Full Idea
Russell's positive argument for universals is that they explain how we can have a priori knowledge, which 'deals exclusively with the relations of universals'.
Clarification
I.e. abstract ideas must exist to permit knowledge by pure thought
Gist of Idea
Russell claims that universals are needed to explain a priori knowledge (as their relations)
Source
report of Bertrand Russell (Problems of Philosophy [1912], Ch. 9) by DH Mellor / A Oliver - Introduction to 'Properties' §3
Book Reference
'Properties', ed/tr. Mellor,D.H. /Oliver,A [OUP 1997], p.6
A Reaction
Unfortunately we can invent the universals, and then delude ourselves that we have a priori knowledge