Man possess a human nature; this “human nature”, which is the concept of that which is human, is found in all men, which means that each man is a particular example of a universal concept – man. In Kant’s works, this universality extends so far as to encompass forest dwellers – man in a state of nature – and the borgeois, meaning that they all possess the same basic qualities. Here again, the essence of man precedes his historically primitive existence in nature.
Atheistic existentialism, which I represent, is more consistent. It states that if God does not exist, there is at least one being in whom existence precedes essence – a being whose existence comes before its essence, a being who exists before he can be defined by any concept of it. That being is man, or, as Heidegger put it, the human reality. What do we mean here by “existence precedes essence”? We mean that man first exists: he materializes in the world, encounters himself, and only afterward defines himself.
If man as existentialists conceive of him cannot be defined, it is because to begin with he is nothing. He will not be anything until later, and then he will be what he makes of himself. Thus, there is no human nature since there is no God to conceive of it. Man is not only that which he conceives himself to be, but that which he wills himself to be, just as he wills himself to be after being thrown into existence, man is nothing other than what he makes of himself. This is the first principle of existentialism.Jean-Paul Sartre (2007). Existentialism Is a Humanism. Yale University Press. p 22.