His most important work is 'L'etre et le neant' or 'Being and Nothingness'.

Sartre describes human conscious-ness as 'no-thingness', in opposition to 'being' or 'thingness', because consciousness is not-matter and therefore escapes determinism. Consiousness it is not completely determined by previously existing causes, therefore it has free will, and it becames each individual's choice to make his or her own life meaningful.

Sartre's philosophy of existentialism expressed his belief that the world was meaningless for man and that the individual should find some direction and meaning for his own personal life.