Full Idea
Not just any operation that makes new things from old is a form of composition! There is no sense in which my parents are part of me, and no sense in which two numbers are parts of their greatest common factor.
Gist of Idea
Composition is not just making new things from old; there are too many counterexamples
Source
David Lewis (Against Structural Universals [1986], 'Variants')
Book Reference
Lewis,David: 'Papers in Metaphysics and Epistemology' [CUP 1999], p.97
A Reaction
One of those rare moments when David Lewis seems to have approached a really sensible metaphysics. Further on he rejects all forms of composition apart from mereology.