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]