Full Idea
The Axiom of Reducibility ...had the effect of making impredicative definitions possible.
Clarification
Impredicative definitions introduce nothing new
Gist of Idea
The Axiom of Reducibility made impredicative definitions possible
Source
A.George / D.J.Velleman (Philosophies of Mathematics [2002], Ch.3)
Book Reference
George,A/Velleman D.J.: 'Philosophies of Mathematics' [Blackwell 2002], p.47