Full Idea
The set of red objects is the set of objects possessing a property: being red. Objects are members of the set in virtue of possessing this property; they do not possess the property in virtue of belonging to the set.
Gist of Idea
Objects join sets because of properties; the property is not bestowed by set membership
Source
John Heil (From an Ontological Point of View [2003], 12.2)
Book Reference
Heil,John: 'From an Ontological Point of View' [OUP 2005], p.129
A Reaction
This seems to be a very effective denial of the claim that universals are sets. However, if 'being a Londoner' counts as a property, you can only have it by joining the London set. Being tall is more fundamental than being a Londoner.