How to be a modal realist

I’m no modal realist myself, mind, and I don’t think you should be either. But it can be important to know how best to do something you have no intention of doing. Modal realism plays a pivotal role in the space of available views about modality: our understanding of that space will be seriously incomplete unless we understand how modal realism can best be made coherent. You probably think that David Lewis has already made it coherent—On the Plurality of Worlds (Lewis 1986: henceforth OPW) stands for many of us as an exemplar of the internal virtues towards which one might aspire in a systematic metaphysics, whatever we may think about the plausibility of the result. I think this judgment needs refining. The book contains the outlines of a marvellously coherent and elegant (albeit otherwise implausible) account of modality and many other subject matters. But it also contains various bits of doctrine that just don’t fit with this picture, including some that might have seemed quite central. My goal in this paper is to track down these aberrant elements and show how the view works once they are eliminated. Let me begin with some puzzles for Lewis. They are not particularly deep: in fact, the correct solution to all three will probably jump out at you long before I have presented it. But they will help us begin to figure out what is really central to the modal realist’s analysis of possibility and necessity. While the most general statement of this analysis is a bit complicated, its application in some simple cases is supposed to be quite straightforward. For example, we have the following analyses: