A person who doubts himself is like a man who would enlist in the ranks of his enemies and bear arms against himself. He makes his failure certain by himself being the first person to be convinced of it.
An honest man can never surrender an honest doubt.
Despair is the conclusion of fools.
Despair is the damp of hell, as joy is the serenity of heaven.
Doubt can only be removed by action.
Doubt is a pain too lonely to know that faith is his twin brother.
Doubt is the beginning, not the end, of wisdom.
Doubt is the father of invention.
Doubt is the incentive to truth and inquiry leads the way.
Doubt is the key to knowledge.
Doubt is the vestibule through which all must pass before they can enter into the temple of wisdom.
Doubt is to certainty as neurosis is to psychosis. The neurotic is in doubt and has fears about persons and things; the psychotic has convictions and makes claims about them. In short, the neurotic has problems, the psychotic has solutions.
Doubt whom you will, but never yourself.
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.
Doubt, of whatever kind, can be ended by action alone.
Doubt, the essential preliminary of all improvement and discovery, must accompany the stages of man's onward progress. The faculty of doubting and questioning, without which those of comparison and judgment would be useless, is itself a divine prerogative of the reason.
Everybody in the world thought we couldn't do it. But we did, dammit. [After winning the America's Cup]
Faith keeps many doubts in her pay. If I could not doubt, I should not believe.
Feed your faith and doubt will starve to death
Great doubts deep wisdom. Small doubts little wisdom.