What we have to discover for ourselves leaves behind in our mind a pathway that can be used on another occasion.

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

If all mankind were suddenly to practice honesty, many thousands of people would be sure to starve.

It is astonishing how much the word infinitely is misused: everything is infinitely more beautiful, infinitely better, etc. The concept must have something pleasing about it, or its misuse could not have become so general.

I ceased in the year 1764 to believe that one can convince one’s opponents with arguments printed in books. It is not to do that, therefore, that I have taken up my pen, but merely so as to annoy them, and to bestow strength and courage on those on our own side, and to make it known to the others that they have not convinced us.

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.

Rational free spirits are the light brigade who go on ahead and reconnoiter the ground which the heavy brigade of the orthodox will eventually occupy.