In §9 of Being and Time GA2
Sein und Zeit
SZ
SuZ
S.u.Z.
Être et temps
Ser e Tempo
Being and Time
Ser y Tiempo
EtreTemps
STMS
STFC
BTMR
STJR
BTJS
ETFV
STJG
ETJA
ETEM Sein und Zeit (1927), ed. Friedrich-Wilhelm von Herrmann, 1977, XIV, 586p. Revised 2018. [GA2] / Sein und Zeit (1927), Tübingen, Max Niemeyer, 1967. / Sein und Zeit. Tübingen : Max Niemeyer Verlag, 1972 , 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 GA2
Sein und Zeit
SZ
SuZ
S.u.Z.
Être et temps
Ser e Tempo
Being and Time
Ser y Tiempo
EtreTemps
STMS
STFC
BTMR
STJR
BTJS
ETFV
STJG
ETJA
ETEM Sein und Zeit (1927), ed. Friedrich-Wilhelm von Herrmann, 1977, XIV, 586p. Revised 2018. [GA2] / Sein und Zeit (1927), Tübingen, Max Niemeyer, 1967. / Sein und Zeit. Tübingen : Max Niemeyer Verlag, 1972 :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 GA2
Sein und Zeit
SZ
SuZ
S.u.Z.
Être et temps
Ser e Tempo
Being and Time
Ser y Tiempo
EtreTemps
STMS
STFC
BTMR
STJR
BTJS
ETFV
STJG
ETJA
ETEM Sein und Zeit (1927), ed. Friedrich-Wilhelm von Herrmann, 1977, XIV, 586p. Revised 2018. [GA2] / Sein und Zeit (1927), Tübingen, Max Niemeyer, 1967. / Sein und Zeit. Tübingen : Max Niemeyer Verlag, 1972 :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 GA2
Sein und Zeit
SZ
SuZ
S.u.Z.
Être et temps
Ser e Tempo
Being and Time
Ser y Tiempo
EtreTemps
STMS
STFC
BTMR
STJR
BTJS
ETFV
STJG
ETJA
ETEM Sein und Zeit (1927), ed. Friedrich-Wilhelm von Herrmann, 1977, XIV, 586p. Revised 2018. [GA2] / Sein und Zeit (1927), Tübingen, Max Niemeyer, 1967. / Sein und Zeit. Tübingen : Max Niemeyer Verlag, 1972 :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]