Full Idea
If every singleton was where its member was, then, in general, classes would be where there members were.
Gist of Idea
If singletons are where their members are, then so are all sets
Source
David Lewis (Parts of Classes [1991], 2.1)
Book Reference
Lewis,David: 'Parts of Classes' [Blackwell 1991], p.32
A Reaction
There seems to be a big dislocation of understanding of the nature of sets, between 'pure' set theory, and set theory with ur-elements. I take the pure to be just an 'abstraction' from the more located one. The empty set has a puzzling location.