[Great scientists] are men of bold ideas, but highly critical of their own ideas: they try to find whether their ideas are right by trying first to find whether they are not perhaps wrong. They work with bold conjectures and severe attempts at refuting their own conjectures.

Definitions.... are never really needed, and rarely of any use

It is part of my thesis that all our knowledge grows only through the correcting of our mistakes.

The survival value of intelligence is that it allows us to extinct a bad idea, before the idea extincts us.

Science is not a system of certain, or -established, statements; nor is it a system which steadily advances towards a state of finality... And our guesses are guided by the unscientific, the metaphysical (though biologically explicable) faith in laws, in regularities which we can uncover-discover. Like Bacon, we might describe our own contemporary science-'the method of reasoning which men now ordinarily apply to nature'-as consisting of 'anticipations, rash and premature' and as 'prejudices'.

We have become makers of our fate when we have ceased to pose as its prophets.

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