Martin Heidegger, answered from the texts and cited to the page.
The existential analytic of Being and Time Division II addresses the Being of Dasein as a structural unity. The first task was the demonstration in Division I that Dasein is being-in-the-world, structurally already engaged in a world of significance through everyday coping.12 Division II asks: what is the Being of this entity that is being-in-the- world?
The answer proceeds through the existential analyses of Sein-zum-Tode (being-toward-death), Gewissen (the call of conscience), and Entschlossenheit (resoluteness). Angst (anxiety) is the fundamental mood that discloses Dasein's finitude: it is the mood in which Dasein finds itself face-to-face with its own ownmost potentiality-for- Being, the possibility of its no-longer-being-able-to-be.
Authentic existence (Eigentlichkeit) is not a moral achievement; it is the proper relation of Dasein to its own finite being, the comportment in which Dasein owns up to its being-toward-death rather than fleeing into the anonymous publicness of das Man. The unity of Dasein's structures is Sorge (care), and the temporal horizon is originary temporality, in which the future has priority over the present and the past.
What this analysis is NOT is Sartre's existentialism as developed in Being and Nothingness: the Sartrean reading takes authenticity as the existentialist hero choosing his values in a meaningless universe and reads Dasein as a sovereign subjectivity. The Letter on Humanism (1947) is the explicit rejection of this misappropriation: authenticity is not the existentialist authentic individual; it is the proper relation of Dasein to Being, which is at the same time openness to what discloses itself in the clearing.3
BEING-IN AS SUCH ] 28. The Task of a Thematic Ana(ysis of Being-in IN the preparatory stage of the existential analytic ofDasein, we have for our leading theme this entity's basic state, Being-in-the-World. Our first aim is to bring into relief phenomenally the unitary primordial structure of Dasein's Being, in terms of which its possibilities and the ways for it 'to be' are ontologicallydetermined.Being and Time, pp. 169–170
V I CARE A S THE BEING O F DASEIN å 39· The Question of the Primordial Totality of Dasein's Structural Whole BEING-IN-THE-WORLD is a structure which is primordially and constantly whole. In the preceding chapters (Division One, Chapters 2-5) this structure has been elucidated phenomenally as a whole, and also in its constitutive items, though always on this basis.Being and Time, p. 225
I I BE ING-IN-THE-WO RLD IN GENE RAL AS THE BA S I C S T A T E OF D A S E I N Ʋ 12. A Preliminary Sketch of Being-in-the-World, in terms of an Orientation towards Being-in as such IN our preparatory discussions (Section g) we have brought out some characteristics of Being which will provide us with a steady light for our further investigation, but which will at the same time become structurally 53 concrete as that…Being and Time, p. 78
The most-read part of Heidegger's writing in the post-war Anglophone reception, largely because of Sartre's existentialist appropriation in Being and Nothingness (1943). The Letter on Humanism (1947) is the explicit response, rejecting the existentialist reading and reframing the BT analytic as the preparation for the Kehre's thinking of Being. Hubert Dreyfus's Being-in-the-World (1991) confines its commentary to Division I and treats Division II as 'less carefully worked out' than the analysis of being-in- the-world; Dreyfus and Jane Rubin's appendix engages Division II through Kierkegaard and the later Heidegger. Charles Guignon's work and the contemporary scholarly conversation around authenticity treat Division II as philosophically central but recognize its difficulty; the Kierkegaardian inheritance through Anxiety and the call of conscience is genuine. Hannah Arendt's appropriation of Augenblick (the moment of vision) as the category of political action gives the analytic a different inheritance in twentieth-century political theory.