Full Idea
Universals can explain the one-over-many problem, and easily explain similarity relations between objects, and explain the similar behaviour of similar objects.
Gist of Idea
Universals explain one-over-many relations, and similar qualities, and similar behaviour
Source
John Heil (From an Ontological Point of View [2003], 13.1)
Book Reference
Heil,John: 'From an Ontological Point of View' [OUP 2005], p.137
A Reaction
A useful summary. If you accept it, you seem to be faced with a choice between Plato (who has universals existing independently of particulars) and Armstrong (who makes them real, but existing only in particulars).