Every great advance in natural knowledge has involved the absolute rejection of authority.

'Infidel' is a term of reproach, which Christians and Mohammedans, in their modesty, agree to apply to those who differ from them.

A man has no reason to be ashamed of having an ape for his grandfather. If there were an ancestor whom I should feel shame in recalling it would rather be a man who plunges into scientific questions with which he has no real acquaintance, only to obscure them by an aimless rhetoric.

My experience of the world is that things left to themselves don't get right.

Logical consequences are the scarecrows of fools and the beacons of wise men.

It is far better for a man to go wrong in freedom than to go right in chains.

If there is anything in the world which I do firmly believe in, it is the universal validity of the law of causation.