“As a philosopher, the existentialist is supposed to make some sense out of the absurdity of existence, and to relate it to Being. Unlike the scientist, the philosopher can take nothing for granted, which is what Valéry meant by stating that science consists in pretending that you know what you do not know, and philosophy in pretending that you do not know what you know. The first statement may or may not apply to modern science, but it is a fact that one of the most arduous tasks facing the traditional philosopher is to prove the reality of his existence and of the world around him or, to use his own expression, ‘to get out of solipsism.’ This is not as easy as it seems. Among the arguments which Greek philosophy delighted in working out from close analysis of movement, multiplicity, and time, to prove that existence is an illusion, the well-know Heraclitean paradox concerning time will suffice for our purpose: the past is no more, the future is not yet, and present, closely considered, immediately splits between past and future, which proves that we really are not at any time. Montaigne, who developed this argument among others during his skeptic mood, concluded that God alone Is, outside of time, space, and change. Such reasoning, on a purely philosophical plane, leads to the distinction between Existence, which really Is Not, and Being, which Does Not Exist, since it is outside of space, time and change.”
Jacques L. Salvan, The Scandalous Ghost: Sartre’s Existentialism as Related to Vitalism, Humanism, Mysticism, Marxism (Detroit: Wayne State University Press, 1967), 18-19.