If we wish our civilization to survive we must break with the habit of deference to great men.

Unlimited tolerance must lead to the disappearance of tolerance. If we extend unlimited tolerance even to those who are intolerant, if we are not prepared to defend a tolerant society, then the tolerant will be destroyed, and tolerance with them. We should therefore claim, in the name of tolerance, the right not to tolerate the intolerant.

No particular theory may ever be regarded as absolutely certain.... No scientific theory is sacrosanct.

The defence of democracy must consist in making anti-democratic experiences too costly for those who try them; much more costly than a democratic compromise

It is not possible to write clearly enough to avoid being misrepresented by people who are sufficiently determined to do so.

You cannot have a rational discussion with a man who prefers shooting you to being convinced by you.

The game of science is, in principle, without end. He who decides one day that scientific statements do not call for any further test, and that they can be regarded as finally verified, retires from the game.