The future is always present, as a promise, a lure and a temptation.

Good tests kill flawed theories; we remain alive to guess again.

A system is empirical or scientific only if it is capable of being tested by experience. These considerations suggest that not the verifiability but the falsifiability of a system is to be taken as a criterion of demarcation... It must be possible for an empirical or scientific system to be refuted by experience.

We must plan for freedom, and not only for security, if for no other reason than only freedom can make security more secure.

Whenever we propose a solution to a problem, we ought to try as hard as we can to overthrow our solution, rather than defend it.

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 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.