Full Idea
I am not suggesting a reduction of number theory to set theory ...There are only sets with number properties; number theory is part of the theory of finite sets.
Gist of Idea
Number theory doesn't 'reduce' to set theory, because sets have number properties
Source
Penelope Maddy (Sets and Numbers [1981], V)
Book Reference
'Philosophy of Mathematics: anthology', ed/tr. Jacquette,Dale [Blackwell 2002], p.353