Full Idea
The phrase 'the concept "horse"' can be the subject of a sentence, and ought to denote an object. But it clearly denotes the concept "horse". Yet Fregean concepts are said to be 'incomplete' objects, which led to confusion.
Gist of Idea
'The concept "horse"' denotes a concept, yet seems also to denote an object
Source
report of Gottlob Frege (On Sense and Reference [1892]) by Vann McGee - Logical Consequence 4
Book Reference
'Bloomsbury Companion to Philosophical Logic', ed/tr. Horsten,L/Pettigrew,R [Bloomsbury 2014], p.36
A Reaction
This is the notorious 'concept "horse"' problem, which was bad news for Frege's idea of a concept.
Related Idea
Idea 18899 Frege takes the existence of horses to be part of their concept [Frege, by Sommers]