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But Heidegger's thought belied the vague sentences and the contradictory behavior. In the classic study of existential philosophy, Irrational Man, New York University Professor William Barrett wrote: "Both atheist and theist have to reckon with Heidegger's thought for he is dealing with matters with which both will have to come to terms." The "matters" of existence and the nature of God immediately affected such modern theologians as Rudolf Bultmann and Reinhold Niebuhr. On the other side of the existential divide, Jean-Paul Sartre's thought is marked with Heideggerian references to being and nothingness, despair and negation. It is typical that Heidegger, having helped to found a modern school of philosophy, disavowed it. In a celebrated open letter he announced that he was not an existentialist. He wrote that he was concerned not with man, but with Being.
The statement was typically ambiguous. Thought cannot be divorced from writer, nor can action be severed from responsibility. Heidegger himself was living proof. Reclusive by nature—he refused even to own a telephone—he nonetheless was married and the father of three children. His lectures attracted huge crowds, and he continued, into his ninth decade, to write books not only about Being but for man. In those books he declared that every great thinker thinks but one great thought. The vast outpourings of Heidegger's mind may, in the end, be reduced to a single concept: the quest for the meaning of Being. He made it a great expedition.