# Axiom:Axiom of Infinity/Class Theory/Formulation 3

## Axiom

Not every set is a natural number.

## Historical Note

Raymond M. Smullyan and Melvin Fitting introduce this formulation of the Axiom of Infinity in their Set Theory and the Continuum Problem, Revised ed. of $2010$ as:

something rather amusing.

It would not normally be expected that this formulation be used as the basis of serious work in this field of mathematics.