What science cannot discover, mankind cannot know.

Any pleasure that does no harm to other people is to be valued.

If the ordinary wage-earner worked four hours a day, there would be enough for everybody, and no unemployment — assuming a certain very moderate amount of sensible organization. This idea shocks the well-to-do, because they are convinced that the poor would not know how to use so much leisure.

If one lived for ever the joys of life would inevitably in the end lose their savour. As it is, they remain perennially fresh.

When one admits that nothing is certain one must, I think, also admit that some things are much more nearly certain than others.

The pure mathematician, like the musician, is a free creator of his world of ordered beauty.

So far I have been speaking of theoretical science, which is an attempt to understand the world. Practical science, which is an attempt to change the world, has been important from the first, and has continually increased in importance, until it has almost ousted theoretical science from men's thoughts.