Full Idea
If abstract entities are not located, then a set of things does seem to have a location, though perhaps a divided one; and universals, if they are wholly present in each particular, are where their instances are, so negation can't define abstraction.
Gist of Idea
If abstractions are non-spatial, then both sets and universals seem to have locations
Source
David Lewis (On the Plurality of Worlds [1986], 1.7)
Book Reference
Lewis,David: 'On the Plurality of Worlds' [Blackwell 2001], p.83
A Reaction
He admits that non-spatial accounts of sets and universals are possible, but the jury is out on both of them, and more cautious theories, even if they are realist, will give them both locations. A good argument.