SARTRE
THE EXISTENTIALIST
Mind over Matter
For Sartre, there are two regions of being within consciousness. In one realm, the being of "in-itself" ("en soi"), the objects of consciousness simply exist, independent and unaware of the consciousness that interacts with them. The being of "for-itself" ("pour soi"), what most would consider the conscious being, is conscious of both itself and of objects in the realm of the "in-itself". But since this level of consciousness is simply an awareness of other objects, it is essentially empty, transparent - a nothingness.
"The necessary condition for our saying not is that non-being be a perpetual presence in us and outside of us, that nothingness haunts our being."
"Being is what it is... the being of for itself is defined, on the contrary, as being what it is not and not being what it is."
("Being and Nothingness")
So, not only does Nothingness exist (an idea that Parmenides would have found startling in the extreme), but it enters the world through the human consciousness. To be a conscious being is to be aware of the gap between my consciousness and its intended objects. It is to be in the world and yet to be aware of not being one of the causally determined objects of the world. My conscious being is ever aware of a distance, a void, separating me from the realm of things. The thing that makes us human also makes the human condition one of isolation from the "real world", so to speak.