So natural to mankind is intolerance ... that religious freedom has hardly anywhere been practically realized.

The idea is essentially repulsive, of a society held together only by the relations and feelings arising out of pecuniary interest.

The real advantage which truth has, consists in this, that when an opinion is true, it may be extinguished once, twice, or many times, but in the course of ages there will generally be found persons to rediscover it

Every established fact which is too bad to admit of any other defence is always presented to us as an injunction of religion.

Induction is a process of inference; it proceeds from the known to the unknown.

The test of real and vigorous thinking, the thinking which ascertains truths instead of dreaming dreams, is successful application to practice.

He who knows only his own side of the case (argument) knows little of that. His reasons may be good, and no one may have been able to refute them. But if he is equally unable to refute the reasons on the opposite side, if he does not so much as know what they are, he has no ground for preferring either opinion