Axiom:Axiom of Pairing

Axiom
For any two sets, there exists a set to which only those two sets are elements:


 * $\forall A: \forall B: \exists x: \forall y: \left({y \in x \iff y = A \lor y = B}\right)$

Thus it is possible to create a set containing any two sets that you have already created.

Otherwise known as the Axiom of the Unordered Pair.

This can be deduced from the Axiom of Infinity and the Axiom of Replacement.