Many things can be other than they are. Many other things cannot. But what are statements like these about? One answer to this question is that we are speaking of possible worlds: if something can be other than it is, then it actually is that way in some possible world. If something cannot be otherwise, it is not otherwise in any world. This answer is presently dominant in analytical philosophy of language and logic. What are these worlds? David Lewis famously claimed that every world exists, just like ours does: there is no difference between the other worlds and ours. In contrast, the medieval thinker John Buridan understands modal logic in terms of objects and causal powers in this world: if something can be other than it is, then there is a causal power that can make it that way. If it cannot, then no causal power—not even God—can alter it, at least without destroying its nature. As we’ll see, the Lewisian plurality is not possible on Buridan’s account; accordingly, a basic tenet of classical theism is untenable on Lewis’s metaphysics. In short, either the Lewisian plurality is incoherent, or a core monotheistic tenet is impossible.