 It is a very widely held view among contemporary philosophers that we need to appeal to entities known as possible worlds in order to account for various phenomena. There are dozens of phenomena that philosophers have thought should be explained in terms of possible worlds, but to name just one, it is often argued that semantic theory is best carried out in terms of possible worlds. Consider, for example, the attempt to state the truth conditions of sentences of the form ‘It is necessary that S’ and ‘It is possible that S’ (where S is any sentence). It is widely believed that the best theory here is that a sentence of the form ‘It is necessary that S’ is true if and only if S is true in all possible worlds, and a sentence of the form ‘It is possible that S’ is true if and only if S is true in at least one possible world. Now, if we add to this theory the premise that at least one sentence of the form ‘It is possible that S’ is true — and this seems undeniable — then we are led to the result that possible worlds exist.