On Fri, 29 Mar 2013 18:24:47 -0400, Shmuel (Seymour J.) Metz <firstname.lastname@example.org> wrote:
>In <email@example.com>, on 03/29/2013 > at 06:07 AM, quasi <firstname.lastname@example.org> said: > >>There are certain concepts for which sets are inadequate and classes >>come to the rescue. For example, we need the class concept if we >>want to define an equivalence on the collection of all groups, since >>that collection is not a set. > >There's no such set in ZFC, but there are set theories in which it >exists aqnd is a set.