  Whether recombination completely captures our modal intuitions regarding plenitude is still a matter of some dispute.[24] However, even if it doesn't, it is less than clear whether this counts against the success of Lewis's reductionist project. For, as a realist about worlds, Lewis does not seem to be under any obligation to “derive” plenitude from more fundamental principles.  Hence, there is no obvious reason why he cannot respond to charges of incompleteness by saying that it is simply a presupposition of his theory that logical space has no gaps, that there are always enough worlds to satisfy the concretist truth condition for any intuitive modal truth.[25] So understood, the role of recombination for a realist about worlds like Lewis is something like the role of such axioms as powerset and replacement for a realist about sets: given some sets, these principles provide us with a detailed — but always less than complete — characterization of what further sets there are. Their role, therefore, is to give us insight into the richness and diversity of set theoretic space, not a complete mechanism for proving which particular sets do or do not exist. Likewise recombination vis-à-vis worlds and logical space.  