Jean-Paul Sartre (1905–1980)

Sartre's philosophical career focuses, in its first phase, upon the construction of a philosophy of existence known as 'Existentialism'. 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 facticity and freedom in an indifferent world.

From Internet Encyclopedia of Philosophy 
Also see Wikipedia 

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