The central tenet in the ontology Sartre describes and seeks to defend in Being and Nothingness is that being divides into the for-itself and the in-itself. Self-consciousness characterizes being-for-itself and distinguishes it from being-in-itself. What it means for a being to exist for itself is that it is self-conscious. How Sartre characterizes self-consciousness in Being and Nothingness is, however, a question that remains to be asked. There is no simple answer to this question. For Sartre, there are really several levels of self-consciousness: the self-consciousness of consciousness at the pre-reflective level, at the level of reflection (both pure and impure) and at the level of being-for-others. There is a profound difference between the self-consciousness of being-for-others and impure reflection, on the one hand, and the self-consciousness of reflection and pre-reflective consciousness, on the other. With being-for-others and impure reflection, self-consciousness involves the attempt to grasp the self as an object for consciousness. Although the nature of this attempt and the reasons for its ultimate failure differ at each level, these levels are bound together by a common sense of self-consciousness as a consciousness of the self as an object.