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These considerations led me in the winter of 1919-20 to conclusions which I may now reformulate as follows:
- It is easy to obtain confirmations, or verifications, for nearly every theory -- if we look for confirmations.
- Confirmations should count only if they are the result of risky predictions; that is to say, unenlightened by the theory in question, we should have expected an event which was incompatible with the theory -- an event which would have refuted the theory.
- Every “good” scientific theory is a prohibition: it forbids certain things to happen. The more a theory forbids, the better it is.
- A theory which is not refutable by any conceivable event is nonscientific. Irrefutability is not a virtue of a theory (as people often think) but a vice.
- Every genuine test of a theory is an attempt to falsify it, or to refute it. Testability is falsifiability; but there are degrees of testability: some theories are more testable, more exposed to refutation, than others; they take, as it were, greater risks.
- Confirmation evidence should not count except when it is the result of genuine test of the theory; and this means that it can be presented as a serious but unsuccessful attempt to falsify the theory. (I now speak in such cases of “corroborating evidence.”)
- Some genuinely testable theories, when found to be false, are still upheld by their admirers -- for example by introducing ad hoc some auxiliary assumption, or by reinterpreting the theory ad hoc in such a way that it escapes refutation. Such as procedure is always possible, but it rescues the theory from refutation only at the price of destroying, or at least lowering, its scientific status. (I later described such as rescuing operation as a “conventionalist twist” or a “conventionalist stratagem.”)
One can sum up all this by saying that the criterion of the scientific status of a theory is its falsifiability, or refutability, or testability.Karl Popper, Conjectures and Refutations: The Growth of Scientific Knowledge, 1963