Full Idea
Renouncing classes means rejecting mathematics. That will not do. Mathematics is an established, going concern. Philosophy is as shaky as can be.
Gist of Idea
Giving up classes means giving up successful mathematics because of dubious philosophy
Source
David Lewis (Parts of Classes [1991], 2.8)
Book Reference
Lewis,David: 'Parts of Classes' [Blackwell 1991], p.58
A Reaction
This culminates in his famous 'Who's going to tell the mathematicians? Not me!'. He has just given four examples of mathematics that seems to entirely depend on classes. This idea sounds like G.E. Moore's common sense against scepticism.