Full Idea
If abstractions were defined by whether they could affect human sense-organs, light-waves would be concrete but radio waves abstract.
Gist of Idea
We can't say that light is concrete but radio waves abstract
Source
Michael Dummett (Frege Philosophy of Language (2nd ed) [1973], Ch.14)
Book Reference
Dummett,Michael: 'Frege Philosophy of Language' [Duckworth 1981], p.480
A Reaction
This is a pretty good baseline example. No account should draw an abstract/concrete line through the electromagnetic spectrum.