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STRUCTURE and revolution are rightly up front in the title of Thomas Kuhn's The Structure of Scientific Revolutions. He was convinced that not only are there scientific revolutions but also that they have a structure. He laid out this structure with great care: normal science (routine scientific work), with its specific accompanying paradigm and a dedication to solving puzzles; followed by serious anomalies produced by research, which lead to a crisis; and finally resolution of that crisis by the creation of a new paradigm.

Puzzle-solving makes us think of crossword puzzles, jigsaws, sudoku - pleasant ways to keep busy when one is not up to useful work. A lot of scientific readers were a bit shocked, then had to admit that this is how it is in much of their daily work. Kuhn wrote: "The most striking feature... is how little they aim to produce major novelties, conceptual or phenomenal." Nowadays, many scientists have great respect for his account of normal science.

Kuhn single-handedly changed the currency of the word "paradigm", so a reader now attaches very different connotations to the word than were available in 1962. As Kuhn stated in his postscript to the book: "The paradigm... is the central element of what I now take to be the most novel and least understood aspect of this book." On the same page he suggested "exemplar" as a substitute. In another essay, he admitted he had "lost control of the word". In later life he abandoned it. But we, the readers of Structure 50 years later, can, I hope, restore it to prominence.

Normal science, then, is characterised by a paradigm, which legitimises the puzzles and problems on which the community works. All is well until the methods legitimised by that paradigm cannot cope with the anomalies that emerge; a crisis results and persists until a new achievement redirects research and serves as a new paradigm. This is a paradigm shift.

By way of illustration, there are moving quotations in Structure from Wolfgang Pauli, a few months before Heisenberg's paper on matrix mechanics showed the way to a new quantum theory, and a few months after. In the former, Pauli feels physics is falling apart and wishes he were in another trade; a few months later, the way ahead is clear. Many had the same feeling: at the height of the crisis the community was falling apart as its paradigm was under challenge.

Obviously, Kuhn's structure is too neat. History is not like that. But it was precisely Kuhn's instinct as a physicist that led him to find a simple and insightful structure. It was a picture of science the general reader could pick up. It also had the merit of being to some extent testable. Historians of science could see the extent to which momentous changes in their fields did conform to Kuhn's structure. Unfortunately, it was also abused by sceptical intellectuals who called the idea of truth into question. Kuhn had no such intention. He was a fact lover and a truth seeker.

So much for structure. As for revolutions, we think first of revolution in political terms. Everything is overthrown; a new world order begins. The first thinker to extend this notion to the sciences may have been Immanuel Kant. In the preface to the second edition of The Critique of Pure Reason, Kant speaks of two revolutionary events. One was the transition in mathematical practice in which techniques familiar in Babylonia and Egypt were transformed in Greece to proofs derived from postulates. The second was the emergence of the experimental method and the laboratory.

At the time Kuhn was writing, scientific revolution of the 17th century was much in vogue: Francis Bacon was its prophet, Galileo its lighthouse, and Newton its sun. But Kuhn was not talking about the scientific revolution. That was quite a different kind of event from the revolutions whose structure Kuhn postulated. Indeed, shortly before writing Structure, he proposed a "second scientific revolution", which took place during the early 19th century when new fields were mathematicised. Heat, light, electricity, and magnetism acquired their own paradigms: suddenly, a whole mass of unsorted phenomena began to make sense. But neither this second revolution nor the first one exhibited the "structure" Kuhn had in mind.

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