Full Idea
If from a black cat and a white cat we disregard colour, then posture, then location, ..we finally derive something which is completely without restrictions on content; but what is derived from the objects does differ, although it is not easy to say how.
Gist of Idea
Disregarding properties of two cats still leaves different objects, but what is now the difference?
Source
Gottlob Frege (Review of Husserl's 'Phil of Arithmetic' [1894], p.324)
Book Reference
-: 'Mind July 1972' [-], p.324
A Reaction
This is a key objection to abstractionism for Frege - we are counting two cats, not two substrata of essential catness, or whatever. But what makes a cat countable? (Key question!) It isn't its colour, or posture or location.