Full Idea
A sentence is factually significant to any given person, if, and only if, he knows how to verify the proposition which it purports to express.
Gist of Idea
A sentence is factually significant to someone if they know how to verify its proposition
Source
A.J. Ayer (Language,Truth and Logic [1936], Ch.1)
Book Reference
Ayer,A.J.: 'Language, Truth and Logic' [Penguin 1974], p.48
A Reaction
'I can't verify it, but I know a bloke who can'? 'If only I could think of a way to verify x'? 'This is unverifiable, but it is the only remaining possibility'? 'X is unverifiable, but it would nice if it was true'? Etc.