Rationalist Plenitude

David Lewis’ modal realism has two components. First, there is an ontological thesis that there is a plenitude of all possible island universes. Second, there is a reductionist thesis that modal truths reduce to truths about these island universes. My goal is to argue for the first ontological thesis in a way that does not depend on the second reductionist thesis. In particular, I will argue that there are two plausible theses about fundamentality that entail the ontological thesis. Moreover, I will argue that the resulting view is particularly well-suited to satisfy a ‘rationalist’ impulse in metaphysics, according to which it is fully explicable why the world is (and indeed must be) the way it is.