Reason, observation, and experience; the holy trinity of science.

If you have ever clothed another with woe, as with a garment of pain, you will never be quite as happy as though you had not done that thing.

There can be but little liberty on earth while men worship a tyrant in heaven.

If, with all the time at my disposal, with all the wealth of the resources of this vast universe, to do with as I will, I could not produce a better scheme of life than now prevails, I would be ashamed of my efforts and consider my work a humiliating failure.

As a man develops, he places a greater value upon his own rights. Liberty becomes a grander and diviner thing. As he values his own rights he begins to value the rights of others. And when all men give to all others all the rights they claim for themselves, this world will be civilized.

Logic is not satisfied with assertion. It cares nothing for the opinions of the great; nothing for the prejudices of the many, and least of all for the superstitions of the dead.

Whoever imagines himself a favorite with God holds others in contempt.