Full Idea
If 'it is not raining' means 'the sentence "it is raining" is false', that makes it almost impossible to understand how a sentence containing the word 'not' can be found true by observation.
Gist of Idea
It is hard to explain how a sentence like 'it is not raining' can be found true by observation
Source
Bertrand Russell (Human Knowledge: its scope and limits [1948], 9)
Book Reference
Russell,Bertrand: 'Human Knowledge' [Routledge 2009], p.111
A Reaction
Russell goes on to explore the general difficulty of deciding negative truths by observation. The same problem arises for truthmaker theory. Obviously I can observe that it isn't raining, but it seems parasitic on observing when it is raining.
Related Idea
Idea 16489 Is it possible to state every possible truth about the whole course of nature without using 'not'? [Russell]