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

The dogma of the infallibility of the Bible is no more self-evident than is that of the infallibility of the popes.

It is because the body is a machine that education is possible. Education is the formation of habits, a superinducing of an artificial organization upon the natural organization of the body.

I am content with nothing, restless and ambitious... and I despise myself for the vanity, which formed half the stimulus to my exertions. Oh would that I were one of those plodding wise fools who having once set their hand to the plough go on nothing doubting.

Skepticism is the highest duty and blind faith the one unpardonable sin.

Every living creature commences its existence under a form different from, and simpler than, that which it eventually attains.

If a man cannot see a church, it is preposterous to take his opinion about its altar-piece or painted window.