Full Idea
A whole is an extra item in our ontology only in the minimal sense that it is not identical to any of its proper parts; but it is not distinct from them either, so when we believe in the parts it is no extra burden to believe in the whole.
Gist of Idea
A whole is distinct from its parts, but is not a further addition in ontology
Source
David Lewis (Against Structural Universals [1986], 'The pictorial')
Book Reference
Lewis,David: 'Papers in Metaphysics and Epistemology' [CUP 1999], p.91
A Reaction
A little confusing, to be 'not identical' and yet 'not different'. As Lewis says elsewhere, the whole is one, and the parts are not. A crux. Essentialism implies a sort of holism, that parts with a structure constitute a new thing.