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State/Axiom of extensionality
Axiom: (Axiom of Extensionality) If $X$ and $Y$ are sets, then $X = Y$ if and only if for every set $z$, $z \in X$ if and only if $z \in Y$.
In other words, two sets are equal if they have the same elements (every element of one set is an element of the other).
This is one of the Zermelo-Fraenkel axioms of set theory.
This statement logically relies on the following definitions and statements:
To visualize the logical connections between this statements and other items of mathematical knowledge, you can visit the following cluster(s), and click the "Visualize" tab: Clust/ZFC axioms of set theory