Volume 3, 1977
In his paper "Why a Class Can't Change Its Members," Richard Sharvy appears to establish the impossibility of the existence of a variable class—that is, a class that at one time has a member that is not a member of it at another time. I first indicate the importance of Sharvy's argument for our understanding of the concept of identity in the contexts of time and modality, and I summarize his argument. Sharvy says that a class C that has one (non-variable) group of members at t2 and another (nonvariable) group of members at t2 would be identical with both the class C1 that always has the first group as members and the class C2 that always has the second group as members. This is an impossibility, since in general, one thing cannot be identical with two.
I then criticize Sharvy's argument by pointing out a weakness in the defense of the claim that C = C1 and C = C2. This weakness is due to an ambiguity in Sharvy's Principle of Extensionality.