Full Idea
Frege employs the notion of 'concrete' (wirklich, literally 'actual') objects, in arguing that not every object is concrete, but it does not work; abstract objects are just as much objects as concrete ones.
Gist of Idea
Frege refers to 'concrete' objects, but they are no different in principle from abstract ones
Source
report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §26,85) by Michael Dummett - Frege Philosophy of Language (2nd ed) Ch.14
Book Reference
Dummett,Michael: 'Frege Philosophy of Language' [Duckworth 1981], p.480
A Reaction
See Idea 10516 for why Dummett is keen on the distinction. Frege strikes me as being wildly irresponsible about ontology.
Related Ideas
Idea 10516 A realistic view of reference is possible for concrete objects, but not for abstract objects [Dummett, by Hale]
Idea 14002 Possible worlds must be abstract, because two qualitatively identical worlds are just one world [Markosian]