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.

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.

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

Some scientists find, or so it seems, that they get their best ideas when smoking; others by drinking coffee or whisky. Thus there is no reason why I should not admit that some may get their ideas by observing, or by repeating observations.

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

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

Those who promise us paradise on earth never produced anything but a hell.