Ideas from 'works' by Ernst Zermelo [1920], by Theme Structure

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4. Formal Logic / F. Set Theory ST / 4. Axioms for Sets / a. Axioms for sets
Zermelo made 'set' and 'member' undefined axioms
                        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.
For Zermelo's set theory the empty set is zero and the successor of each number is its unit set
                        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