The proposition that existence precedes essence (French: l'existence précède l'essence) is a central claim of existentialism, which reverses the traditional philosophical view that the essence (the nature) of a thing is more fundamental and immutable than its existence (the mere fact of its being). To existentialists, human beings—through their consciousness—create their own values and determine a meaning for their life because the human being does not possess any inherent identity or value. That identity or value must be created by the individual. By posing the acts that constitute him or her, they make their existence more significant.
The idea can be found in the works of philosopher Søren Kierkegaard in the 19th century, but was explicitly formulated by philosopher Jean-Paul Sartre in the 20th century. The three-word formula originated in his 1946 lecture "Existentialism Is a Humanism", though antecedent notions can be found in Heidegger's Being and Time.
Sartre's view
The Sartrean claim is best understood in contrast to an established principle of metaphysics‹See TfD› that essence precedes existence, i.e. a typical claim for this traditional thesis would be that man is essentially selfish, or that he is a rational being.
To Sartre, "existence precedes essence" means that a personality is not built over a previously designed model or a precise purpose, because it is the human being who chooses to engage in such enterprise. While not denying the constraining conditions of human existence, he answers to Spinoza who affirmed that man is determined by what surrounds him. Therefore, to Sartre an oppressive situation is not intolerable in itself, but once regarded as such by those who feel oppressed the situation becomes intolerable. So by projecting my intentions onto my present condition, “It is I who freely transform it into action”. When he said that “the world is a mirror of my freedom”, he meant that the world obliged me to react, to overtake myself. It is this overtaking of a present constraining situation by a project to come that Sartre names transcendence. He added that “we are condemned to be free”.
When it is said that man defines himself, it is often perceived as stating that man can "wish" to be something - anything, a bird, for instance - and then be it. According to Sartre's account, however, this would be a kind of bad faith. What is meant by the statement is that man is (1) defined only insofar as he acts and (2) that he is responsible for his actions. To clarify, it can be said that a man who acts cruelly towards other people is, by that act, defined as a cruel man and in that same instance, he (as opposed to his genes, for instance) is defined as being responsible for being this cruel man. Of course, the more positive therapeutic aspect of this is also implied: You can choose to act in a different way, and to be a good person instead of a cruel person. Here it is also clear that since man can choose to be either cruel or good, he is, in fact, neither of these things essentially.
To claim that existence precedes essence is to assert that there is no such predetermined essence to be found in humans, and that an individual's essence is defined by him or her through how he or she creates and lives his or her life. As Sartre puts it in his Existentialism is a Humanism: "man first of all exists, encounters himself, surges up in the world – and defines himself afterwards."
Existentialism tends to focus on the question of human existence and the conditions of this existence. What is meant by existence is the concrete life of each individual, and their concrete ways of being in the world. Even though this concrete individual existence must be the primary source of information in the study of people, certain conditions are commonly held to be "endemic" to human existence. These conditions are usually in some way related to the inherent meaninglessness or absurdity of the earth and its apparent contrast with our pre-reflexive lived lives which normally present themselves to us as meaningful. A central theme is that since the world "in-itself" is absurd, that is, not "fair", then a meaningful life can at any point suddenly lose all its meaning. The reasons why this happens are many, ranging from a tragedy that "tears a person's world apart," to the results of an honest inquiry into one's own existence. Such an encounter can make a person mentally unstable, and avoiding such instability by making people aware of their condition and ready to handle it is one of the central themes of existentialism. Albert Camus, for instance, famously claimed in Le Mythe de Sisyphe that "there is only one truly serious philosophical problem, and that is suicide."
Aside from these "psychological" issues, it is also claimed that these encounters with the absurd are where we are most in touch with our condition as humans. Such an encounter cannot be without philosophical significance, and existentialist philosophers derive many metaphysical theories from these encounters. These are often related to the self, consciousness and freedom as well as the nature of meaning.