Of all the inventions of man I doubt whether any was more easily accomplished than that of a Heaven.

If all else fails, the character of a man can be recognized by nothing so surely as by a jest which he takes badly.

If you are going to build something in the air it is always better to build castles than houses of cards.

Doubt must be no more than vigilance, otherwise it can become dangerous.

The sure conviction that we could if we wanted to is the reason so many good minds are idle.

One might call habit a moral friction: something that prevents the mind from gliding over things but connects it with them and makes it hard for it to free itself from them.

The great trick of regarding small departures from the truth as the truth itself - on which is founded the entire integral calculus - is also the basis of our witty speculations, where the whole thing would often collapse if we considered the departures with philosophical rigour.