspective I develop can be described as originary inauthenticity. As should become evident, such an interpretation of Sein und Zeit is not without political consequences.