Full Idea
In the superior realist and simple theory of types, the place of the axiom of reducibility is not taken by the axiom of classes, Zermelo's Aussonderungsaxiom.
Gist of Idea
In simple type theory the axiom of Separation is better than Reducibility
Source
report of Kurt Gödel (Russell's Mathematical Logic [1944], p.140-1) by Bernard Linsky - Russell's Metaphysical Logic 6.1 n3
Book Reference
Linsky,Bernard: 'Russell's Metaphysical Logic' [CSLI 1999], p.91
A Reaction
This is Zermelo's Axiom of Separation, but that too is not an axiom of standard ZFC.