Full Idea
Most sets cannot be regarded as abstractions by equivalence: most sets are equivalence classes only under thoroughly artificial equivalence. (And the empty set is not an equivalence class at all).
Gist of Idea
For most sets, the concept of equivalence is too artificial to explain abstraction
Source
David Lewis (On the Plurality of Worlds [1986], 1.7)
Book Reference
Lewis,David: 'On the Plurality of Worlds' [Blackwell 2001], p.85
A Reaction
[Recorded for further investigation..] My intuitions certainly cry out against such a thin logical notion giving a decent explanation of such a rich activity as abstraction.