Full Idea
Lewis says that the parts of a thing are identical with the whole they compose, calling his view 'composition as identity', which is the claim that a physical object is 'nothing over and above its parts'.
Gist of Idea
Lewis affirms 'composition as identity' - that an object is no more than its parts
Source
report of David Lewis (Parts of Classes [1991], p.84-7) by Trenton Merricks - Objects and Persons §I.IV
Book Reference
Merricks,Trenton: 'Objects and Persons' [OUP 2003], p.20
A Reaction
The ontological economy of this view is obviously attractive, but I don't agree with it. You certainly can't say that all identity consists entirely of composition by parts, because the parts need identity to get the view off the ground.