Doubt everything at least once, even the proposition that two times two equals four.

- Georg Christoph Lichtenberg

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.

- Francis Bacon

God is a comedian playing to an audience too afraid to laugh.

- Voltaire

I am like a slave who, enjoying an imaginary liberty during sleep, begins to suspect that his liberty is only a dream; he fears to wake up and conspires with his pleasant illusions to retain them longer. So insensibly to myself I fall into my former opinions; and am slow to wake up from this slumber for fear that the labors of waking life which will have to follow the tranquility of this sleep, instead of leading me into the daylight of the knowledge of truth, will be insufficient to dispell the darkness of all the difficulties which have just been raised.

- René Descartes from Meditations on the First Philosophy

Doubt concerning the things most important for us to know is a stress too great for the human mind to endure very long. We cannot help putting an end to our doubt in one way or another, because we would rather be mistaken than believe nothing.

- Rousseau