Freeman Dyson in the New York Review of Books:
Science consists of facts and theories. Facts and theories are born in different ways and are judged by different standards. Facts are supposed to be true or false. They are discovered by observers or experimenters. A scientist who claims to have discovered a fact that turns out to be wrong is judged harshly. One wrong fact is enough to ruin a career.
Theories have an entirely different status. They are free creations of the human mind, intended to describe our understanding of nature. Since our understanding is incomplete, theories are provisional. Theories are tools of understanding, and a tool does not need to be precisely true in order to be useful. Theories are supposed to be more-or-less true, with plenty of room for disagreement. A scientist who invents a theory that turns out to be wrong is judged leniently. Mistakes are tolerated, so long as the culprit is willing to correct them when nature proves them wrong.
Brilliant Blunders, by Mario Livio, is a lively account of five wrong theories proposed by five great scientists during the last two centuries. These examples give for nonexpert readers a good picture of the way science works. The inventor of a brilliant idea cannot tell whether it is right or wrong. Livio quotes the psychologist Daniel Kahneman describing how theories are born: “We can’t live in a state of perpetual doubt, so we make up the best story possible and we live as if the story were true.” A theory that began as a wild guess ends as a firm belief. Humans need beliefs in order to live, and great scientists are no exception. Great scientists produce right theories and wrong theories, and believe in them with equal conviction.