Full Idea
When mathematicians abstract one thing from others, they take an equivalence class. ....But it is only superficially a one; underneath, a class are still many.
Gist of Idea
Mathematicians abstract by equivalence classes, but that doesn't turn a many into one
Source
David Lewis (Against Structural Universals [1986], 'The pictorial')
Book Reference
Lewis,David: 'Papers in Metaphysics and Epistemology' [CUP 1999], p.93
A Reaction
This is Frege's approach to abstraction, and it is helpful to have it spelled out that this is a mathematical technique, even when applied by Frege to obtaining 'direction' from classes of parallels. Too much philosophy borrows inappropriate techniques.