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Full Idea
Axiom of Extension: Two sets are equal if and only if they have the same elements.
Gist of Idea
Extensionality: Two sets are equal if and only if they have the same elements
Source
Brian Clegg (Infinity: Quest to Think the Unthinkable [2003], Ch.15)
Book Ref
Clegg,Brian: 'Infinity' [Robinson 2003], p.205