Abstract

The fundamental thesis of David Lewis's "Parts of Classes" is that the nonempty subsets of a set are mereological parts of it. This paper shows that Lewis's considerations in favor of this thesis are unpersuasive. First, common speech provides no support. Second, the formal analogy between mereology and the Boolean algebra of sets can be explained without accepting the thesis. Third, it is very doubtful that the thesis is fruitful. Certainly, Lewis's claim that it helps us understand set theory is unwarranted