Actualism and possible worlds

THE IDEA of possible worlds has both promised and, I believe, delivered understanding and insight in a wide range of topics. Pre-eminent here, I think, is the topic of broadly logical possibility, both de t the function of proper names and definite descriptions; the nature of counterfactuals; time and temporal relations; causal determinism; in philosophical theology, the ontological argument, theological determinism, and the problem of evil (see [7], chapters IV-X). In one respect, however, the idea of possible worlds may seem to have contributed less to clarity than to confusion; for if we take this idea seriously, we may find outselves committed to the dubious notion that there are or could have been things that do not exist. Let me explain.