Full Idea
What is true of the many is not exactly what is true of the one. After all they are many while it is one. The number of the many is six, whereas the number of the fusion is one. The singletons of the many are distinct from the singleton of the one.
Gist of Idea
The many are many and the one is one, so they can't be identical
Source
David Lewis (Parts of Classes [1991], 3.6)
Book Reference
Lewis,David: 'Parts of Classes' [Blackwell 1991], p.87
A Reaction
I wouldn't take this objection to be conclusive. 'Some pebbles' seem to be many, but a 'handful of pebbles' seem to be one, where the physical situation might be identical. If they are not identical, then the non-identity is purely conceptual.
Related Idea
Idea 14747 'Composition as identity' says that an object just is the objects which compose it [Sider]