Full Idea
We can reintroduce 'not' by a definition: the words 'this is not blue' are defined as expressing disbelief in what is expressed by the words 'this is blue'. In this way the need of 'not' as an indefinable constituent of facts is avoided.
Gist of Idea
If we define 'this is not blue' as disbelief in 'this is blue', we eliminate 'not' as an ingredient of facts
Source
Bertrand Russell (Human Knowledge: its scope and limits [1948], 9)
Book Reference
Russell,Bertrand: 'Human Knowledge' [Routledge 2009], p.114
A Reaction
This is part of Russell's programme of giving a psychological account of logical connectives. See other ideas from his 1940 and 1948 works. He observes that disbelief is a state just as positive as belief. I love it.