Transworld Similarity and Transworld Belief

Relations of transworld similarity play an essential role in Lewis's system. Analysis reveals that they involve the possibility of detailed transworld belief. Such belief is problematic within Lewis's framework. He has an answer to the problems raised, but it relies on a dubious distinction between natural and mere properties. Replacing that distinction with a respectable one undermines an essential part of his case against one of his chief opponents, the linguistic ersatzist.