Full Idea
Imagine a pair of worlds, one in which there are the universals and their instances and one in which there are just the instances (a world of modes). How would the absence of universals make itself felt?
Clarification
'Modes' are similar to tropes, which are particular instances of properties
Gist of Idea
How could you tell if the universals were missing from a world of instances?
Source
John Heil (From an Ontological Point of View [2003], 13.7)
Book Reference
Heil,John: 'From an Ontological Point of View' [OUP 2005], p.148
A Reaction
A nice question for Plato, very much in the spirit of Aristotle's string of questions. Compare 'suppose the physics remained, but someone removed the laws'. Either chaos ensues, or you realise they were redundant. Same with Forms.