Full Idea
It isn't possible to believe something for which you cannot imagine some kind of verification.
Gist of Idea
You can't believe it if you can't imagine a verification for it
Source
Ludwig Wittgenstein (Philosophical Remarks [1930], p.200), quoted by J. Alberto Coffa - The Semantic Tradition from Kant to Carnap 13 'Constr'
Book Reference
Coffa,J.Alberto: 'The Semantic Tradition from Kant to Carnap' [CUP 1993], p.257
A Reaction
In 1930 LW was calling this his 'old principle'. As it stands here it is too vague to assert very much.