 For any set of principles governing the fiction of possible worlds, the question obviously arises as to why that set should be chosen. Alternatively, there are many stories we could tell about other worlds (whether they are cosmoi or fictional abstract objects), and the question arises as to why one rather than any other be chosen to be the modal fiction. (One could of course be more pluralist than this, and allow that different fictions are suitable for different purposes. The relativised question may still be asked, however: why this fiction, rather than any other, for this particular purpose?). This task of selecting and justifying one or more particular stories about worlds will be one of the challenges discussed in section 4. However, the next distinction to be discussed makes a big difference to what sort of answer to this question will be acceptable.