When scientists put forward hypotheses, they sometimes involve new kinds of entities, which we can call 'hypothetical entities.' Hypothetical entities are pervasive in the sciences, and some examples include caloric and, up until very recently, the Higgs boson. Some hypothetical entities are discovered, as was the case with the Higgs boson, while scientists conclude that others, like caloric, do not exist. Hypothetical entities pose a number of important challenges for the philosophy of science, and my goal is to develop and (...) defend what I will call the suppositionalist view of hypothetical entities. In chapter 1, I examine the extant views of hypothetical entities, which I draw from the scientific realism debate. I argue that these views are all committed to the claim that terms for hypothetical entities putatively refer to empirical entities. In chapter 2, I develop the suppositionalist view of hypothetical entities. On this view, terms for hypothetical entities refer to what are called 'objects of supposition.' Examples of such objects from other domains include fictional characters like Superman and mathematical objects like the natural numbers. I draw from analogies with fiction and mathematics in order to develop the suppositionalist view in the scientific domain. In chapter 3, I give a history of a hypothetical entity that I will later use as a test case for views of hypothetical entities. In the late-eighteenth century, Antoine Lavoisier hypothesized that muriatic acid is composed of oxygen and a hypothetical entity called the 'muriatic radical.' In the early-nineteenth century, Humphry Davy's work on muriatic acid showed that it is actually composed of hydrogen and chlorine, and so muriatic acid is hydrochloric acid. Finally, in chapter 4, I use the history of the muriatic radical in order to argue against the extant views, and for the suppositionalist view. I argue that the former are committed to giving a history of the muriatic radical that is either whiggish or incomplete. The latter, however, can give us a non-whiggish history that is more complete, and hence it is preferable to the extant views. (shrink)