Irrationally held truths may be more harmful than reasoned errors.

The only question which any wise man can ask himself, and which any honest man will ask himself, is whether a doctrine is true or false.

There are savages without God in any proper sense of the word, but none without ghosts.

Deduction, which takes us from the general proposition to facts again-teaches us, if I may so say, to anticipate from the ticket what is inside the bundle.

It is not what we believe, but why we believe it. Moral responsibility lies in diligently weighing the evidence. We must actively doubt; we have to scrutinize our views, not take them on trust. No virtue attached to blindly accepting orthodoxy, however 'venerable'.

The student of Nature wonders the more and is astonished the less, the more conversant he becomes with her operations; but of all the perennial miracles she offers to his inspection, perhaps the most worthy of admiration is the development of a plant or of an animal from its embryo.

The ultimate court of appeal is observation and experiment... not authority.