Sartre uses pour-soi to describe human beings, who are defined by their possession of consciousness and, more specifically, by their consciousness of their own existence—and, as Sartre writes, by their consciousness of lacking the complete, definable essence of the en-soi.
Sartre defines two types, or ways, of being: en-soi, or being-in-itself, and pour-soi, or being-for-itself. He uses the first of these, en-soi, to describe things that have a definable and complete essence yet are not conscious of themselves or their essential completeness. Trees, rocks, and birds, for example, fall into this category. Sartre uses pour-soi to describe human beings, who are defined by their possession of consciousness and, more specifically, by their consciousness of their own existence—and, as Sartre writes, by their consciousness of lacking the complete, definable essence of the en-soi. This state of being-for-itself is not just defined by self-consciousness—it would not exist without that consciousness. In Sartre’s philosophical system, the interplay and difference between these two manners of being is a constant and indispensable point of discussion.