 In chapter 3 Lewis looks at the alternatives to his kind of modal realism. He takes himself to have established that we need to have possible worlds of some kind in our ontology, but not that these possible worlds must be concrete. In particular, they can be abstract, or what he calls “ersatz” possible worlds. Lewis does not have a single knock-down argument against all forms of ersatzism. Instead he divides the space of possible ersatzist positions into three, and launches different attacks against different ones.