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.

• In science, as in art, and, as I believe, in every other sphere of human activity, there may be wisdom in a multitude of counsellors, but it is only in one or two of them.
• Science is simply common sense at its best, that is, rigidly accurate in observation, and merciless to fallacy in logic.
• The best men of the best epochs are simply those who make the fewest blunders and commit the fewest sins.
• My business is to teach my aspirations to confirm themselves to fact, not to try and make facts harmonize with my aspirations.
• It is because the body is a machine that education is possible. Education is the formation of habits, a superinducing of an artificial organization upon the natural organization of the body.
• Proclaim human equality as loudly as you like, Witless will serve his brother.
• My experience of the world is that things left to themselves don't get right.
• The improver of natural knowledge absolutely refuses to acknowledge authority, as such. For him, skepticism is the highest of duties blind faith the one unpardonable sin.
• The man of science has learned to believe in justification, not by faith, but by verification.
• The results of political changes are hardly ever those which their friends hope or their foes fear.