The Structure of Scientific Revolutions

We may, to be more precise, have to relinquish the notion, explicit or implicit, that changes of paradigm carry scientists and those who learn from them closer and closer to the truth.

– Thomas Kuhn


The Structure of Scientific Revolutions (1962) is a book about the history of science by the philosopher Thomas S. Kuhn. Its publication was a landmark event in the history, philosophy, and sociology of scientific knowledge. Kuhn challenged the then prevailing view of progress in “normal science”. Normal scientific progress was viewed as “development-by-accumulation” of accepted facts and theories. Kuhn argued for an episodic model in which periods of such conceptual continuity in normal science were interrupted by periods of revolutionary science. The discovery of “anomalies” during revolutions in science leads to new paradigms. New paradigms then ask new questions of old data, move beyond the mere “puzzle-solving” of the previous paradigm, change the rules of the game and the “map” directing new research.

For example, Kuhn’s analysis of the Copernican Revolution emphasized that, in its beginning, it did not offer more accurate predictions of celestial events, such as planetary positions, than the Ptolemaic system, but instead appealed to some practitioners based on a promise of better, simpler, solutions that might be developed at some point in the future. Kuhn called the core concepts of an ascendant revolution its “paradigms” and thereby launched this word into widespread analogical use in the second half of the 20th century. Kuhn’s insistence that a paradigm shift was a mélange of sociology, enthusiasm and scientific promise, but not a logically determinate procedure, caused an uproar in reaction to his work. Kuhn addressed concerns in the 1969 postscript to the second edition. For some commentators, The Structure of Scientific Revolutions introduced a realistic humanism into the core of science, while for others the nobility of science was tarnished by Kuhn’s introduction of an irrational element into the heart of its greatest achievements.

In a sense that I am unable to explicate further, the proponents of competing paradigms practice their trades in different worlds.


The Structure of Scientific Revolutions was first published as a monograph in the International Encyclopedia of Unified Science, then as a book by University of Chicago Press in 1962. In 1969, Kuhn added a postscript to the book in which he replied to critical responses to the first edition. A 50th Anniversary Edition (with an introductory essay by Ian Hacking) was published by the University of Chicago Press in April 2012.

Kuhn dated the genesis of his book to 1947 when he was a graduate student at Harvard University and had been asked to teach a science class for humanities undergraduates with a focus on historical case studies. Kuhn later commented that until then, “I’d never read an old document in science.” Aristotle’s Physics was astonishingly unlike Isaac Newton’s work in its concepts of matter and motion. Kuhn wrote “… as I was reading him, Aristotle appeared not only ignorant of mechanics, but a dreadfully bad physical scientist as well. About motion, in particular, his writings seemed to me full of egregious errors, both of logic and of observation.” This was in an apparent contradiction with the fact that Aristotle was a brilliant mind. While perusing Aristotle’s Physics, Kuhn formed the view that in order to properly appreciate Aristotle’s reasoning, one must be aware of the scientific conventions of the time. Kuhn concluded that Aristotle’s concepts were not “bad Newton,” just different. This insight was the foundation of The Structure of Scientific Revolutions.

Prior to the publication of Kuhn’s book, a number of ideas regarding the process of scientific investigation and discovery had already been proposed. Ludwik Fleck developed the first system of the sociology of scientific knowledge in his book The Genesis and Development of a Scientific Fact (1935). He claimed that the exchange of ideas led to the establishment of a thought collective, which, when developed sufficiently, served to separate the field into esoteric (professional) and exoteric (laymen) circles. Kuhn wrote the foreword to the 1979 edition of Fleck’s book, noting that he read it in 1950 and was reassured that someone “saw in the history of science what I myself was finding there.”

Kuhn was not confident about how his book would be received. Harvard University had denied his tenure, a few years before. However, by the mid-1980s, his book had achieved blockbuster status.

One theory to which Kuhn replies directly is Karl Popper’s “falsificationism,” which stresses falsifiability as the most important criterion for distinguishing between that which is scientific and that which is unscientific. Kuhn also addresses verificationism, a philosophical movement that emerged in the 1920s among logical positivists. The verifiability principle claims that meaningful statements must be supported by empirical evidence or logical requirements.

Scientists work from models acquired through education and through subsequent exposure to the literature often without quite knowing or needing to know what characteristics have given these models the status of community paradigms.

Basic Approach

Kuhn’s approach to the history and philosophy of science focuses on conceptual issues like the practice of normal science, influence of historical events, emergence of scientific discoveries, nature of scientific revolutions and progress through scientific revolutions. What sorts of intellectual options and strategies were available to people during a given period? What types of lexicons and terminology were known and employed during certain epochs? Stressing the importance of not attributing traditional thought to earlier investigators, Kuhn’s book argues that the evolution of scientific theory does not emerge from the straightforward accumulation of facts, but rather from a set of changing intellectual circumstances and possibilities. Such an approach is largely commensurate with the general historical school of non-linear history.

Kuhn did not see scientific theory as proceeding linearly from an objective, unbiased accumulation of all available data, but rather as paradigm-driven. “The operations and measurements that a scientist undertakes in the laboratory are not ‘the given’ of experience but rather ‘the collected with difficulty.’ They are not what the scientist sees—at least not before his research is well advanced and his attention focused. Rather, they are concrete indices to the content of more elementary perceptions, and as such they are selected for the close scrutiny of normal research only because they promise opportunity for the fruitful elaboration of an accepted paradigm. Far more clearly than the immediate experience from which they in part derive, operations and measurements are paradigm-determined. Science does not deal in all possible laboratory manipulations. Instead, it selects those relevant to the juxtaposition of a paradigm with the immediate experience that that paradigm has partially determined. As a result, scientists with different paradigms engage in different concrete laboratory manipulations.”

History, if viewed as a repository for more than anecdote or chronology, could produce a decisive transformation in the image of science by which we are now possessed.

Historical Examples of Chemistry

Kuhn explains his ideas using examples taken from the history of science. For instance, eighteenth century scientists believed that homogenous solutions were chemical compounds. Therefore, a combination of water and alcohol was generally classified as a compound. Nowadays it is considered to be a solution, but there was no reason then to suspect that it was not a compound. Water and alcohol would not separate spontaneously, nor will they separate completely upon distillation (they form an azeotrope). Water and alcohol can be combined in any proportion.

Under this paradigm, scientists believed that chemical reactions (such as the combination of water and alcohol) did not necessarily occur in fixed proportion. This belief was ultimately overturned by Dalton’s atomic theory, which asserted that atoms can only combine in simple, whole-number ratios. Under this new paradigm, any reaction which did not occur in fixed proportion could not be a chemical process. This type world-view transition among the scientific community exemplifies Kuhn’s paradigm shift.

To my complete surprise, that exposure to out-of-date scientific theory and practice radically undermined some of my basic conceptions about the nature of science and the reasons for its special success.

Copernican Revolution

A famous example of a revolution in scientific thought is the Copernican Revolution. In Ptolemy’s school of thought, cycles and epicycles (with some additional concepts) were used for modeling the movements of the planets in a cosmos that had a stationary Earth at its center. As accuracy of celestial observations increased, complexity of the Ptolemaic cyclical and epicyclical mechanisms had to increase to maintain the calculated planetary positions close to the observed positions. Copernicus proposed a cosmology in which the Sun was at the center and the Earth was one of the planets revolving around it. For modeling the planetary motions, Copernicus used the tools he was familiar with, namely the cycles and epicycles of the Ptolemaic toolbox. Yet Copernicus’ model needed more cycles and epicycles than existed in the then-current Ptolemaic model, and due to a lack of accuracy in calculations, his model did not appear to provide more accurate predictions than the Ptolemy model. Copernicus’ contemporaries rejected his cosmology, and Kuhn asserts that they were quite right to do so: Copernicus’ cosmology lacked credibility.

Kuhn illustrates how a paradigm shift later became possible when Galileo Galilei introduced his new ideas concerning motion. Intuitively, when an object is set in motion, it soon comes to a halt. A well-made cart may travel a long distance before it stops, but unless something keeps pushing it, it will eventually stop moving. Aristotle had argued that this was presumably a fundamental property of nature: for the motion of an object to be sustained, it must continue to be pushed. Given the knowledge available at the time, this represented sensible, reasonable thinking.

Galileo put forward a bold alternative conjecture: suppose, he said, that we always observe objects coming to a halt simply because some friction is always occurring. Galileo had no equipment with which to objectively confirm his conjecture, but he suggested that without any friction to slow down an object in motion, its inherent tendency is to maintain its speed without the application of any additional force.

The Ptolemaic approach of using cycles and epicycles was becoming strained: there seemed to be no end to the mushrooming growth in complexity required to account for the observable phenomena. Johannes Kepler was the first person to abandon the tools of the Ptolemaic paradigm. He started to explore the possibility that the planet Mars might have an elliptical orbit rather than a circular one. Clearly, the angular velocity could not be constant, but it proved very difficult to find the formula describing the rate of change of the planet’s angular velocity. After many years of calculations, Kepler arrived at what we now know as the law of equal areas.

Galileo’s conjecture was merely that — a conjecture. So was Kepler’s cosmology. But each conjecture increased the credibility of the other, and together, they changed the prevailing perceptions of the scientific community. Later, Newton showed that Kepler’s three laws could all be derived from a single theory of motion and planetary motion. Newton solidified and unified the paradigm shift that Galileo and Kepler had initiated.

Only when they must choose between competing theories do scientists behave like philosophers.


One of the aims of science is to find models that will account for as many observations as possible within a coherent framework. Together, Galileo’s rethinking of the nature of motion and Keplerian cosmology represented a coherent framework that was capable of rivaling the Aristotelian/Ptolemaic framework.

Once a paradigm shift has taken place, the textbooks are rewritten. Often the history of science too is rewritten, being presented as an inevitable process leading up to the current, established framework of thought. There is a prevalent belief that all hitherto-unexplained phenomena will in due course be accounted for in terms of this established framework. Kuhn states that scientists spend most (if not all) of their careers in a process of puzzle-solving. Their puzzle-solving is pursued with great tenacity, because the previous successes of the established paradigm tend to generate great confidence that the approach being taken guarantees that a solution to the puzzle exists, even though it may be very hard to find. Kuhn calls this process normal science.

As a paradigm is stretched to its limits, anomalies — failures of the current paradigm to take into account observed phenomena — accumulate. Their significance is judged by the practitioners of the discipline. Some anomalies may be dismissed as errors in observation, others as merely requiring small adjustments to the current paradigm that will be clarified in due course. Some anomalies resolve themselves spontaneously, having increased the available depth of insight along the way. But no matter how great or numerous the anomalies that persist, Kuhn observes, the practicing scientists will not lose faith in the established paradigm until a credible alternative is available; to lose faith in the solvability of the problems would in effect mean ceasing to be a scientist.

In any community of scientists, Kuhn states, there are some individuals who are bolder than most. These scientists, judging that a crisis exists, embark on what Kuhn calls revolutionary science, exploring alternatives to long-held, obvious-seeming assumptions. Occasionally this generates a rival to the established framework of thought. The new candidate paradigm will appear to be accompanied by numerous anomalies, partly because it is still so new and incomplete. The majority of the scientific community will oppose any conceptual change, and, Kuhn emphasizes, so they should. To fulfill its potential, a scientific community needs to contain both individuals who are bold and individuals who are conservative. There are many examples in the history of science in which confidence in the established frame of thought was eventually vindicated. It is almost impossible to predict whether the anomalies in a candidate for a new paradigm will eventually be resolved. Those scientists who possess an exceptional ability to recognize a theory’s potential will be the first whose preference is likely to shift in favor of the challenging paradigm. There typically follows a period in which there are adherents of both paradigms. In time, if the challenging paradigm is solidified and unified, it will replace the old paradigm, and a paradigm shift will have occurred.

Suddenly the fragments in my head sorted themselves out in a new way, and fell into place together. My jaw dropped, for all at once Aristotle seemed a very good physicist indeed, but of a sort I’d never dreamed possible. Now I could understand why he had said what he’d said, and what his authority had been.


Kuhn explains the process of scientific change as the result of various phases of paradigm change.

Phase 1 – It exists only once and is the pre-paradigm phase, in which there is no consensus on any particular theory. This phase is characterized by several incompatible and incomplete theories. Consequently, most scientific inquiry takes the form of lengthy books, as there is no common body of facts that may be taken for granted. If the actors in the pre-paradigm community eventually gravitate to one of these conceptual frameworks and ultimately to a widespread consensus on the appropriate choice of methods, terminology and on the kinds of experiment that are likely to contribute to increased insights.

Phase 2 – Normal science begins, in which puzzles are solved within the context of the dominant paradigm. As long as there is consensus within the discipline, normal science continues. Over time, progress in normal science may reveal anomalies, facts that are difficult to explain within the context of the existing paradigm. While usually these anomalies are resolved, in some cases they may accumulate to the point where normal science becomes difficult and where weaknesses in the old paradigm are revealed.

Phase 3 – If the paradigm proves chronically unable to account for anomalies, the community enters a crisis period. Crises are often resolved within the context of normal science. However, after significant efforts of normal science within a paradigm fail, science may enter the next phase.

Phase 4 – Paradigm shift, or scientific revolution, is the phase in which the underlying assumptions of the field are reexamined and a new paradigm is established.

Phase 5- Post-Revolution, the new paradigm’s dominance is established and so scientists return to normal science, solving puzzles within the new paradigm.

A science may go through these cycles repeatedly, though Kuhn notes that it is a good thing for science that such shifts do not occur often or easily.

Philosophers of science have repeatedly demonstrated that more than one theoretical construction can always be placed upon a given collection of data. History of science indicates that, particularly in the early developmental stages of a new paradigm, it is not even very difficult to invent such alternates.


According to Kuhn, the scientific paradigms preceding and succeeding a paradigm shift are so different that their theories are incommensurable — the new paradigm cannot be proven or disproven by the rules of the old paradigm, and vice versa. (A later interpretation by Kuhn of ‘commensurable’ versus ‘incommensurable’ was as a distinction between languages, namely, that statements in commensurable languages were translatable fully from one to the other, while in incommensurable languages, strict translation is not possible.) The paradigm shift does not merely involve the revision or transformation of an individual theory, it changes the way terminology is defined, how the scientists in that field view their subject, and, perhaps most significantly, what questions are regarded as valid, and what rules are used to determine the truth of a particular theory. The new theories were not, as the scientists had previously thought, just extensions of old theories, but were instead completely new world views. Such incommensurability exists not just before and after a paradigm shift, but in the periods in between conflicting paradigms. It is simply not possible, according to Kuhn, to construct an impartial language that can be used to perform a neutral comparison between conflicting paradigms, because the very terms used are integral to the respective paradigms, and therefore have different connotations in each paradigm. The advocates of mutually exclusive paradigms are in a difficult position: “Though each may hope to convert the other to his way of seeing science and its problems, neither may hope to prove his case. The competition between paradigms is not the sort of battle that can be resolved by proofs.” Scientists subscribing to different paradigms end up talking past one another.

Kuhn states that the probabilistic tools used by verificationists are inherently inadequate for the task of deciding between conflicting theories, since they belong to the very paradigms they seek to compare. Similarly, observations that are intended to falsify a statement will fall under one of the paradigms they are supposed to help compare, and will therefore also be inadequate for the task. According to Kuhn, the concept of falsifiability is unhelpful for understanding why and how science has developed as it has. In the practice of science, scientists will only consider the possibility that a theory has been falsified if an alternative theory is available that they judge credible. If there is not, scientists will continue to adhere to the established conceptual framework. If a paradigm shift has occurred, the textbooks will be rewritten to state that the previous theory has been falsified.

Kuhn further developed his ideas regarding incommensurability in the 1980s and 1990s. In his unpublished manuscript The Plurality of Worlds, Kuhn introduces the theory of kind concepts: sets of interrelated concepts that are characteristic of a time period in a science and differ in structure from the modern analogous kind concepts. These different structures imply different “taxonomies” of things and processes, and this difference in taxonomies constitutes incommensurability. This theory is strongly naturalistic and draws on developmental psychology to “found a quasi-transcendental theory of experience and of reality.”

Scientific revolutions are inaugurated by a growing sense… that an existing paradigm has ceased to function adequately in the exploration of an aspect of nature to which that paradigm itself had previously led the way.


Kuhn introduced the concept of an exemplar in a postscript to the second edition of The Structure of Scientific Revolutions (1970). He noted that he was substituting the term ‘exemplars’ for ‘paradigm’, meaning the problems and solutions that students of a subject learn from the beginning of their education. For example, physicists might have as exemplars the inclined plane, Kepler’s laws of planetary motion, or instruments like the calorimeter.

According to Kuhn, scientific practice alternates between periods of normal science and revolutionary science. During periods of normalcy, scientists tend to subscribe to a large body of interconnecting knowledge, methods, and assumptions which make up the reigning paradigm. Normal science presents a series of problems that are solved as scientists explore their field. The solutions to some of these problems become well known and are the exemplars of the field.

Those who study a scientific discipline are expected to know its exemplars. There is no fixed set of exemplars, but for a physicist today it would probably include the harmonic oscillator from mechanics and the hydrogen atom from quantum mechanics.

By now it may be clear that the position I’m developing is a sort of post-Darwinian Kantianism.

Kuhn on Scientific Progress

The first edition of The Structure of Scientific Revolutions ended with a chapter titled “Progress through Revolutions”, in which Kuhn spelled out his views on the nature of scientific progress. Since he considered problem solving to be a central element of science, Kuhn saw that for a new candidate paradigm to be accepted by a scientific community, “First, the new candidate must seem to resolve some outstanding and generally recognized problem that can be met in no other way. Second, the new paradigm must promise to preserve a relatively large part of the concrete problem solving activity that has accrued to science through its predecessors.

While the new paradigm is rarely as expansive as the old paradigm in its initial stages, it must nevertheless have significant promise for future problem-solving. As a result, though new paradigms seldom or never possess all the capabilities of their predecessors, they usually preserve a great deal of the most concrete parts of past achievement and they always permit additional concrete problem-solutions besides.

In the second edition, Kuhn added a postscript in which he elaborated his ideas on the nature of scientific progress. He described a thought experiment involving an observer who has the opportunity to inspect an assortment of theories, each corresponding to a single stage in a succession of theories. What if the observer is presented with these theories without any explicit indication of their chronological order? Kuhn anticipates that it will be possible to reconstruct their chronology on the basis of the theories’ scope and content, because the more recent a theory is, the better it will be as an instrument for solving the kinds of puzzle that scientists aim to solve. Kuhn remarked: “That is not a relativist’s position, and it displays the sense in which I am a convinced believer in scientific progress.”

The man who is striving to solve a problem defined by existing knowledge and technique is not, however, just looking around. He knows what he wants to achieve, and he designs his instruments and directs his thoughts accordingly. Unanticipated novelty, the new discovery, can emerge only to the extent that his anticipations about nature and his instruments prove wrong… There is no other effective way in which discoveries might be generated.

If these out-of date beliefs are to be called myths, then myths can be produced by the same sorts of methods and held for the same sorts of reasons that now lead to scientific knowledge.

See Also: Science as Mythology

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