The fatal tendency of mankind to leave off thinking about a thing when it is no longer doubtful is the cause of half their errors.

Whatever crushes individuality is despotism.

To mistake money for wealth, is the same sort of error as to mistake the highway which may be the easiest way of getting to your house or lands, for the house and lands themselves.

The beliefs which we have most warrant for have no safeguard to rest on but a standing invitation to the whole world to prove them unfounded. If the challenge is not accepted, or is accepted and the attempt fails, we are far enough from certainty still; but we have done the best that the existing state of human reason admits of; we have neglected nothing that could give the truth a chance of reaching us.

The philosophy of reasoning, to be complete, ought to comprise the theory of bad as well as of good reasoning.

If opponents of all important truths do not exist, it is indispensable to imagine them and supply them with the strongest arguments which the most skillful devil's advocate can conjure up.

Let not any one pacify his conscience by the delusion that he can do no harm if he takes no part, and forms no opinion. Bad men need nothing more to compass their ends, than that good men should look on and do nothing.