In explaining the meaning of existentialism -- the philosophical school of thought of which Sartre was the modern progenitor -- Sartre once wrote: "If man, as the existentialist conceives him, is indefinable, it is because at first he is nothing. Only afterward will he be something, and he himself will have made what he will be. Thus there is no human nature, since there is no God to conceive it. Not only is man what he conceives himself to be, but he is also only what he wills himself to be after this thrust toward existence."