Full Idea
Axiom of Specification: For every set and every condition, there corresponds a set whose elements are exactly the same as those elements of the original set for which the condition is true. So the concept 'number is even' produces a set from the integers.
Gist of Idea
Specification: a condition applied to a set will always produce a new set
Source
Brian Clegg (Infinity: Quest to Think the Unthinkable [2003], Ch.15)
Book Reference
Clegg,Brian: 'Infinity' [Robinson 2003], p.205
A Reaction
What if the condition won't apply to the set? 'Number is even' presumably won't produce a set if it is applied to a set of non-numbers.