For every set there is a set whose members are exactly the subsets of .
Every finite ordinal belongs to , so but . An internal power set of would contain itself. Yet implies , making that impossible. This example illustrates why one must check which subsets the structure sees when proving an axiom fails.

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The Axiom of Power Set is one of the axioms in set theory, specifically within the Zermelo-Fraenkel set theory (ZF), which is a foundational system for much of modern mathematics. The axiom states that for any set \( A \), there exists a set \( P(A) \), called the power set of \( A \), which contains all the subsets of \( A \).