"We demand guaranteed rigidly defined areas of doubt and uncertainty." - Douglas Adams

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

"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." - Alexandre Dumas, père

"Doubt is the father of invention." - Galileo

"Men become civilised, not in proportion to their willingness to believe, but in proportion to their readiness to doubt." - Henry Louis Mencken

"It is evident that scepticism, while it makes no actual change in man, always makes him feel better." - Henry Louis Mencken

"Doubt begins only at the last frontiers of what is possible." - Giovanni Jacopo Casanova Seingalt

"Doubt is not a pleasant condition, but certainty is an absurd one." - Voltaire, letter (to Frederick the Great, 1767)

"When you doubt, abstain." - Zoroaster

"All we have gained then by our unbelief
Is a life of doubt diversified by faith,
For one of faith diversified by doubt:
We called the chess-board white,
we call it black."
- ROBERT BROWNING, Bishop Blougram's Apology

"If you would be a real seeker after truth, it is necessary that at least once in your life you doubt, as far as possible, all things." - RENÉ DESCARTES, Principles of Philosophy

"Doubt grows with knowledge." - JOHANN WOLFGANG VON GOETHE, Proverbs in Prose

"Modest doubt is call'd
The beacon of the wise."
- SHAKESPEARE, Troilus and Cressida

"Our doubts are traitors
And make us lose the good we oft might win
By fearing to attempt."
- SHAKESPEARE, Measure for Measure

"There lives more faith in honest doubt,
Believe me, than in half the creeds."
- ALFRED, LORD TENNYSON, In Memoriam

"Life is doubt,
And faith without doubt is nothing but death."
- MIGUEL DE UNAMUNO, Salmo II

"Give me the benefit of your convictions, if you have any; but keep your doubts to yourself, for I have enough of my own." - Goethe, Johann Wolfgang von (1749-1832)

"I respect faith, but doubt is what gives you an education." - Mizner, Wilson

"Those who believe that they believe in God, but without passion in their hearts, without anguish in mind, without uncertainty, without doubt, without an element of despair even in their consolation, believe in the God idea, not God himself." - Unamuno, Miguel de (1864-1936)