If a little knowledge is dangerous, where is the man who has so much as to be out of danger?

It is wrong for a man to say that he is certain of the objective truth of any proposition unless he can produce evidence which logically justifies that certainty.

Extinguished theologians lie about the cradle of every science, as strangled snakes beside that of Hercules

The foundation of all morality is to have done, once and for all, with lying; to give up pretending to believe that for which there is no evidence, and repeating unintelligible propositions about things beyond the possibilities of knowledge.

The rung of a ladder was never meant to rest upon.

Whatever evil voices may rage, Science, secure among the powers that are eternal, will do her work and be blessed.

The struggle for existence holds as much in the intellectual as in the physical world. A theory is a species of thinking, and its right to exist is coextensive with its power of resisting extinction by its rivals.