Full Idea
Empiricists are in general rather suspicious with respect to any kind of abstract entities like properties, classes, relations, numbers, propositions etc. They usually feel more sympathy with nominalists than with realists (in the medieval sense).
Gist of Idea
Empiricists tend to reject abstract entities, and to feel sympathy with nominalism
Source
Rudolph Carnap (Empiricism, Semantics and Ontology [1950], 1)
Book Reference
Carnap,Rudolph: 'Meaning and Necessity (2nd ed)' [Chicago 1988], p.205
A Reaction
The obvious reason is that you can't have sense experiences of abstract entities. I like the question 'what are they made of?' rather than the question 'how can I experience them?'.