Axiom of Extensionality

Last updated Nov 1, 2022

$$\forall A \forall B [ \forall x(x \in A \Leftrightarrow x \in B) \Rightarrow A = B]$$ This Sentence is stating that for all Sets $A, B$, if they share the same elements, then they are the same Sets.

1. Enderton - Elements of Set Theory - Ch2 page 17