After World War II, Sartre wrote a small, popular book explaining the tenets of existentialism. In Existentialism and Human Emotions (1947) Sartre sets forth the novel and exciting idea of human freedom that identified him as one of the most renowned postwar philosophers of the twentieth century. The phrase “existence precedes essence” says that there is no such thing as a given human nature. On the contrary, it is your own choices and acts that make up your identity. Man first exists and his choices then define his essence.

Sartre's dictum that “existence precedes essence” separates him from several major classical philosophers, beginning with Plato and Aristotle. For Plato, truth is eternal, unchangeable, and absolute, and knowing it is the central goal of philosophy. According to Plato, there are eternally existing forms and your job is to discover them through philosophical contemplation, through reason. Human beings have a common eternal “essentialist” nature defined by reason.

Aristotle, too, said that humans are rational animals. He said:

Reason is the true self of every person, since it is the supreme and better part. It will be strange, then, if he should choose not his own life, but some other's. … What is naturally proper to every creature is the highest and pleasantest for him. And so, to man, this will be the life of reason, since reason is, in the highest sense, a man's self.

In one way or another, all the major philosophical systems from Plato through the Middle Ages down to Descartes, Leibniz, Kant, and Hegel carried on this essentialist tradition. Truth is outside of you, and your job is to use reason to discover it.

But Sartre says, “There is no human nature…. Man is nothing but what he makes of himself.” Sartre expressed nature in the formula, “man's existence precedes essence.” By this he means that you have no fixed nature and have not been created for any particular purpose. Things like paper cutters and hammers have set natures, since they have been created to fulfill a set purpose. But mankind is not created by God or evolution or anything else; you simply find yourself existing by no choice of your own and have to decide what to make of yourself. Each person must create her own essence.