Full Idea
The axiom of Reducibility ...is crucial in the reduction of classes to logic, ...and seems to be a quite legitimate logical notion for Russell.
Gist of Idea
Russell saw Reducibility as legitimate for reducing classes to logic
Source
comment on B Russell/AN Whitehead (Principia Mathematica [1913]) by Bernard Linsky - Russell's Metaphysical Logic 6.4
Book Reference
Linsky,Bernard: 'Russell's Metaphysical Logic' [CSLI 1999], p.101
A Reaction
This is an unusual defence of the axiom, which is usually presumed to have been kicked into the long grass by Quine. If one could reduce classes to logic, that would destroy the opposition to logicism in a single neat coup.