Full Idea
Doubt can exist only where a question exists, a question only where an answer can exist, and an answer only where something can be said.
Gist of Idea
Doubts can't exist if they are inexpressible or unanswerable
Source
Ludwig Wittgenstein (Tractatus Logico-Philosophicus [1921], 6.51)
Book Reference
Wittgenstein,Ludwig: 'Tractatus Logico-Philosophicus (Pears)', ed/tr. Pears,D. /McGuinness,B. [RKP 1961], p.73
A Reaction
I don't agree with any of that. It is typical of the phase when philosophers were mesmerised by language. Cats look puzzled sometimes. A glimmering of doubt may be pre-linguistic, inexpressible and unanswerable, but still feels like a doubt.