Realism about possible worlds bears analytical fruit. The prize plum, perhaps, is the analysis of modality, in particular, the analysis of modal operators as quantifiers over possible worlds. But if the goal is the elimination, and not just the systematization, of primitive modality, it won't do for the realist simply to "take possible worlds as primitive." For, what sort of primitive is "possible world"? It wears its modality right on its face! Nor will it do for the realist to "take the worldmate relation as primitive," the relation that holds between things inhabiting one and the same possible world. On a realist account, 'possible world' and 'worldmate' are trivially interdefinable. No, the realist must provide an analysis of possible world (and worldmate) -or forfeit the prize.
The modern champion of realism about possible worlds, of course, is David Lewis; but a realist need not accept all of Lewis's "modal realism. ''l The core of realism about possible worlds, I think, is captured by the following five claims. (1) Worlds exist. 2 (2) Worlds are individuals rather than classes, or functions, or mathematical structures. (3) Worlds are particulars rather than properties or universals. (4) Worlds are "concrete" in this sense: they are fully determinate in all qualitative respects. (5) Worlds are (for the most part) mereologically complex rather than simple -for example, many worlds have parts that stand in spatiotemporal relations to one another. 3 The mereological complexity of worlds suggests that worlds can, and should, be analyzed.
How should the realist analysis of world proceed? Let reality be the sum of whatever exists. Let logical space be that portion of reality over which (alethic) modal operators range, in other words, the sum ofpossibilia. There are two tasks for the realist, two distinct