Full Idea
The fact that Zermelo and Von Neumann disagree on which particular sets the numbers are is fatal to the view that each number is some particular set.
Gist of Idea
Numbers can't be sets if there is no agreement on which sets they are
Source
Paul Benacerraf (What Numbers Could Not Be [1965], II)
Book Reference
'Philosophy of Mathematics: readings (2nd)', ed/tr. Benacerraf/Putnam [CUP 1983], p.279
A Reaction
I agree. A brilliantly simple argument. There is the possibility that one of the two accounts is correct (I would vote for Zermelo), but it is not actually possible to prove it.