Two Concepts of Modality: Modal Realism and Modal Reductionism

Necessary and contingent propositions, objects with accidental and essential properties, possible worlds, individual essences-these are the phenomena of modality. I shall contrast two opposed conceptions of modal phenomena1; one of them, as I see it, is properly thought of as modal realism; the other is modal reductionism. 'Modal realism', as I use the term, has nothing to do with whether certain sentences or propositions have truth values; it has equally little to do with the question whether it is possible that our most cherished theories should in fact be false. I speak rather of existential realism and antirealism.2 The existential realist with respect to universals, for example, holds that there really are such things as universals; the antirealist holds that there are no such things, and may add that the role said by some to be played by them is in fact played by entities of some other sort. The existential realist with respect to socalled theoretical entities in science-quarks or chromosomes, sayclaims that there really are things with at least roughly the properties scientists say such things have; the antirealist denies this. In the first part of this paper, I shall sketch a version of modal realism; in the second I shall outline and briefly explain modal reductionism. My chief example of reductionism will be the important modal theory of David Lewis: I shall argue that Lewis is a modal realist and/or a realist about possible worlds in approximately the sense in which William of Ockham is a realist about universals: namely, not at all.