Sartre attempted to answer this question using the phenomenonological approach. Here’s some background from the wiki on “Being and Nothingness”.
Based on an examination of the nature of phenomena, he describes the nature of two types of being, being-in-itself (the being of things) and being-for-itself. While being-in-itself is something that can only be approximated by human being, being-for-itself is the being of consciousness.
https://en.m.wikipedia.org/wiki/Being_and_Nothingness
Every question brings up the possibility of a negative answer, of non-being, e.g. "Who is entering? No one." For Sartre, this is how nothingness can exist at all.
Interesting for the fact that Nothingness is a meditative attainment in Buddhism. Sartre also saw that it was a type of “being”.
Phenomenological ontology:
In Sartre's opinion, consciousness does not make sense by itself: it arises only as an awareness of objects. Consciousness is therefore always and essentially consciousness of something, whether this "something" is a thing, a person, an imaginary object, etc.
And there’s the concept/ question you’re inquiring into: being-in-itself
https://en.m.wikipedia.org/wiki/Being_in_itself
In Sartrean existentialism, being-in-itself (être-en-soi) is also contrasted with the being of persons, which he describes as a combination of, or vacillation or tension between, being-for-itself (être-pour-soi) and being-for-others (I'être-pour-autrui).[citation needed]
Being-in-itself refers to objects in the external world — a mode of existence that simply is. It is not conscious so it is neither active nor passive and harbors no potentiality for transcendence. This mode of being is relevant to inanimate objects, but not to humans, who Sartre says must always make a choice.[1]
Sartre is careful to never go into what being-in-itself “is”. He refers to it as “a plethora of being”. Something you might find in the attainment of “neither perception nor non perception”.
But Sartre does state that (at some place in the universe) being-in-itself had an upsurge in which it tried to “establish” itself by reflecting upon itself. There was a necessary upsurge where “being-in-itself-for-itself” was attempted.
Now, being in itself successfully created a “reflection” of itself. But that immediately caused the in-itself to be exactly what it was not - “an appearance”. This created a great division.
By causing itself to arise as an appearance, the “being-for-itself” came into “the picture”. The being-for-itself was able to be conscious of the “appearance” of being-in-itself. In no way was this consciousness “being-in-itself” (nor was the appearance. The appearance simply was consciousness - a for-itself on the face of the in-itself).
But that’s how the whole thing started. Being-in-itself attempted to “understand its own reality” and failed - in the process giving rise to appearance and consciousness which are simply nothingnesses on the face of reality.
We are nothingness. And this is the source of our anguish. Because we must always choose to be something that we are not - and not be something that we are.
The origins of the in-itself are never discussed. Only the relationship between appearance and consciousness on the face of an unknowable “plethora of being” (which, incidentally has no way of knowing itself. We, as humans, approximate it. Unless we transcend it. Ie. cessation of perception and feeling).
Its good if we can clarify the terminology used 1st.
