In the world of letters, learning and knowledge are one, and books are the source of both; whereas in science, as in life, learning and knowledge are distinct, and the study of things, and not of books, is the source of the latter.

I took thought, and invented what I conceived to be the appropriate title of 'agnostic'.

A man who speaks out honestly and fearlessly that which he knows, and that which he believes, will always enlist the good will and the respect, however much he may fail in winning the assent, of his fellow men.

Education is the instruction of the intellect in the laws of Nature, under which name I include not merely things and their forces, but people and their ways; and the fashioning of the affections and of the will into an earnest and loving desire to move in harmony with those laws.

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

A well-worn adage advises those who set out upon a great enterprise to count the cost, yet some of the greatest enterprises have succeeded because the people who undertook them did not count the cost.

History warns us that it is the customary fate of new truths to begin as heresies and to end as superstitions.