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Heidegger's the nothing

Can somebody give me a relatively straightforward account of Heidegger's concept of nothing? Especially in relation to his essay What is Metaphysics?
gpt
The key to this text is the line where Heidegger says that the sciences "look at beings a whole, an nothing else," and repeats a similar line three times. He then says: "Yet what is *this* nothing?" That is, the "nothing" indicated by this phrase "nothing else." As the essay eventually reveals, in reference to Hegel's *Logic*, "the Nothing" is another way for thinking about "Being," the main subject of all of Heidegger's essays.

One of Heidegger's most basic insight is the "ontological difference" between beings [*Seiende*] and being [*Sein*]. Previous philosophers, he thinks, have conflated the being of beings (what makes beings *be*) with the beings that are (the things, or entities, that "have" being).

So in sketching out the "nothing" that remains when we look at what lies outside of a total theory of everything, something that Heidegger also connects to Dasein's feeling of angst, Heidegger is attempting to pave a path toward the main topic of his entire philosophy: Being itself.

At the same time, Heidegger is clearly engaging in meontology (a theory of nothingness), and makes some very interesting points about how different conceptions of nothingness inform metaphysics. For example, there's the interesting paragraph of the difference between Greek and Christian meontology, and how this informs the respective theories of being.

Heidegger is also interested in how thinking the idea of "nothingness," the total nihilation of being (the nothing noths) helps us think about the "pure thatness" of beings, the fact that beings are not nothing, but rather reveal themselves in their manifestness. 
In *Being and Time*, Heidegger looked at angst to illustrate how most of the time, we are engaged with the world, interested in it, and care about it. In angst, all significance disappears into nothingness and depression, showing us that most of the time, the opposite is true.

In "What is Metaphysics?", Heidegger makes a similar move on the level of Being itself. Most of the time, beings are manifest, but we usually overlook this simple "thatness," taking it for granted. Only in thinking about the whole of beings against the background of the nothing do we notice the presence and manifestness that we take for granted on a regular basis. That's why Heidegger ends with that Leibniz quote: "Why are their beings, rather than nothing?" The feeling of this question, a sense of wonder at the pure manifestness of beings, attunes us into thinking about Being itself, thus making the ontological difference between Being and beings (what-is) evident. 

So that's the gist of what's going on in "What is Metaphysics?"