Full Idea
Suppose the relation of member to singleton is external. Why must Possum be a member of one singleton rather than another? Why isn't it contingent which singleton is his?
Clarification
Possum was David Lewis's cat
Gist of Idea
If singleton membership is external, why is an object a member of one rather than another?
Source
David Lewis (Parts of Classes [1991], 2.2)
Book Reference
Lewis,David: 'Parts of Classes' [Blackwell 1991], p.37
A Reaction
He cites Van Inwagen for raising this question, and answers it in terms of counterparts. So is the relation internal or external? I think of sets as pairs of curly brackets, not existing entities, so the question doesn't bother me.