Also known as the Aussonderungsaxiom. The unrestricted principle of comprehension leads to contradiction in set theory. The axiom of separation, due to Zermelo, restored consistency by allowing a set of objects to exist when it is the subset of a previous set, and its members meet a condition: (∃y)(∀x)(x ∈ y) iff (x ∈ z & Fx). That is, a set y of objects exists when it is separated out from a previously given set z, as the subset whose members meet a condition F.