Full Idea
Axiom of Pairing: For any two sets there exists a set to which they both belong. So you can make a set out of two other sets.
Gist of Idea
Pairing: For any two sets there exists a set to which they both belong
Source
Brian Clegg (Infinity: Quest to Think the Unthinkable [2003], Ch.15)
Book Reference
Clegg,Brian: 'Infinity' [Robinson 2003], p.205