The philosophical career of Jean Paul Sartre (1905-1980) focuses, in its first phase, upon the construction of a philosophy of existence known as existentialism. Sartre's early works are characterized by a development of classic phenomenology , but his reflection diverges from Husserl’ s on methodology, the conception of the self, and an interest in ethics. These points of divergence are the cornerstones of Sartre’s existential phenomenology, whose purpose is to understand human existence rather than the world as such. Adopting and adapting the methods of phenomenology, Sartre sets out to develop an ontological account of what it is to be human. The main features of this ontology are the groundlessness and radical freedom which characterize the human condition. These are contrasted with the unproblematic being of the world of things. Sartre’s substantial literary output adds dramatic expression to the always unstable co-existence of facts and freedom in an indifferent world.
Sartre’s ontology is explained in his philosophical masterpiece, Being and Nothingness , where he defines two types of reality which lie beyond our conscious experience: the being of the object of consciousness and that of consciousness itself. The object of consciousness exists as "in-itself," that is, in an independent and non-relational way. However, consciousness is always consciousness “of something,” so it is defined in relation to something else, and it is not possible to grasp it within a conscious experience: it exists as "for-itself." An essential feature of consciousness is its negative power, by which we can experience "nothingness." This power is also at work within the self, where it creates an intrinsic lack of self-identity. So the unity of the self is understood as a task for the for-itself rather than as a given.