In this paper I will present an argument that David Lewis’ modal realism is self-refuting, and that the contradiction that makes it self-refuting can only be blocked by modifying certain others of Lewis’ philosophical commitments. My argument will proceed as follows. First, I will briefly describe the motivation for and the main components of Lewisian modal realism. Second, I will explain Lewis’ view of what it is for an individual or a set to exist in a world. Third, I go (...) on to argue that Lewis’ position creates a dilemma for modal realism: either each world contains all the other worlds as parts, which violates the principle that worlds are spatiotemporally isolated, or worlds are reducible to sets, which violates the principle that worlds are individuals. Finally, I briefly discuss a suggestion by Divers that strongly bears on this problem. (shrink)