The property or principle of being extensional; in logic and philosophy, the focus on actual referents or scope rather than intensional meaning or definition.
From 'extensional' plus '-ity', creating an abstract noun. This is a technical term primarily used in formal logic, mathematics, and philosophy.
The 'Axiom of Extensionality' is fundamental in set theory—it says two sets are equal if they have exactly the same members, regardless of how they're described, which seems obvious until you realize how weird things get with infinity!
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