Philosophy when superficially studied, excites doubt, when thoroughly explored, it dispels it.

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.

The fearful unbelief is unbelief in yourself.

Despair is the damp of hell, as joy is the serenity of heaven.

Doubt is the father of invention.

Doubt is a pain too lonely to know that faith is his twin brother.

To have doubted one's own first principles is the mark of a civilized man.

Doubt is the beginning, not the end, of wisdom.

Everybody in the world thought we couldn't do it. But we did, dammit. [After winning the America's Cup]

An honest man can never surrender an honest doubt.

The road to perseverance lies by doubt.

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

Our doubts are traitors and make us lose the good we might win, by fearing to attempt.[Measure For Measure]

When in charge ponder. When in trouble delegate. When in doubt mumble.

When you doubt, abstain.