Full Idea
Frege, rebelling against 'psychologism', identified concepts (and hence 'intensions' or meanings) with abstract entities rather than mental entities.
Gist of Idea
Frege said concepts were abstract entities, not mental entities
Source
report of Gottlob Frege (works [1890]) by Hilary Putnam - Meaning and Reference p.119
Book Reference
'Naming, Necessity, and Natural Kinds', ed/tr. Schwartz,Stephen P. [Cornell 1979], p.119
A Reaction
This, of course, assumes that 'abstract' entities and 'mental' entities are quite distinct things. A concept is presumably a mental item which has content, and the word 'concept' is simply ambiguous, between the container and the contents.