Axiom of Pairing from Axiom of Specification

Theorem
Let it be supposed that there exists a set which contains at least two elements.

Then the Axiom of Pairing is a consequence of the Axiom of Specification.

Proof
Let $A$ be a set which contains at least two elements.

Let $a$ and $b$ be any two elements of $A$.

Let $\map P x$ be the propositional function:
 * $\map P x := \paren {x = a \lor x = b}$

Then we may use the Axiom of Specification to define $B$ as:
 * $x \in B \iff \set {x \in A: \map P x}$

Hence we can define:
 * $B := \set {a, b}$

for any two $a, b \in A$.