I believe four ingredients are necessary for happiness: health, warm personal relations, sufficient means to keep you from want, and successful work.

Men who are unhappy, like men who sleep badly, are always proud of the fact.

The wise use of leisure, it must be conceded, is a product of civilization and education.

Is the set of all sets which are not members of themselves a member of itself?

Do not use power to suppress opinions you think pernicious, for if you do the opinions will suppress you.

In action, in desire, we must submit perpetually to the tyranny of outside forces; but in thought, in aspiration, we are free, free from our fellowmen, free from the petty planet on which our bodies impotently crawl, free even, while we live, from the tyranny of death.

I hold all knowledge that is concerned with things that actually exist - all that is commonly called Science - to be of very slight value compared to the knowledge which, like philosophy and mathematics, is concerned with ideal and eternal objects, and is freed from this miserable world which God has made.