With few exceptions, an existentialist asserts that an individual is determined from within, not from outside forces which determine or limit his freedom and responsibility.
Existentialism posits that existence precedes essence (actus essendi). Essence refers to what man is (his nature); existence denotes that "he is." Thus, "I am a man" is first part existence ("I am") and second part essence ("a man.") This is in opposition to traditional Platonic essentialism (think "Parable of the Cave"), which embraces man's essence prior to his existence ("A Man, I am!"), that man has an eternal, unchangeable human nature. Existentialists refute this by affirming that man's existence cannot coincide with essence; otherwise conjoined, man's essence would be to exist and never die, which is a MAJOR philosophical problem.
Herein lies the problem of essentialism: it glorifies the thing ("man") as if it is an ideal, instead of the individual "I." As a result, it leads to romantic idealism and the denial of death, two major philosophical problems. Sartre, as an existentialist, rebels against these notions.
Sartre believes in the premise "existence precedes essence" because he knows man to be a conscious subject rather than a thing to be predicted or manipulated. According to Sartre, man need not act in accordance with any stipulated essence (nature), generalization, or system. So says Enotes:
What this ["existence precedes essence"] means is that the identity of any one person—their essence—cannot be found by examining what other people are like, but only in what that particular person has done. Because no one can claim that his or her actions are “caused” by anyone else, existentialist literature focuses on freedom and responsibility.