People who have encountered Sartre for the first time recall nothing more of his philosophy than his claims on absolute freedom and the nonexistence of God. Indeed, Sartrean atheism is so well known that it really needs no further elaboration here. In his lecture, Existentialism is a Humanism, Sartre even confesses himself to be an atheist, or a representative of atheistic Existentialism. However, some points need be stated in order to better understand Sartrean philosophy.
Atheism is integral to Sartrean philosophy in the sense that it is the fulcrum on which his philosophy grounds itself. His claim of the precedence of existence over essence is but a logical consequence of his atheism. It is thus somehow ironic for him to prove the nonexistence of God, as if it is a necessary implication of a more primordial principle. Fr. Magin Borrajo, O.P. says:
In spite of the fact that atheism is Sartre's point of departure, coloring his whole life and directing the attitudes he has adopted towards others and towards himself, he has not met squarely the problem of God's existence. Sartrean atheism is not a conclusion arrived at after a detailed investigation: it is rather a postulate or in the words of Merleau Ponty, an "état d'âme," or as Sartre himself says, an "accident," the result of the circumstances of his education and the spiritual indigence of the environment in which he lived.
Sartre asserts that God does not exist for the reason that the concept of God entails self-contradiction. Sartre defines God as a being-in-itself-for-itself. He is an in-itself in so far as the concept of the divine presupposes that He be an existing entity, complete in himself and totally unrelated. On the other hand, he must likewise be a for-itself in so far as He must be completely free and not beholden to anything else. Since such a synthesis is impossible for the reason that it involves a contradiction, then the logical conclusion must be to deny the existing of such a being.
Sartrean humanism may have also contributed to his atheism. The existence of a God limits man's freedom. As Fr. Borrajo puts it, "God's existence would turn our freedom to a mere illusion." The look of God objectifies man, and this objectification takes away from man his capacity to be self-creative. Sartre relates a story encapsulating his relationship with God:
For several years more, I maintained public relations with the Almighty. But privately, I ceased to associate with Him. Only once did I have the feeling that He existed. I had been playing with matches and burned a small rug. I was in the process of covering up my crime when suddenly God saw me. I felt His gaze inside my head and on my hands. I whirled about in the bathroom, horribly visible, a live target. Indignation saved me. I flew into a rage against so crude an indiscretion, I blasphemed, I muttered like my grandfather: "God damn it, God damn it, God damn it." He never looked at me again.
Moreover, Sartre came to the point of positing that man invents God in order to account for meaning in the world. Man is haunted by cosmic meaninglessness, which he alone cannot solve. Man is then necessitated to invent a concept which can explain the unexplainable, including the origin of the world.
Sartre later asserted that man is a useless passion for the reason that he tends towards this impossible synthesis of the in-itself and the for-itself. William Barrett gives a very good summary of Sartrean atheism:
Sartre's atheism states candidly … that man is an alien in the universe, unjustified and unjustifiable, absurd in the simple sense that there is no Leibnizian reason sufficient to explain why he or his universe exists.
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