Full Idea
Frege's mirroring principle (that the structure of thoughts mirrors that of language) has the uncomfortable consequence that since the phrase 'the concept "horse"' is saturated, it cannot refer to something unsaturated, which includes concepts.
Clarification
'Saturated' means all ingredients are settled
Gist of Idea
The phrase 'the concept "horse"' can't refer to a concept, because it is saturated
Source
Michael Potter (The Rise of Analytic Philosophy 1879-1930 [2020], 16 'Conc')
Book Reference
Potter,Michael: 'The Rise of Anaytic Philosophy 1879-1930' [Routledge 2020], p.109