Full Idea
The 'concrete/abstract' distinction has a strong intuitive feel, and can seem to be drawable by familiar contrasts, between what can/cannot be perceived, what can/cannot be involved in causal interactions, and is/is not located in space and time.
Gist of Idea
The abstract/concrete distinction is based on what is perceivable, causal and located
Source
Bob Hale (Abstract Objects [1987], Ch.3.I)
Book Reference
Hale,Bob: 'Abstract Objects' [Blackwell 1987], p.46
A Reaction
Problems arise, needless to say. The idea of an abstraction can be causal, and abstractions seem to change. If universals are abstract, we seem to perceive some of them. They can hardly be non-spatial if they have a temporal beginning and end.