Full Idea
Lewis has shown that set theory may be reduced to a mereological theory in which singletons are the only atoms.
Gist of Idea
Set theory reduces to a mereological theory with singletons as the only atoms
Source
report of David Lewis (Parts of Classes [1991]) by Fraser MacBride - Review of Chihara's 'Structural Acc of Maths' p.80
Book Reference
-: 'Bulletin of Symbolic Logic' [-], p.80
A Reaction
Presumably the axiom of extensionality, that a set is no more than its members, translates into unrestricted composition, that any parts will make an object. Difficult territory, but I suspect that this is of great importance in metaphysics.