"From the very beginning, existentialism defined itself as a philosophy of ambiguity. It was by affirming the irreducible character of ambiguity that Kierkegaard opposed himself to Hegel, and it is by ambiguity that, in our own generation, Sartre, in Being and Nothingness, fundamentally defined man, that being whose being is not to be, that subjectivity which realizes itself only as a presence in the world, that engaged freedom, that surging of the for-oneself which is immediately given for others."