No number of sightings of white swans can prove the theory that all swans are white. The sighting of just one black one may disprove it.

There is an almost universal tendency, perhaps an inborn tendency, to suspect the good faith of a man who holds opinions that differ from our own opinions. It obviously endangers the freedom and the objectivity of our discussion if we attack a person instead of attacking an opinion or, more precisely, a theory.

Simple statements are to be prized more highly than less simple ones because they tell us more; because their empirical content is greater; and because they are better testable.

Optimism is a duty. The future is open. It is not predetermined. No one can predict it, except by chance. We all contribute to determining it by what we do. We are all equally responsible for its success.

If we are not prepared to defend a tolerant society against the onslaught of the intolerant, then the tolerant will be destroyed, and tolerance with them.

I have spoken to Einstein and he admitted to me that his theory was in fact no different from the one of Parmenides.

The use of violence is justified only under a tyranny which makes reforms without violence impossible, and should have only one aim, that is, to bring about a state of affairs which makes reforms without violence possible.