Full Idea
Frege felt that meanings are public property, and identified concepts (and hence 'intensions' or meanings) with abstract entities rather than mental entities.
Gist of Idea
Frege felt that meanings must be public, so they are abstractions rather than mental entities
Source
report of Gottlob Frege (On Concept and Object [1892]) by Hilary Putnam - Meaning and Reference p.150
Book Reference
'Meaning and Reference', ed/tr. Moore,A.W. [OUP 1993], p.150
A Reaction
This is the germ of Wittgenstein's private language argument. I am inclined to feel that Frege approached language strictly as a logician, and didn't really care that he got himself into implausible platonist ontological commitments.