The secret of genius is to carry the spirit of childhood into maturity.

Do what you can to do what you ought, and leave hoping and fearing alone.

Let us have "sweet girl graduates" by all means. They will be none the less sweet for a little wisdom; and the "golden hair" will not curl less gracefully outside the head by reason of there being brains within.

If there is anything in the world which I do firmly believe in, it is the universal validity of the law of causation.

Of all the senseless babble I have ever had occasion to read, the demonstrations of these philosophers who undertake to tell us all about the nature of God would be the worst, if they were not surpassed by the still greater absurdities of the philosophers who try to prove that there is no God.

Life is too short to occupy oneself with the slaying of the slain more than once.

In the world of letters, learning and knowledge are one, and books are the source of both; whereas in science, as in life, learning and knowledge are distinct, and the study of things, and not of books, is the source of the latter.