The Logic of Scientific Discovery

Summary

The Logic of Scientific Discovery is Karl Popper's account of what makes a theory scientific and how scientific knowledge grows. It first appeared in Vienna in 1934, with the imprint date 1935, as Logik der Forschung, in a series edited by Moritz Schlick and Philipp Frank of the Vienna Circle. Popper, then a secondary-school teacher, had cut it down from a much longer manuscript. The English version, translated by Popper with the help of Julius Freed and Lan Freed, was published in London by Hutchinson in 1959 with many new footnotes and appendices. The book remains under copyright, and no full text is included in the library. Popper begins with two problems. The problem of induction, posed by Hume, is that no number of observations can justify a universal statement: however many white swans we have seen, it does not follow that all swans are white. The problem of demarcation is how to distinguish the empirical sciences from mathematics, logic and metaphysics. Popper's answer to both is the same. Science does not proceed by induction but by the deductive testing of bold conjectures. A theory is empirical only if it forbids something, so that some possible observation could contradict it. Verification can never be conclusive, but a single accepted counter-instance can refute a universal law. Falsifiability, not verifiability, is therefore the criterion of demarcation, and Popper insists that it marks off science from other kinds of thought without declaring them meaningless, as some members of the Vienna Circle held. The central chapters develop this view. Methodological rules are proposals, chosen so as to keep theories exposed to refutation; a scientist who saves a theory by ad hoc assumptions or redefinitions, which Popper calls conventionalist stratagems, has left the empirical method. Observation reports, or basic statements, are themselves accepted by decision and could be tested further, so science has no absolute foundation: it rises above a swamp on piles driven in far enough to bear the structure for the time being. Theories can be compared by their degree of falsifiability, and the better theory says more and so risks more; Popper identifies simplicity with this testability. Long chapters treat probability, which he interprets in frequency terms, and the interpretation of quantum mechanics. The final chapter argues that theories are never shown to be true or even probable, but can be corroborated by surviving severe tests. The book made falsifiability one of the best-known ideas in the philosophy of science, and it has been extensively discussed and criticized by Kuhn, Lakatos, Feyerabend and others.

Top Lessons

• A theory must be able to fail

  Popper argues that a scientific theory says something about the world only if it forbids something, so that some observation could prove it wrong. A theory compatible with every possible event explains nothing. The willingness to be refuted is the mark of empirical science.

• Do not rescue a theory by special pleading

  Any theory can be saved from refutation by adding assumptions invented only for that purpose or by changing the meaning of its terms. Popper calls these conventionalist stratagems and proposes rules against them, because a theory protected in this way no longer tells us anything about the world.

• Knowledge without bedrock

  Even observation reports are accepted by decision and could be tested further. Popper concludes that science stands on piles driven into a swamp: firm enough to carry the structure for now, but not resting on any final foundation. Knowledge is conjectural, though it can still grow.

• Prefer bold theories

  Popper argues that theories which say more, and therefore risk more, are the better candidates for science. A highly improbable theory with great content that survives severe tests is worth more than a safe theory that says little.

Verified Quotes

Medical Lens

The book contains no medical subject matter, but its account of testing has been invoked in discussions of method in medicine and psychology. The immunologist Peter Medawar was a prominent admirer of Popper's view of scientific method, and the falsifiability criterion was cited by the US Supreme Court in Daubert v. Merrell Dow Pharmaceuticals (1993), a case about expert testimony on a drug's alleged effects. Popper's later criticisms of psychoanalysis as untestable appear in Conjectures and Refutations rather than in this book. These connections are matters of later influence, not content of the text.