The whole problem with the world is that fools and fanatics are always so certain of themselves, and wiser people so full of doubts. Bertrand Russell
No man likes to have his intelligence or good faith questioned, especially if he has doubts about it himself. Henry Adams
Do not doubt him who tells you he is afraid, but be afraid of him who tells you he has no doubts. Erich Fried
Doubts give birth to truths. Japanese Proverb
The person who trusts is happy; the person who doubts is wise. Hungarian Proverb
He who knows nothing, doubts nothing. Brazilian Proverb