Quotes about doubt

In order to cease being a doubtful case, one has to cease being, that's all.

I have always felt that doubt was the beginning of wisdom, and the fear of God was the end of wisdom.

The two ways of contemplation are not unlike the two ways of action commonly spoken of by the ancients: the one plain and smooth in the beginning, and in the end impassable; the other rough and troublesome in the entrance, but after a while fair and even. So it is in contemplation: If a man will begin with certainties, he shall end in doubts; but if he will be content to begin with doubts he shall end in certainties.

How can I tell," said the man, "that the past isn't a fiction designed to account for the discrepancy between my immediate physical sensations and my state of mind?

What if nothing exists and we're all in somebody's dream? Or what's worse, what if only that fat guy in the third row exists?