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.

It is the rule which says that the other rules of scientific procedure must be designed in such a way that they do not protect any statement in science against falsification.

A theory is just a mathematical model to describe the observations.

It is wrong and dangerous to extol freedom by telling people that they will certainly be all right once they are free. The most we can say of democracy or freedom is that they give our personal abilities a little more influence on our well-being.

I think that there is only one way to science - or to philosophy, for that matter: to meet a problem, to see its beauty and fall in love with it; to get married to it and to live with it happily, till death do ye part - unless you should meet another and even more fascinating problem or unless, indeed, you should obtain a solution. But even if you do obtain a solution, you may then discover, to your delight, the existence of a whole family of enchanting, though perhaps difficult, problem children, for whose welfare you may work, with a purpose, to the end of your days.

Science is perhaps the only human activity in which errors are systematically criticized and, in time, corrected.

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