This chapter examines the ontological presuppositions behind the traditional idea of the dissociation of being from time; it raises, as an alternative, the “ontology of temporal differentiation.” Predominant in everyday understanding and in the natural sciences is the “ontology of the extant,” which identifies the being of beings with their being-extant or full determinacy. To ensure the independence of such a full determinacy from finite human experience, a perfect intellect must be assumed. This, and not the human subject, is considered the measure for the being of beings. The ontology of the extant exhibits two problems. First, it focuses on extant differences at the price of the differentiating processes which allow the differences to “make a difference” in the first place, so that all differences are in fact juxtaposed indifferently in the being. Second, it understands the essence of beings according to the manifestation of beings to the perfect intellect, so that their manifestation to the finite human intellect becomes always “second-hand.” To address the first problem, we need to attend to the process of differentiation, which allows each aspect of a being to be distinguished from other possible aspects while remaining with the latter in an originary unity. It is thanks to this process that the difference “matters” or “makes a difference.” In the movement of contemporary “post-metaphysics,” which rose under the influences of the later Heidegger and Deleuze, a “differential ontology” has been articulated to take differentiation seriously; it understands being as incessant differentiation. Due to a mistrust of modern subjectivity, however, differential ontology takes a naturalist or impersonalist position and must assume a realm of “the virtual,” in which all differences subsist. The manifestation of beings, now construed as actualization from the virtual, becomes once again second-hand experience. This is how differential ontology fails to address the second problem with the ontology of the extant. Faced with this challenge, I propose the “ontology of temporal differentiation.” It says that an originary sense of time must be prepared to make comprehensible the “focus-margin” structure of the field of sense which is at work in the manifestation of beings, as well as the “tenuous presence” of beings which this structure enables.