Full Idea
The stuctural universal 'isobutane' consists of the universal carbon four times over, hydrogen ten times over, and the universal 'bonded' thirteen times over - just like the universal 'butane'.
Gist of Idea
Butane and Isobutane have the same atoms, but different structures
Source
David Lewis (Against Structural Universals [1986], 'Variants')
Book Reference
Lewis,David: 'Papers in Metaphysics and Epistemology' [CUP 1999], p.96
A Reaction
The point is that isobutane and butane have the same components in different structures. At least this is Lewis facing up to the problem of the 'flatness' of mereological wholes.