Full Idea
Trope theory needs a primitive notion - 'instantion' in yet another sense - to say how the tropes that comprise a particle are united.
Gist of Idea
Trope theory needs a primitive notion for what unites some tropes
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
Any theory which says that objects are just 'bundles' of tropes is asking for trouble. But if you say (with Lowe and others) that tropes are 'modes' of existing entities, you still have to give an account of the entity.