Bounded Class is Set
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Theorem
Let $B$ be a class.
Let it be assumed that $B$ is a subclass of a basic universe $V$.
Let $B$ be bounded by a set $x$.
Then $B$ is itself a set.
Proof
By definition, every element of $B$ is a subset of $x$.
Then every element of $B$ is an element of the power set $\powerset x$ of $x$.
Thus $B$ is a subclass of $\powerset x$.
By the Axiom of Powers, $\powerset x$ is a set.
That is, $\powerset x$ is an element of $V$.
As $V$ is a swelled class, then by definition, then every subclass of $\powerset x$ is an element of $V$.
That includes $B$.
That is, $B$ is an element of $V$.
Thus, by definition of $V$, $B$ is a set.
$\blacksquare$
Sources
- 2010: Raymond M. Smullyan and Melvin Fitting: Set Theory and the Continuum Problem (revised ed.) ... (previous) ... (next): Chapter $3$: The Natural Numbers: $\S 4$ A double induction principle and its applications