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 or nature of a thing is more fundamental and immutable than its existence. To existentialists, the human being - through his consciousness - creates his own values and determines a meaning to his life, for in the beginning the human being does not possess any identity or value. By posing the acts that constitute him, he makes his existence more significant..
The idea can be found in the works of Averroes in the 12th century, Mulla Sadra in the 17th century, Søren Kierkegaard in the 19th century, and was later more explicitly formulated by French philosopher Jean-Paul Sartre in the 20th century. Simone de Beauvoir also uses this concept in her feminist existentialism to develop the idea that "one is not born a woman, but becomes one". In Islamic philosophy, whereas previous methods of philosophical thought held that "essence precedes existence", a concept which dates back to Avicenna and Shahab al-Din Suhrawardi, Mulla Sadra substituted a metaphysics of existence for the traditional metaphysics of essences, giving priority ab initio to existence over quiddity.
In western philosophy Sartre flips this around arguing that for humans, existence precedes essence. The three-word formula originates with Sartre in his 1945 lecture "Existentialism Is a Humanism" (though antecedent notions can be found in Heidegger's Being and Time).
Mulla Sadra's philosophy of existence
Sadr al-Din Muhammad Shirazi, commonly known as Mulla Sadra (979 - 1050 AH; 1571/1572 - 1640/1641 AD) claimed that there are no immutable essences, but that each essence is determined and variable according to the degree of intensity of its act of existence. 
Sayyid Jalal Ashtiyani later summarizes Mulla Sadra's concept as follows:
"The existent being that has an essence must then be caused and existence that is pure existence ... is therefore a Necessary Being."
In his view reality is existence, differentiated in a variety of ways, and these different ways look to us like essences. What first affect us are things that exist and we form ideas of essences afterwards, so existence precedes essence. This position referred to as primacy of existence (Arabic: Asalat al-Wujud).
The Sartrean claim is best understood in contrast to an established principle of metaphysics that essence precedes existence, i.e. that there is such a thing as human nature, determined by the cosmic order (or a god), laid down by religious tradition, or legislated by political or social authority. A typical claim for this traditional thesis would be that man is essentially selfish, or that he is a rational being.
To Sartre, the idea that "existence precedes essence" means that a personality is not built over a previous designed model or a precise purpose, because that's the human being who chooses to engage in such entreprise. 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 on my present condition, “it’s me that freely transform it in 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. That’s 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 man, 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 his concrete ways of being in the world. Even though this concrete individual existence must be the primary source of information in the study of man, 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 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.
- Albert Camus, The Myth of Sisyphus, 1948.
- Corbin, Henry (1993 (original French 1964)). History of Islamic Philosophy, Translated by Liadain Sherrard, Philip Sherrard. London; Kegan Paul International in association with Islamic Publications for The Institute of Ismaili Studies. ISBN 0710304161.
- Joseph S. Catalano, A Commentary on Jean-Paul Sartre's Being and Nothingness, University of Chicago Press 1985.
- Leaman, Oliver; Peter S. Groff (2007). Islamic Philosophy A-Z. Edinburgh University Press. ISBN 0748620893.
- Sartre, Existentialism is a Humanism (L'existentialisme est un humanisme) 1946 Lecture
- Stanford Encyclopedia of Philosophy, article Existentialism
- Razavi, Mehdi Amin (1997), Suhrawardi and the School of Illumination, Routledge, ISBN 0700704124
- Stanford Encyclopedia of Philosophy article http://plato.stanford.edu/entries/existentialism/#ExiPreEss
- ↑ (fr) (Dictionary) "L'existencialisme" - see "l'identité de la personne"
- ↑ (fr) Encyclopédie de la jeunesse, 1979, p.567
- ↑ 3.0 3.1 Irwin, Jones (Autumn 2002), "Averroes' Reason: A Medieval Tale of Christianity and Islam", The Philosopher LXXXX (2)
- ↑ (Razavi 1997, p. 130)
- ↑ Kierkegaard, Søren. Philosophical Fragments, 1844.
- ↑ (Razavi 1997, p. 129)
- ↑ 7.0 7.1 Corbin (1993), pp. 342 and 343
- ↑ Stanford Encyclopedia of Philosophy
- ↑ Sartre, in Being and Nothingness (1943), credits a slightly longer version of the claim to Heidegger: "Now freedom has no essence. It is not subject to any logical necessity; we must say of it what Heidegger said of the Dasein in general: 'In it existence precedes and commands essence.'" However, Sartre gives no page reference for this citation. In Being and Time, Heidegger writes: "The 'essence' of human-being lies in its existence." ("Das 'Wesen' des Daseins liegt in seiner Existenz", Sein und Zeit, p. 42.)
- ↑ (Razavi 1997, pp. 129-30)
- ↑ Leaman (2007), p.35
- ↑ (fr) Philagora.net -Notions de philosophie, L'existencialisme: Jean-Paul Sartre (Notions of Philosophy, Existentialism)
- ↑ Catalano p. 81
- ↑ Sartre, Existentialism is a humanism
- ↑ Camus, The Myth of Sisyphus