Full Idea
We could not, without structures, uphold the principle that every truth has a truthmaker. If Fa is true, the truthmaker is not F, not a, nor both together; not their mereological sum; not a set-theoretic construction. These would exist just the same.
Gist of Idea
We could not uphold a truthmaker for 'Fa' without structures
Source
David Lewis (Comment on Armstrong and Forrest [1986], p.109)
Book Reference
Lewis,David: 'Papers in Metaphysics and Epistemology' [CUP 1999], p.109
A Reaction
This point ought to trouble Lewis, as well as Armstrong and Forrest. If we assert 'Fa', we must (in any theory) have some idea of what unites them, as well as of their separate existence. It must a fact about 'a', not a fact about 'F'.