Overview
In the introduction, Sartre sketches his own theory of consciousness, being, and phenomena through criticism of both earlier phenomenologists (most notably Husserl and Heidegger) as well as idealists, rationalists, and empiricists. According to him, one of the major achievements of modern philosophy is phenomenology because it disproved the kinds of dualism that set the existent up as having a "hidden" nature (such as Kant's noumenon); Phenomenology has removed "the illusion of worlds behind the scene."
Based on an examination of the nature of phenomena, he describes the nature of two types of being, being-in-itself 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.
Read more about this topic: Being And Nothingness