When you doubt, abstain.
- Zoroaster

A person who doubts himself is like a man who would enlist in the ranks of his enemies and bear arms agains himself. He makes his failure certain by himself being the first person to be convinced of it.
- Dumas, Alexandre (p?re)

Doubt, indulged and cherished, is in danger of becoming denial; but if honest, and bent on thorough investigation, it may soon lead to full establishment of the truth.
- Edwards, Tyron

Doubt is the father of invention.
- Galileo

To have doubted one's own first principles is the mark of a civilized man.
- Holmes Sr., Oliver Wendell

Who never doubted, never half believed. Where doubt is, there truth is - it is her shadow.
- Bailey, Gamaliel

Men become civilized, not in proportion to their willingness to believe, but in proportion to their readiness to doubt.
- Mencken, H.L.

It is evident that skepticism, while it makes no actual change in man, always makes him feel better.
- Mencken, H.L.

Doubt begins only at the last frontiers of what is possible.
- Seingalt, Giovanni J.

Our doubts are traitors, and make us lose the good we oft might win by fearing to attempt.
- Shakespeare, William

We should never ever doubt what nobody is sure about.
- Wonka, Willy