 After arguing that we are best off in all these areas of philosophy if we accept unactualised possibilities, Lewis spends the rest of chapter 1 saying what possible worlds are on his view. He isn’t yet arguing for this way of thinking about possible worlds; that will come in chapter 3. For now he is just describing what he takes to be the best theory of possible worlds. He holds that possible worlds are isolated; no part of one is spatio-temporally related to any other world. Indeed, he holds that lack of spatio-temporal relation (or something like it) is what marks individuals as being in different worlds. So his theory has the somewhat odd consequence that there could not have been two parts of the world that aren’t spatio-temporally connected. He holds that worlds are concrete, though spelling out just what the abstract/concrete distinction comes to in this context isn’t a trivial task. And he holds that worlds are plenitudinous, there are enough of them to do the work needed for an analysis of modality. This requires at least that they satisfy a principle of recombination. But since it is possible that things are different in kind to anything that actually exists, plenitude implies that there are worlds that can’t be made by simply putting parts of this world together. (Alastair Wilson (forthcoming) has a good survey of the complicated relationship between plenitude and recombination.)