"Sartre's slogan—“existence precedes essence”—may serve to introduce what is most distinctive of existentialism, namely, the idea that no general, non-formal account of what it means to be human can be given, since that meaning is decided in and through existing itself. Existence is “self-making-in-a-situation” (Fackenheim 1961:37). In contrast to other entities, whose essential properties are fixed by the kind of entities they are, what is essential to a human being—what makes her who she is—is not fixed by her type but by what she makes of herself, who she becomes. The fundamental contribution of existential thought lies in the idea that one's identity is constituted neither by nature nor by culture, since to “exist” is precisely to constitute such an identity. It is in light of this idea that key existential notions such as facticity, transcendence (project), alienation, and authenticity must be understood.
At first, it seems hard to understand how one can say much about existence as such. Traditionally, philosophers have connected the concept of existence with that of essence in such a way that the former signifies merely the instantiation of the latter. If “essence” designates what a thing is and “existence” that it is, it follows that what is intelligible about any given thing, what can be thought about it, will belong to its essence. It is from essence in this sense—say, human being as rational animal or imago Dei—that ancient philosophy drew its prescriptions for an individual's way of life, its estimation of the meaning and value of existence. Having an essence meant that human beings could be placed within a larger whole, a kosmos, that provided the standard for human flourishing. Modern philosophy retained this framework even as it abandoned the idea of a “natural place” for man in the face of the scientific picture of an infinite, labyrinthine universe. In what looks like a proto-existential move, Descartes rejected the traditional essential definitions of man in favor of a radical, first-person reflection on his own existence, the “I am.” Nevertheless, he quickly reinstated the old model by characterizing his existence as that of a substance determined by an essential property, “thinking.” In contrast, Heidegger proposes that “I” am “an entity whose what [essence] is precisely to be and nothing but to be” (Heidegger 1985:110; 1962:67). Such an entity's existing cannot, therefore, be thought as the instantiation of an essence, and consequently what it means to be such an entity cannot be determined by appeal to pre-given frameworks or systems—whether scientific, historical, or philosophical."