Nature's deepest laws, her only true laws, are her invisible ones.

There are two freedoms - the false, where a man is free to do what he likes; the true, where he is free to do what he ought.

You must not talk about 'ain't and can't' when you speak of this great wonderful world round you, of which the wisest man knows only the very smallest corner, and is, as the great Sir Isaac Newton said, only a child picking up pebbles on the shore of a boundless ocean.

Make it a rule and pray to God to help you keep it . . . never, if possible, to lie down at night without being able to say "I have made one human being at least a little wiser, a little happier, or a little better this day."

For to be discontented with the divine discontent, and to be ashamed with the noble shame, is the very germ and first upgrowth of all virtue.

Do noble things, not dream them all day long.

If I am ever obscure in my expressions, do not fancy that therefore I am deep. If I were really deep, all the world would understand, though they might not appreciate. The perfectly popular style is the perfectly scientific one. To me an obscurity is a reason for suspecting a fallacy.