Subclass of Set is Set

Theorem
Let $A$ be a set.

Let $\phi \left({x}\right)$ be a condition in which $x$ is taken to be a set.

Then there exists a set that consists of all of the elements of $A$ that satisfies this condition.

In ZF, this result is known as the Axiom of Subsets.

Proof
By the axiom of class comprehension, let $B$ be the class defined as:

Aiming for contradiction, suppose that $B$ is not a set.

Then $B$ must be a proper class.

It is easily seen that $B \subseteq A$.

So by the Axiom of Powers, $B \in \mathcal P \left({A}\right)$ where $\mathcal P \left({A}\right)$ is defined as the power set of $A$.

But by Proper Class is not Element of Class, this is a contradiction.

Therefore by contradiction it follows that $B$ is a set.