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.
In his essay, "Existentialism is a Humanism," Jean-Paul Sartre purports his view on the human condition: human beings exist prior to having any meaning to their existence. That is, humans are responsible for creating their own meaning in their lives; only they can define their own nature or essence, that by which a being is what it is.
This view of existence and essence is contrary to the theological view of the human condition in which the essence of man is conceived in the mind of God before man's existence. Sartre reverses the theological idea in what is known as atheistic existentialism, contending that man first exists, then has complete responsibility for defining himself (i.e.creating his essence or who he is). Essence is not absolute or eternal, so man is in a constant state of changing and defining himself until he no longer can find a way of defining himself--death. Then, at the end of a man's life, he is the sum total of his actions and, of course, can no longer be changed.