A man who knows a subject thoroughly, a man so soaked in it that he eats it, sleeps it and dreams it- this man can always teach it with success, no matter how little he knows of technical pedagogy.

The essence of science is that it is always willing to abandon a given idea, however fundamental it may seem to be, for a better one; the essence of theology is that it holds its truths to be eternal and immutable. To be sure, theology is always yielding a little to the progress of knowledge, and only a Holy Roller in the mountains of Tennessee would dare to preach today what the popes preached in the Thirteenth Century, but this yielding is always done grudgingly, and thus lingers a good while behind the event.

Moral certainty is always a sign of cultural inferiority. The more uncivilized the man, the surer he is that he knows precisely what is right and what is wrong. All human progress, even in morals, has been the work of men who have doubted the current moral values, not of men who have whooped them up and tried to enforce them. The truly civilized man is always skeptical and tolerant.

Hygiene is the corruption of medicine by morality.

What restrains us from killing is partly fear of punishment, partly moral scruple, and partly what may be described as a sense of humor

Never let your inferiors do you a favor - it will be extremely costly.

The time must come inevitably when mankind shall surmount the imbecility of religion, as it has surmounted the imbecility of religion's ally, magic. It is impossible to imagine this world being really civilized so long as so much nonsense survives. In even its highest forms religion embraces concepts that run counter to all common sense. It can be defended only by making assumptions and adopting rules of logic that are never heard of in any other field of human thinking.