The problem of induction and Popper's answer
In the eighteenth century David Hume argued that no number of observations can logically justify a general law about what will happen next. Seeing thousands of white swans never proves that all swans are white, yet a single black swan refutes the claim. In The Logic of Scientific Discovery (originally 1934) Karl Popper built on this asymmetry: scientific theories can never be verified, only falsified. A good theory therefore sticks its neck out by ruling things out, and a theory that survives serious attempts at refutation is merely corroborated for now.
Drawing the line
In Conjectures and Refutations (1963) Popper held up Einstein's general relativity as the model case. It predicted exactly how much starlight passing the Sun would bend, and the 1919 eclipse expeditions led by Eddington (published in 1920) could have sunk it. By contrast, Popper argued, psychoanalysis and Marxist theories of history could explain away any outcome after the fact and so were unfalsifiable. Falsifiability became, alongside parsimony, one of the standard yardsticks for judging theories, and its logic echoes the way statistical hypothesis testing tries to reject a null hypothesis.
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Criticism and later debate
Pierre Duhem and W. V. O. Quine noted that theories are always tested together with auxiliary assumptions, so a failed prediction does not pinpoint what went wrong (the Duhem–Quine problem). Thomas Kuhn's The Structure of Scientific Revolutions (1962) observed that working scientists rarely drop a paradigm over anomalies, and Imre Lakatos proposed judging research programmes by whether they predict novel facts (progressive) or only patch themselves up (degenerating). Paul Feyerabend rejected any universal method at all. Today many philosophers favor Bayesian updating of degrees of belief, and since the 2015 reproducibility project in psychology, practices such as preregistration have been promoted to make claims more open to refutation.
FAQ
- Does unfalsifiable mean false?
- No. For Popper, an unfalsifiable claim is simply not scientific; it is not thereby false. Meaningful ideas in metaphysics or ethics can be unfalsifiable.
- Is evolution falsifiable?
- Popper once raised doubts but later revised his view. Fossils consistently found in the wrong geological layers, for example, would seriously challenge evolutionary theory, so it is generally regarded as falsifiable.
Sources reviewed: 2026-10-26
References
- Hume, D. (1748). An Enquiry Concerning Human Understanding. London: A. Millar.
- Duhem, P. (1906). La théorie physique: son objet et sa structure. Paris: Chevalier & Rivière.
- Dyson, F. W., Eddington, A. S., & Davidson, C. (1920). A determination of the deflection of light by the Sun's gravitational field. Philosophical Transactions of the Royal Society A, 220, 291–333.
- Popper, K. R. (1934). Logik der Forschung. Wien: Springer (Eng. The Logic of Scientific Discovery, 1959).
- Quine, W. V. O. (1951). Two dogmas of empiricism. The Philosophical Review, 60(1), 20–43.
- Kuhn, T. S. (1962). The Structure of Scientific Revolutions. Chicago: University of Chicago Press.
- Popper, K. R. (1963). Conjectures and Refutations. London: Routledge & Kegan Paul.
- Lakatos, I. (1970). Falsification and the methodology of scientific research programmes. In I. Lakatos & A. Musgrave (Eds.), Criticism and the Growth of Knowledge (pp. 91–196). Cambridge University Press.
- Feyerabend, P. (1975). Against Method. London: New Left Books.
- Open Science Collaboration (2015). Estimating the reproducibility of psychological science. Science, 349(6251), aac4716.