Quotes on doubt

The latest issue of eSkeptic has an essay/quote collection on doubt, by Michael Canfield:

I think we ought always to entertain our opinions with some measure of doubt. I shouldn’t wish people dogmatically to believe any philosophy, not even mine.
— Bertrand Russell

To have doubted one’s own first principles is the mark of a civilized man.
— Oliver Wendell Holmes Jr

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.
— Descartes

But,

The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts.
— Bertrand Russell

The best lack all convictions, while the worst / Are full of passionate intensity.
— Yeats

Doubt is not a pleasant condition, but certainty is absurd.
— Voltaire

Therefore,

Doubt ‘til thou canst doubt no more … doubt is thought and thought is life. Systems which end doubt are devices for drugging thought.
— Albert Guerard