Full Idea
Set theory has its unofficial axioms, traditional remarks about the nature of classes. They are never argued, but are passed heedlessly from one author to another. One of these says that the classes are nowhere: they are outside space and time.
Gist of Idea
Set theory has some unofficial axioms, generalisations about how to understand it
Source
David Lewis (Parts of Classes [1991], 2.1)
Book Reference
Lewis,David: 'Parts of Classes' [Blackwell 1991], p.31
A Reaction
Why don't the people who write formal books on set theory ever say things like this?