Full Idea
In simple type theory, there is a null set of type 1, a null set of type 2, a null set of type 3..... (Quine has expressed his distaste for this).
Gist of Idea
In simple type theory there is a hierarchy of null sets
Source
Charles Chihara (A Structural Account of Mathematics [2004], 07.4)
Book Reference
Chihara,Charles: 'A Structural Account of Mathematics' [OUP 2004], p.192
A Reaction
It is bad enough trying to individuate the unique null set, without whole gangs of them drifting indistinguishably through the logical fog. All rational beings should share Quine's distaste, even if Quine is wrong.