Sickness is mankind's greatest defect.

A vacuum of ideas affects people differently than a vacuum of air, otherwise readers of books would be constantly collapsing.

If an angel were to tell us about his philosophy, I believe many of his statements might well sound like '2 x 2= 13'.

There is no more important rule of conduct in the world than this: attach yourself as much as you can to people who are abler than you and yet not so very different that you cannot understand them.

We often have need of a profound philosophy to restore to our feelings their original state of innocence, to find our way out of the rubble of things alien to us, to begin to feel for ourselves and to speak ourselves, and I might almost say to exist ourselves.

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.

A good method of discovery is to imagine certain members of a system removed and then see how what is left would behave: for example, where would we be if iron were absent from the world: this is an old example.