Full Idea
I propose that ...numbers are properties of sets, analogous, for example, to lengths, which are properties of physical objects.
Gist of Idea
Numbers are properties of sets, just as lengths are properties of physical objects
Source
Penelope Maddy (Sets and Numbers [1981], III)
Book Reference
'Philosophy of Mathematics: anthology', ed/tr. Jacquette,Dale [Blackwell 2002], p.349
A Reaction
Are lengths properties of physical objects? A hole in the ground can have a length. A gap can have a length. Pure space seems to contain lengths. A set seems much more abstract than its members.