Sartre´s masterwork and major philosophical work of his life is Being and Nothingness, which became the core of Existentialism. He speaks of consciousness, bad faith, the existence of “nothingness”, free will and authenticity. The idea that individuals can always choose their own actions, even in situations which appear to enslave them.
He begins with the origin of negation, the empty nothingness or opposite of being. Our conscious existence introduces the idea of nothingness into the world. What this means is that we are able to conjure up things that aren’t physically visible to us.
For example, you might see your bed because its right in front of you, but you can also not see a pyramid, you can imagine it being there, and thus your experience of the current room is altered and structured around the fact that there is not a pyramid in it.
Sartre believes consciousness involves making ongoing distinctions between things and yourself. He explains that making these distinctions that make things appear as they do in our experience, also involves their continuing to not appear to be other things. It is a process of negation.
A table continues to be a table, it is not an animal, an automobile, or an abstract formula. In other words, things are what they are by their continuing to not be what they are not.
Thus, perception is a negative process and consciousness affects it by nihilating, to encase in a shell of non-being.
The Being For-itself and The Being In-itself

The theory of nothingness is central to Sartre’s philosophy. He distinguishes between two kinds of being: consciousness or what he calls the Being For-Itself, which is the source of all meaning. And on the other hand, a mode of existence that simply is, which is not conscious and is relevant only to inanimate objects, the Being In-Itself.
One of the problems of human existence for Sartre is the desire to attain Being-In-Itself, which he describes as the desire to be God, a longing for full control over one’s destiny and for absolute identity, only attainable by achieving full control over the destiny of all existence.
The world is meaningful to us because we, the For-Itself, give meaning to the In-Itself. The For-Itself uses the world to try to give itself some kind of definition, but it is pure nothingness.
Sartre doesn’t believe you can define humanity, whatever we are is so free that we can constantly redefine whatever we are. Nothing could ever become necessary for us. Therefore, we are a kind of nullity. But we must have some kind of content, we need to become, what he calls an In-Itself For-Itself. That is, we need to become conscious of having some meaning and content.
All of our activity is understood by trying to cover up our nothingness and delude ourselves into thinking that we have an identity, some kind of content and meaning in our lives. But since we really don’t and can’t have it because we are pure freedom and nothingness, we are a futile passion or in despair. So, we are constantly in bad faith, we are the kind of being that needs something that we can’t have.
The Being For-Others

In addition, he later adds the Being For-Others, which englobes the whole of society. He states that many relationships are created by people’s attraction not to another person, but rather how that person makes them feel about themselves by how they look at them. Whenever Sartre thought about what other people were thinking when they were looking at him is fundamental to his existence and to all his writing.