You may read any quantity of books, and you may almost as ignorant as you were at starting, if you don't have, at the back of yourminds, the change for words in definite images which can only be acquired through the operation of your observing faculties on the phenomena of nature.

The question of all questions for humanity, the problem which lies behind all others and is more interesting than any of them, is that of the determination of man's place in nature and his relation to the cosmos.

The foundation of morality is to have done, once and for all, with lying.

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

The only question which any wise man can ask himself, and which any honest man will ask himself, is whether a doctrine is true or false.

As if to demonstrate, by a striking example, the impossibility of erecting any cerebral barrier between man and the apes, Nature has provided us, in the latter animals, with an almost complete series of gradations from brains little higher than that of a Rodent, to brains little lower than that of Man.

It is better to read a little and thoroughly than cram a crude undigested mass into my head, though it be great in quantity.