Full Idea
Number-abstraction simply has the wonderful and very fruitful property of making things absolutely the same as one another without altering them. Something like this is possible only in the psychological wash-tub.
Gist of Idea
Number-abstraction somehow makes things identical without changing them!
Source
Gottlob Frege (Review of Husserl's 'Phil of Arithmetic' [1894], p.332)
Book Reference
-: 'Mind July 1972' [-], p.332
A Reaction
Frege can be awfully sarcastic. I don't really see his difficulty. For mathematics we only need to know what is countable about an object - we don't need to know how many hairs there are on the cat, only that it has identity.