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.

All that makes existence valuable to any one depends on the enforcement of restraints upon the actions of other people.

Is there any moral enormity which might not be justified by imitation of such a Deity?

A [psychological] difficulty is not an impossibility.

We often hear the teachers of all creeds lamenting the difficulty of keeping up in the minds of believers a lively apprehension of the truth which they nominally recognize, so that it may penetrate the feelings, and acquire a real mastery over the conduct.... When it has come to be a hereditary creed, and to be received passively, not actively ... there is a progressive tendency to forget all of the belief except the formularies ... until it almost ceases to connect itself at all with the inner life of the human being.

Everything must be free to be written and published without restraint.

If two or more instances of the phenomenon under investigation have only one circumstance in common, the circumstance in which alone all the instances agree is the cause (or effect) of the given phenomenon.