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.

To err is human also in so far as animals seldom or never err, or at least only the cleverest of them do so.

Nothing puts a greater obstacle in the way of the progress of knowledge than thinking that one knows what one does not yet know.

Food probably has a very great influence on the condition of men. Wine exercises a more visible influence, food does it more slowly but perhaps just as surely. Who knows if a well-prepared soup was not responsible for the pneumatic pump or a poor one for a war?

Do not say hypothesis, and even less theory: say way of thinking.

If there were only turnips and potatoes in the world, someone would complain that plants grow the wrong way.

There is a great difference between believing in something and believing in it again.