Full Idea
For Zermelo the successor of n is {n} (rather than Von Neumann's successor, which is n U {n}).
Gist of Idea
For Zermelo the successor of n is {n} (rather than n U {n})
Source
report of Ernst Zermelo (Investigations in the Foundations of Set Theory I [1908]) by Penelope Maddy - Naturalism in Mathematics I.2 n8
Book Reference
Maddy,Penelope: 'Naturalism in Mathematics' [OUP 2000], p.24
A Reaction
I could ask some naive questions about the comparison of these two, but I am too shy about revealing my ignorance.