Full Idea
Corresponding to every concept there is a class (some classes will be sets, the others proper classes).
Clarification
Sets are more precisely defined than classes
Gist of Idea
Corresponding to every concept there is a class (some of them sets)
Source
A.George / D.J.Velleman (Philosophies of Mathematics [2002], Ch.4)
Book Reference
George,A/Velleman D.J.: 'Philosophies of Mathematics' [Blackwell 2002], p.90