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2 ideas

4. Formal Logic / F. Set Theory ST / 4. Axioms for Sets / a. Axioms for sets
For Zermelo's set theory the empty set is zero and the successor of each number is its unit set [Zermelo, by Blackburn]
     Full Idea: For Zermelo's set theory the empty set is zero and the successor of each number is its unit set.
     From: report of Ernst Zermelo (works [1920]) by Simon Blackburn - Oxford Dictionary of Philosophy p.280
Zermelo made 'set' and 'member' undefined axioms [Zermelo, by Chihara]
     Full Idea: The terms 'set' and 'is a member of' are primitives of Zermelo's 1908 axiomatization of set theory. They are not given model-theoretic analyses or definitions.
     From: report of Ernst Zermelo (works [1920]) by Charles Chihara - A Structural Account of Mathematics 7.5
     A reaction: This looks like good practice if you want to work with sets, but not so hot if you are interested in metaphysics.