This thesis explores the question: what is the past? I take up the radical view that the past exists, but the present and future do not. The four main theories of time have described the past in a variety of ways: Presentism says it is non-existent; eternalism describes it as on a par with the present and future; the growing block and moving spotlight views describe it as being ontologically on a par with the present or future, respectively. I investigate how these theories have addressed the nature of the past question using a logico-semantic approach, which generates ontological problems of absence because the past objects that statements about the past denote are absent from their ontologies. Presentism, which accepts that the past is non-existent, faces the greatest challenges concerning the past because it cannot account for the truthmaker of statements about the past. Eternalism, along with the growing block and moving spotlight views, must reconcile the significant differences in our experiences of the past, present, and future with their view that all or some of them are ontologically on a par. I consider the traditional logico-semantic approaches to responding to the nature of the past question and argue that actualist versions of presentism are ill-equipped to answer it. Of the theories of time that do not rely upon a logico-semantic analysis, four seem promising. Meinongian presentism, which argues that there is a non-existent past accepts an ontological response to the question of the past, but I argue that it incorrectly defines the point of contrast between the existent and the non-existent. Although eternalism, the growing block, and moving spotlight views can explain the ontological ground of references and truths about the past, we may question whether there is a past since these views treat its nature as ontologically on par with that of the present or future. This steers the investigation towards intentionality, the directedness of our thoughts toward objects. I analyse thoughts about the past, as well as those about the present and future to understand their nature and their relationship to an external mind-independent world. I argue that the past’s uniquely immutable nature differentiates it from the mutable present and future, and that immutability entails substantiveness, while mutability entails nonsubstantiveness. The past’s substantive nature entails its existence while the nonsubstantiveness of the present and future entail their non-existence.