Thomas Kuhn was an American physicist turned historian of science whose The Structure of Scientific Revolutions (1962) is among the most cited academic books ever written, and whose central term escaped into general use almost immediately.

Normal science

Kuhn's history showed him that scientists do not spend their time attempting to refute their theories. Most work happens inside a settled framework — a paradigm — which supplies the questions worth asking, the methods that count, and the standard of a good answer.

Normal science is puzzle-solving within that frame. An anomaly is not treated as a refutation but as a puzzle not yet solved, which Kuhn argued is the correct response: a framework abandoned at the first difficulty would never be developed far enough to be useful.

Revolutions

Anomalies accumulate. Eventually enough resist solution that confidence in the framework itself weakens, and a crisis opens in which alternatives get a hearing. A revolution replaces the paradigm, and with it the questions, the methods, and what counts as data.

This is a paradigm shift, and Kuhn's claim is that it is not a simple accumulation of knowledge. Afterwards, scientists are in some respects working in a different world.

Incommensurability

The contested part. Kuhn argued that competing paradigms cannot be fully compared, because the terms shift meaning between them — mass means something different before and after relativity — and because each supplies its own standard of what a good theory does.

If so, choosing between paradigms is not settled by evidence alone, since what counts as evidence is partly what is in dispute. Critics read this as making science irrational. Kuhn spent much of his later career denying that, arguing that good reasons operate without a decision procedure that compels agreement.

Against Popper

The disagreement with Popper is about description before it is about norms. Popper said science advances by attempted refutation. Kuhn replied that the historical record shows something else, and that a discipline behaving as Popper recommended would never get anywhere.