It often takes more courage to change one's opinion than to stick to it.

I have often noticed that when people come to understand a mathematical proposition in some other way than that of the ordinary demonstration, they promptly say, "Oh, I see. That's how it must be." This is a sign that they explain it to themselves from within their own system.

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

Sickness is mankind's greatest defect.

There is something in our minds like sunshine and the weather, which is not under our control. When I write, the best things come to me from I know not where.

What is the good of drawing conclusions from experience? I don't deny we sometimes draw the right conclusions, but don't we just as often draw the wrong ones?

He marvelled at the fact that the cats had two holes cut in their fur at precisely the spot where their eyes were.