To look for nails at low tide.

Neither in this world nor elsewhere is there any happiness in store for him who always doubts.

Doubt makes the mountain which faith can move.

Doubt is the key to knowledge.

He who knows nothing, doubts nothing.

I doubt, therefore I think; I think therefore I am.

Dubito ergo cogito; cogito ergo sum.

When in doubt, tell the truth.

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

Love and doubt have never been on speaking terms.

When in doubt, don't.

Doubt is uncomfortable, certainty is ridiculous.

There is nothing more dreadful than the habit of doubt. Doubt separates people. It is a poison that disintegrates friendships and breaks up pleasant relations. It is a thorn that irritates and hurts; it is a sword that kills.

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

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.

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

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.