Full Idea
There is no need to admit as a fundamental notion any form of implication not expressible as a truth-function.
Gist of Idea
All forms of implication are expressible as truth-functions
Source
Bertrand Russell (Introduction to Mathematical Philosophy [1919], XIV)
Book Reference
Russell,Bertrand: 'Introduction to Mathematical Philosophy' [George Allen and Unwin 1975], p.154
A Reaction
Note that this is from a book about 'mathematical' philosophy. Nevertheless, it seems to have the form of a universal credo for Russell. He wasn't talking about conditionals here. Maybe conditionals are not implications (in isolation, that is).