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.

True ignorance is not the absence of knowledge, but the refusal to acquire it.

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

We have the right not to tolerate the intolerant. We should tolerate even them whenever we can do so without running a great risk; but the risk may become so great that we cannot allow ourselves the luxury.

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.

The method of science depends on our attempts to describe the world with simple theories: theories that are complex may become untestable, even if they happen to be true. Science may be described as the art of systematic over-simplification-the art of discerning what we may with advantage omit.

The history of science, like the history of all human ideas, is a history of irresponsible dreams, of obstinacy, and of error.