Full Idea
The initial foundations should be immediately clear, natural and not open to question. This is satisfied by the notion of integer and by inductive inference, by it is not satisfied by the axioms of Zermelo, or anything else of that kind.
Gist of Idea
Integers and induction are clear as foundations, but set-theory axioms certainly aren't
Source
Thoralf Skolem (Remarks on axiomatised set theory [1922], p.299)
Book Reference
'From Frege to Gödel 1879-1931', ed/tr. Heijenoort,Jean van [Harvard 1967], p.299
A Reaction
This is a plea (endorsed by Almog) that the integers themselves should be taken as primitive and foundational. I would say that the idea of successor is more primitive than the integers.