Full Idea
The ban on 'impredicative' definitions says you can't define a class in terms of a totality to which that class must be seen as belonging.
Gist of Idea
'Impredictative' definitions fix a class in terms of the greater class to which it belongs
Source
Bernard Linsky (Russell's Metaphysical Logic [1999], 1)
Book Reference
Linsky,Bernard: 'Russell's Metaphysical Logic' [CSLI 1999], p.4
A Reaction
So that would be defining 'citizen' in terms of the community to which the citizen belongs? If you are asked to define 'community' and 'citizen' together, where do you start? But how else can it be done? Russell's Reducibility aimed to block this.
Related Idea
Idea 21705 Reducibility says any impredicative function has an appropriate predicative replacement [Linsky,B]