Doubleton Class can be Formed from Two Sets

Theorem
Let $V$ be a basic universe.

Let $a, b \in V$ be sets.

Then the doubleton class $\set {a, b}$ can be formed, which is a subclass of $V$.

Proof
Using the axiom of specification, let $A$ be the class defined as:
 * $A := \set {x: x \in V \land \paren {x = a \lor x = b} }$

That is:
 * $A = \set {a, b}$

By the axiom of extension, $\set {a, b}$ is the only such class which has $a$ and $b$ as elements.

Also see

 * Definition:Doubleton Class