Full Idea
If abstracting from the difference between my house and my neighbour's, I were to regard both houses as mine, the defect of the abstraction would soon be made clear. It may, though, be possible to obtain a concept by means of abstraction...
Gist of Idea
If we abstract the difference between two houses, they don't become the same house
Source
Gottlob Frege (Grundgesetze der Arithmetik 2 (Basic Laws) [1903], §99)
Book Reference
Frege,Gottlob: 'Translations from the Writings of Gottlob Frege', ed/tr. Geach,P/Black,M [Blackwell 1980], p.174
A Reaction
Note the important concession at the end, which shows Frege could never deny the abstraction process, despite all the modern protests by Geach and Dummett that he totally rejected it.