Karl Popper’s 17 theses regarding scientific knowledge

1. All scientific knowledge is hypothetical or conjectural.
2. The growth of knowledge, and especially of scientific knowledge, consists in learning from our mistakes.
3. What may be called the method of science consists in learning from our mistakes systematically: first, by taking risks, by daring to make mistakes–that is, by boldly proposing new theories; and secondly, by searching systematically for the mistakes we have made — that is, by the critical discussion and the critical examination of our theories.
4. Among the most important arguments that are used in this critical discussion are arguments from experimental tests.
5. Experiments are constantly guided by theory, by theoretical hunches of which the experimenter is often not conscious, by hypotheses concerning possible sources of experimental errors, and by hopes or conjectures about what will be a fruitful experiment. (By theoretical hunches I mean guesses that experiments of a certain kind will be theoretically fruitful.)
6. What is called scientific objectivity consists solely in the critical approach: in the fact that if you are biased in favour of your pet theory, some of your friends and colleagues (or failing these, some workers of the next generation) will be eager to criticize your work — that is to say, to refute your pet theories if they can.
7. This fact should encourage you to try to refute your own theories yourself — that is to say, it may impose some discipline upon you.
8. In spite of this, it would be a mistake to think that scientists are more ‘objective’ than other people. It is not the objectivity or detachment of the individual scientist but of science itself (what may be called ‘the friendly-hostile cooperation of scientists’ — that is, their readiness for mutual criticism) which makes for objectivity.
9. There is even something like a methodological justification for individual scientists to be dogmatic and biased. Since the method of science is that of critical discussion, it is of great importance that the theories criticized should be tenaciously defended. For only in this way can we learn their real power. And only if criticism meets resistance can we learn the full force of a critical argument.
10. The fundamental role played in science by theories or hypotheses or conjectures makes it important to distinguish between testable (or falsifiable) and non-testable (or non-falsifiable) theories.
11. Only a theory which asserts or implies that certain conceivable events will not, in fact, happen is testable. The test consists in trying to bring about, with all the means we can muster, precisely these events which the theory tells us cannot occur.
12. Thus, every testable theory may be said to forbid the occurrence of certain events. A theory speaks about empirical reality only in so far as it sets limits to it.
13. Every testable theory can thus be put into the form ‘such and such cannot happen’. For example, the second law of thermodynamics can be formulated as saying that a perpetual motion machine of the second kind cannot exist.
14. No theory can tell us anything about the empirical world unless it is in principle capable of clashing with the empirical world. And this means, precisely, that it must be refutable.
15. Testability has degrees: a theory which asserts more, and thus takes greater risks, is better testable than a theory which asserts very little.
16. Similarly, tests can be graded as being more or less severe. Qualitative tests, for example, are in general less severe than quantitative tests. And tests of more precise quantitative predictions are more severe than tests of less precise predictions.
17. Authoritarianism in science was linked with the idea of establishing, that is to say, of proving or verifying, its theories. The critical approach is linked with the idea of testing, that is to say, or trying to refute, or to falsify, its conjectures.

From The Myth of the Framework, Chapter 4, page 93 to 94