Full Idea
Russell insisted on the vicious circle principle, and thus rejected impredicative definitions, which resulted in an unwieldy ramified type theory, with the ad hoc axiom of reducibility. Ramsey's simpler theory was impredicative and avoided the axiom.
Clarification
See Idea 8730 for impredicative definitions
Gist of Idea
To avoid vicious circularity Russell produced ramified type theory, but Ramsey simplified it
Source
report of B Russell/AN Whitehead (Principia Mathematica [1913]) by Stewart Shapiro - Thinking About Mathematics 5.2
Book Reference
Shapiro,Stewart: 'Thinking About Mathematics' [OUP 2000], p.127
A Reaction
Nowadays the theory of types seems to have been given up, possibly because it has no real attraction if it lacks the strict character which Russell aspired to.
Related Idea
Idea 8730 'Impredicative' definitions refer to the thing being described [Shapiro]