George J. Seidel on the catch in metaphysics.

Metaphysics, the truth “of” beings, is the denial of the truth “of” Seyn, which denial comes from Seyn. Seyn denies itself in the first beginning. It does not appear. The overcoming of metaphysics also comes from Seyn, taking back the denial. The quiet of the first beginning is overcome with the ringing in with the other beginning.30 In a much later piece, Das Wesen des Nihilismus (1946–1948), Heidegger puts the matter in even clearer terms: in metaphysics, Seyn remains unthought. This is, perhaps, the first thing that thinking needs to learn. The fact that Seyn is unthought means that the truth (Unverborgenheit, unconcealment) of things also remains unthought.31 After all, Seyn fails to appear. However, this is true of Sein as well. In metaphysics, the ontological difference is not thought; which means that neither is being thought. Heidegger’s point is that if the distinction between Sein and Seyn is not made, then the distinction between Sein and Seienden, which finds its basis in the former, will not be made either; which means, further, that being gets forgotten. And there is no way for metaphysics, which is essentially conceptual thinking, to catch the initial revelation (anfängliche Wesen) of Seyn, and with it Seyn, which is hidden from metaphysical thinking (MN, 171–172). It is a thoroughly Catch-22 situation for metaphysical thinking. Metaphysics should think being, but it is unable to get beyond beings; and when it does, all it gets is a Supreme Being. It can get to the ontological difference between being and beings, and thus Sein, only if it can think through to Seyn, which it is unable to do. Logos historical thinking is non-conceptual. Thus, the essence of Seyn, as also Sein, is inexplicable, hidden from metaphysical thinking. For Kierkegaard the Paradox cannot be thought. For Heidegger it is the Seyn, the Logos of Da-sein, that cannot be thought; which means that being does not get thought either. Hence, “Nihilism.”

From "Musing with Kierkegaard: Heidegger’s Besinnung".

Yossarian: "That’s some catch, that Catch-22."  ¶ 3:08 AM