Full Idea
Trope theory cannot analyse similarity, because duplication of tropes is itself a primitive relation of similarity.
Gist of Idea
Tropes need a similarity primitive, so they cannot be used to explain similarity
Source
David Lewis (On the Plurality of Worlds [1986], 1.5)
Book Reference
Lewis,David: 'On the Plurality of Worlds' [Blackwell 2001], p.65
A Reaction
A reasonable reply to this one, I think, is that no one can explain or analyse similarity. To say that the same universal (or bunch of graded universals) is instantiated explains nothing. Maybe type-identity must be primitive in any theory?
Related Idea
Idea 15749 Trope theory (unlike universals) needs a primitive notion of being duplicates [Lewis]