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The various reasons why Heidegger held back SZ I.3 are the subject of much commentary by scholars and very few words by Heidegger himself. In 1946 (“Letter on Humanism”) he wrote famously that in this projected third division the plan was to be that “everything turns about” (umkehrt), both as regards what was to be thought and how it was to be thought.17 He is referring here to the transition that had been planned from the very beginning of Being and Time and was to be carried out in SZ I.3 from ex-sistence as structurally opening up and sustaining the clearing to the clearing itself insofar as it makes possible the meaningfulness of things. In a lecture course at Freiburg in 1952 (on 13 February, precisely twenty-five years after Being and Time was published) Heidegger said:
Soon after Sein und Zeit was printed I got a shock. As regards In-der-Welt-Sein (which I have labored over for quite a long time and which Sartre, for example, has completely misunderstood), Sein is certainly always touched upon along with other things and certainly is there, but in the formulation it sort of limps along behind.18
Here Heidegger (as he often and confusingly did) is using “Sein” to refer to the clearing. The clearing certainly was mentioned in Being and Time, where it was identified with ex-sistence itself as being-in-the-world; but it was to come into its own only in SZ I.3, where the transition would take place from ex-sistence as sustaining the clearing to the clearing that is sustained by ex-sistence. But we must avoid the subject-object mistake of thinking of ex-sistence as standing on one side and the clearing on the other as a phenomenon that is separate (or even distinct) from ex-sistence. “They,” in fact, are the same phenomenon seen from two viewpoints—that of the sustaining and that of the sustained—but “both” are ex-sistence as thrown-open. The clearing is the οὗ ἕνεκα of ex-sistence, its very raison d’être, that for the sake of which ex-sistence ex-sists at all. The planned change of focus in SZ I.3 would concentrate on the same phenomenon as in SZ I.1–2, but now on the thrown-open clearing with regard to how it makes all forms of meaningfulness possible. But from SZ I.1–2 to SZ I.3 the same fundamental principle would hold: No ex-sistence, no clearing; no clearing, no ex-sistence.
17. GA 9: 328.1 with note = 250.1 with note.
18. GA 83: 650.30–651.3.