Full Idea
Maddy takes a natural number to be a certain property of sui generis sets, the property of having a certain number of members.
Gist of Idea
A natural number is a property of sets
Source
report of Penelope Maddy (Realism in Mathematics [1990], 3 §2) by Alex Oliver - The Metaphysics of Properties
Book Reference
-: 'Mind' [-], p.19
A Reaction
[I believe Maddy has shifted since then] Presumably this will make room for zero and infinities as natural numbers. Personally I want my natural numbers to count things.