  The key insight that defines and unites existentialism as a philosophical position, despite all the divergences between the authors included under that denomination, is the emphasis on the radical nature of human freedom, and the metaphysical and ontological imports of that freedom. The metaphysical and ontological significance of freedom precedes its moral, ethical, and political aspects, since the ways in which human beings “hook up” to the world should be considered before issues of duty or justice. For existentialism, human freedom grounds the very possibility of knowledge in its deepest form, i.e., the capacity of human beings to reveal something about reality. A Christian existentialist like Gabriel Marcel interprets the metaphysical reach of human freedom in terms of the capacity and responsibility of individuals to make themselves “available” to the mystery of their participation in creation, in particular by responding to the appeal of the great “Thou” (see in particular Marcel 1960b). Atheistic existentialists (Camus, Sartre, de Beauvoir, Merleau-Ponty), on the contrary, do not ground freedom in faith and the hope of accessing the transcendent; instead they emphasise the difficulty of assuming that freedom, since nothing can ensure that our attempts at finding meaning in the world will actually yield something objectively present in it. But in all cases freedom is the ultimate ground of human beings’ capacity to relate to the world.