8. What Heidegger calls Seinsvergessenheit is the forgottenness not of Sein but what makes possible Sein and Seinsverständnis.
a. It is impossible for human beings to forget "being" without ceasing to be human. Indeed, metaphysics has never forgotten "being" or the ontological difference. Even the failure to thematize one's unthematic awareness of "is" does not count as what Heidegger means by Seinsvergessenheit (cf. GA 9, 263.28-30).
b. Seinsvergessenheit is shorthand for "the forgottenness of what allow for taking-as and thus 'being'" (cf. SD 40.6, 18-24). Seinsvergessenheit means overlooking our hidden-withdrawn finitude and the Welt/Lichtung/Da that it holds open.
c. This lack is already intrinsically hidden and withdrawn (= the lethe), but its hiddenness is "Redoubled" (doubly and improperly hidden) when it is overlooked or forgotten in Seinsvergessenheit (SD 44.14-19, 26-30).
d. However, one can also understand and embrace that intrinsically hidden finitude--as one's own essence, as what generates the open clearing, and as the source of all taking-as and occurence of "is." In the early Heidegger the act of understanding and embracing own's finitude is called "resolve" (Entschlossenheit: GA 2, 393.36). In the later work it is described as returning to and settling into one's opened-ness (die Einkehr in das Ereignis: SD 44.16-17, 26).
In Being and Time (1927) [the relation between Da-sein and Be-ing] first grasped as “understanding of being,” whereby understanding is grasped as projecting-open –- and the opening-throwing as thrown, and that means: belonging to en-ownment by be-ing itself.
But if we fail to recognize in advance the strangeness and uniqueness (incomparability) of be-ing -- and together with it what is ownmost to Da-sein -- then we succumb all too easily to the opinion that this 'relation" corresponds to or is even commensurate with the relation between subject and object. But Da-sein has overcome all subjectivity; and be-ing is never an object, re-presentable. It is in every case only a being that is capable of becoming an object -- and even here not every being.
Heidegger writes "Entschlossenheit" ("resoluteness" or "decisivesness") as "Ent-schlossenheit" ("un-closedness") in order to emphasize that the existential "resoluteness" whereby Dasein finds a way to authentically choose the commitments which define it (after having been radically individualized in being-toward-death) does not entail deciding on a particular course of action ahead of time and obstinately sticking to one's guns come what may, but rather requires an "openness" whereby we continue to be responsive to the emerging solicitations of our particular existential "situation." The existential situation in general is thus not unlike a living puzzle we must continually "re-solve."