In §9 of Being and Time, whose aim is the preliminary presentation of “The Theme of the Analytic of Dasein” (Analytik des Daseins) (the title of the section), Heidegger explains “existence” (Existenz) thus: “The ‘essence’ (Wesen) of Dasein lies in its existence.. . . All of this entity’s being-so (Sosein) is primarily being” (SZ:42). Or, as he puts it in a slightly different way, “this entity (Seiende), in its being, comports itself to its being. … It is being that is in each case at issue for this entity” (SZ:41-2). He enlarges on this definition later, this time introducing the language of understanding (Verstehen): “Dasein is the entity that in its being comports itself understandingly to this being. The formal concept of existence is herewith indicated” (SZ:52-3). Dasein is the entity whose being is always at issue in what it does, that is, the entity who always has an understanding of itself (Selbstverständlichkeit), and whose self-understanding is constitutive of its “being-so,” its being what or who it is.

This central, existentialist thesis lies at the heart of Heidegger’s conception of human being. Let us formulate it thus:

The Existentiality Thesis: If Dasein is A, then it is A because it understands itself as A.
(BHTI:32)