One of the reasons that you possess freedom is that you possess what objects do not — consciousness. Being the Cartesian that he was, Sartre followed Descartes in distinguishing between extended substance and mental substance. One characteristic of mental substance is freedom.

Sartre said that when you analyze what appears to you, you discover two modes of being. First, there is the sort of being manifested in objects. This is called the in-itself (l'en-soi). The term in-itself signifies that objects are self-contained or self-identical. Sartre considers a manufactured object like a paper cutter. It is a man-made object designed for a purpose. Nothing in this object — or any objects — transcends what it currently is. A bottle, a mountain, or a house simply exists.

The second sort of being is what characterizes human consciousness. Sartre calls this sort of being the for-itself (le pour-soi). The term for-itself signifies that such beings are conscious and self-aware. Rather than being determined by external causes, as objects are bound by gravity, you are capable of spontaneous freedom and living your life in terms of future possibilities. You are “subjectivity,” Sartre wrote in Existentialism and Human Emotions, “Man first of all is the being who hurls himself toward the future and who is conscious of imagining himself as being in the future.”

Sartre's greatest work was Being and Nothingness (1943). The title captures the two modes of being. Being in-itself is simply there, without possibilities. When consciousness is present, however, “nothingness” is introduced into the world. Only the for-itself can become separate from the bare existence of things in the causal order in this way. Consciousness has the ability to separate itself from things and to live in the “what-is-not” (the realm of possibilities). You become aware of your own consciousness by being aware of the gap between yourself and the world of causally determined objects.