Full Idea
The nerve of the Intuitionist theory, led by Brouwer, is the denial of the law of excluded middle; it holds that a proposition can only be accounted true or false when there is some method of ascertaining which of these it is.
Gist of Idea
Intuitionism says propositions are only true or false if there is a method of showing it
Source
Bertrand Russell (My Philosophical Development [1959], Ch.2)
Book Reference
Russell,Bertrand: 'My Philosophical Development' [Routledge 1993], p.82
A Reaction
He cites 'there are three successive sevens in the expansion of pi' as a case in point. This seems to me an example of the verificationism and anti-realism which is typical of that period. It strikes me as nonsense, but Russell takes it seriously.