 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 1925 [1985, 110]; 1927 [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.