What is corroboration?

Karl Popper, answered from the texts and cited to the page.

Karl Popper answers

Corroboration is my term for the evaluation of how well a theory has stood up to empirical tests — and I chose the word deliberately, precisely to avoid the misleading associations that cluster around "confirmation." The root of "confirmation" suggests making a hypothesis more certain, or even finally securing it; that is the verificationist picture, and it is not mine.¹

Corroboration carries no such implication. The fundamental idea is this: the degree of corroboration of a theory is an evaluation of the results of the empirical tests it has undergone.² There are two attitudes one can take toward the relation between a theory and experience — one may look for confirmation, or one may look for refutation.² Scientific tests are always attempted refutations; corroboration is what we record when the attempt fails.²

A theory is better corroborated the more severe the tests it has passed, and the better it has passed them.³ Now, corroboration is not a probability, and this point is not a technicality — it is the whole philosophical weight of the concept.⁴ Tautologies have the maximum degree of logical probability, yet they are precisely what empirical science finds unacceptable; establishing that an alleged empirical theory is tautological always amounts to a decisive criticism of it.⁴

Interesting scientific theories, including those we now regard as our best, have always had negligible — if not zero — logical probability, because their content is high and their claims are bold.⁴ Probability and acceptability-in-science therefore run in opposite directions, which is why corroboration cannot be a probability measure. The corroborative appraisal itself is not a hypothesis.

It can be derived once we are given the theory and the accepted basic statements: it asserts that those statements do not contradict the theory, and it does so with due regard to the degree of testability of the theory and the severity of the tests to which it has been subjected up to a stated period of time.⁵ Mere compatibility — the bare fact that a theory has not yet been falsified — is not sufficient for a positive degree of corroboration.⁵

Something more is required: the tests must have been genuinely severe, genuinely capable of refuting the theory had it been false. One further asymmetry matters. A corroborative appraisal made at a later date, after new basic statements have been accepted, can replace a positive degree of corroboration with a negative one — but not the reverse.⁶

The arrow runs one way. And it is always the experiment, in the end, that saves us from following a track that leads nowhere, that challenges us to find a new way, even though it is always the theory and not the observation that opens the path to new knowledge in the first place.⁶

Sources

1. I do prefer the label 'degree of corroboration' to 'degree of confirmation'. For the term 'confirmation' may easily suggest a wrong idea. It contains the root 'firm', and it suggests either a process of making a hypothesis by degrees more certain, or even a process of making it finally secure. In other words, the term 'confirmation' has strong verificationist associations.

   Realism and the Aim of Science, pp. 272–273

2. The degree of corroboration of a theory is an evaluation of the results of the empirical tests it has undergone... There are two attitudes, two ways of looking at the relations between a theory and experience: one may look for confirmation, or for refutation... Scientific tests are always attempted refutations.

   Realism and the Aim of Science, pp. 285–286

3. A theory will be said to be the better corroborated the more severe the tests it has passed (and the better it has passed them).

   Realism and the Aim of Science, pp. 285–286

4. Tautologies have the maximum degree of probability but they are unacceptable to empirical science: establishing that an alleged empirical theory is tautological (or almost so) always amounts to a decisive criticism of it... interesting scientific theories have always a negligible (if not zero) probability—including those which are at present generally accepted.

   Realism and the Aim of Science, pp. 285–286

5. The appraisal of the corroboration is not a hypothesis, but can be derived if we are given the theory as well as the accepted basic statements. It asserts the fact that these basic statements do not contradict the theory, and it does this with due regard to the degree of testability of the theory, and to the severity of the tests to which the theory has been subjected, up to a stated period of time.

   The Logic of Scientific Discovery, p. 264

6. A corroborative appraisal made at a later date—that is, an appraisal made after new basic statements have been added to those already accepted—can replace a positive degree of corroboration by a negative one, but not vice versa. And although I believe that in the history of science it is always the theory and not the experiment, always the idea and not the observation, which opens up the way to new knowledge, I also believe that it is always the experiment which saves us from following a track that leads nowhere.

   The Logic of Scientific Discovery, pp. 266–268