David Lewis‘s Parts of Classes is a great book, in all respects. But one of its most interesting thesis, in my mind, is not its core thesis that standard set theory — ZFC — reduces to classical mereology + plural quantification + a primitive singleton-relation, but rather its sub-thesis of how to understand classical mereology, what Lewis calls the thesis of Composition as Identity: (CAI): a whole is the same portion of reality as its many parts taken together; it is (...) them collectively, they collectively are it. CAI is needed as an assumption for the core thesis of Parts of Classes — the reduction of ZFC — on pain of it being unmotivated.1 But CAI is the most interesting in its own right. It is also as such it is presented by Lewis, and received in the literature. In what follows, I critically assess CAI as Lewis presents it in Parts of Classes. I first argue that Lewis‘s presentation of CAI has been misunderstood in the literature (section 1). I then argue that the best (if not the only) way to understand it entails a slightly revisionary semantics for a certain form of predication (section 2). I finally end by showing that this might create more trouble than it solves for Lewis (section 3). (shrink)

Some believe that there is a God who is the source of all things; and some believe that there are necessarily existing abstract objects. But can one believe both these things? That is the question of this Element. First, Einar Duenger Bøhn clarifies the concepts involved, and the problem that arises from believing in both God and abstract objects. Second, he presents and discusses the possible kinds of solutions to that problem. Third, Bøhn discusses a new kind of solution to (...) the problem, according to which reality is most fundamentally made of information. (shrink)