You may read any quantity of books, and you may almost as ignorant as you were at starting, if you don't have, at the back of yourminds, the change for words in definite images which can only be acquired through the operation of your observing faculties on the phenomena of nature.

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. This is what Agnosticism asserts; and, in my opinion, it is all that is essential to Agnosticism. That which Agnostics deny and repudiate, as immoral, is the contrary doctrine, that there are propositions which men ought to believe without logically satisfactory evidence; and that reprobation ought to attach to the profession of disbelief in such inadequately supported propositions.

Not far from the invention of fire we must rank the invention of doubt.

Our reverence for the nobility of manhood will not be lessened by the knowledge that man is in substance and in structure, one with the brutes; for he alone possesses the marvellous endowment of intelligible and rational speech whereby he has slowly accumulated and organized the experience which is almost wholly lost with the cessation of individual life in other animals; so that he now stands raised above it as on a mountain-top, far above the level of his humble fellows, and transfigured from his grosser nature by reflecting, here and there, a ray from the infinite source of truth.

The Bible has been the Magna Charta of the poor and of the oppressed.

Skepticism is the highest duty and blind faith the one unpardonable sin.

The great tragedy of science - the slaying of a beautiful hypothesis by an ugly fact.