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The Structure of Scientific Revolutions (Thomas Kuhn, 1962) is an analysis of the history of science. Its publication was a landmark event in the sociology of knowledge, and popularized the terms paradigm and paradigm shift.

Cover of 3rd edition, paperback

The book was initially published as a monograph in the International Encyclopedia of Unified Science, then as a book by University of Chicago Press in 1962 (ISBN 0-226-45808-3). (All page numbers below refer to the third edition of the text, 1996). In 1969, Kuhn added a postscript to the book in which he replied to critical responses to the first edition of the book.

Kuhn traced the origin of the 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 the focus being historical case studies. Kuhn later said 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 concluded that Aristotle's concepts were not "bad Newton" but, rather, simply different.


Basic approach

Kuhn's approach to the history and philosophy of science has been labeled as one focused on "conceptual" issues: what sorts of ideas were "thinkable" at a given period in time, what sorts of intellectual options and strategies were available to people in a time, and emphasizing the need to avoid imposing modern conceptions onto historical actors. Taking this stance, Kuhn's book as a whole argues that theory change in science is not a simple accumulation of facts, but rather a set of changing intellectual circumstances and possibilities.

Historical examples

In the book, Kuhn explains his ideas by discussing examples from the history of science.

At some stage in the history of chemistry, some chemists began to explore the idea of atomism. Generally, when substances are heated they fall apart in their constituent elements, and often, but by no means always, the elements would be found to only combine in certain proportions. At the time, a mixture of water and alcohol was generally classified as a compound. Nowadays it is thought to be a mixture, but at the time there was no reason to suspect it was not a compound. Water and alcohol would not separate spontaneously, but they could be separated when heated. Water and alcohol can be combined in any proportion.

Now if a chemist is inclined to go with atomic theory, then all the instances of compounds with their elements in fixed proportion would be viewed as compounds that exhibit normal behavior, and all the known exceptions to that normal behavior would be viewed as anomalies, that presumably will be explained in due course. On the other hand, if a chemist is inclined to feel that theories of atomicity of matter are a dead end, then all the instances of compounds with their elements in fixed proportion would be viewed as compounds that exhibit anomalous behavior, that hopefully will be explained in due course, and all the compounds that can have their elements mix in any ratio would be seen as the normal behavior of compounds.

We now believe that the atomists were on the right track. But if you restrict yourself to thinking about chemistry using only the knowledge available at the time, you find that at the time either point of view was quite defensible.

The Copernican Revolution

Arguably the most famous example of a revolution in science was the Copernican Revolution. The tools of the school of thought of Ptolemy were to use cycles and epicycles (and some other means) to model the movements of the planets in a cosmos with a stationary Earth at its center. Given the knowledge at the time, this was the best approach possible. As observational accuracy increased, the complexity of the mechanisms of cycles and epicycles and other means had to be increased to keep the calculated planetary positions close to observed positions. Copernicus proposed a cosmology with the Sun at the center and the Earth as one of the planets revolving around the Sun. For modeling the planetary motions, Copernicus used the tools he was familiar with, the cycles and epicycles etc, of the Ptolemeic toolbox. Copernicus' model needed more cycles and epicycles than the Ptolemeic model current during that time. Copernicus' contemporaries rejected his cosmology, and Kuhn asserts that they were quite right in doing so. Copernicus' cosmology had no credibility.

Thomas Kuhn illustrates how later a paradigm shift could occur by describing the new ideas that Galileo Galilei introduced into thinking about motion. Intuitively, it seems that always when an object is set in motion, it soon comes to a halt. A well made cart will come a long way before coming to a halt, but unless you keep pushing, it will come to halt. Presumably, Aristotle argued, that is a fundamental property of nature: in order to sustain motion, you need to keep pushing. And given the knowledge at the time, that was good, reasonable thinking. Galilei suggested a bold conjecture: suppose he said, we always see objects come to a halt just because there's always some friction at play. Galilei had no equipment to seek objective confirmation of his conjecture, but he suggested that the actual nature of motion is that without friction to slow an object down, the object will sustain its speed, without any force.

The ptolemeic approach of using cycles and epicycles was getting strained. There seemed to be no end to the growth in complexity. Johannes Kepler was the first to departure from the tools of the ptolemeic paradigm. Kepler started to explore an elliptic orbit for the planet Mars, rather than circular. Clearly, the angular velocity could not be constant, but it was terribly difficult to find a formula for the rate of change of the angular velocity of the planet. After many years of non-stop calculations reaching dead end after dead end, Kepler discovered the law of equal areas.

Galilei's conjecture was just that, a conjecture. So was Kepler's cosmology. But each added credibility to the other. Together, the two conjectures swung the hearts of the scientific community. Later, Newton showed that the three laws of Kepler could all three be derived from a single theory of motion and planetary motion. Newton solidified and unified the paradigm shift that Galilei and Kepler had started.


The aim of science is to find a model that will account for as much of the observations as possible in a coherent framework. Galilei's rethinking of the nature of motion and Keplerian cosmology together constituted a coherent framework that could rival the Aristotelian/Ptolemeic framework.

Once a paradigm shift has taken place, the schoolbooks are rewritten, often rewriting the history of science presenting history as inevitably building up to the established frame of thought. There is belief that in due course all phenomena will be accounted for in terms of the established framework. Kuhn points out that this is what scientists spend most if not all of their careers doing, a process of puzzle solving. The puzzle-solving is pursued with great tenacity, for the previous successes of the established paradigm instill great confidence that the approach that is taken guarantees that a solution to the puzzle exists, if very hard to find. Kuhn calls this process Normal science.

As a paradigm is explored to the limits of its scope, anomalies — failures of the current paradigm to take into account observed phenomena — accumulate. Their significance is judged by the practitioners of the discipline. Some may be dismissed as errors in observation, others as only requiring small adjustments to the current paradigm, to be elucidated in due course. Sometimes anomalies "dissolve" spontaneously, with deepening insight. But no matter how many or how large the anomalies that persist, Kuhn observes, the practicing scientists will not lose faith in the established paradigm, as long as no credible alternative is available. To lose faith that the problems are solvable would be to cease being a scientist.

In any community of scientists, Kuhn describes, there are individuals that are more bold than most. These scientists, judging a crisis is on, embark on what Thomas Kuhn calls revolutionary science, exploring alternatives to long held, obvious-seeming assumptions. Occasionally this results in a candidate for a challenge to the established frame of thought. The new candidate will appear to come with a lot of anomalies, simply because it is so new and incomplete. The majority of the scientific community will oppose any change of mind, and, emphasizes Kuhn, they should. In order to fulfill its potential, a scientific community must consist of both people who are bold and people who are conservative. There are many examples in the history of science where confidence in the established frame of thought was eventually vindicated. Whether the anomalies of the candidate for a new paradigm will be solvable is almost impossible to predict. The scientists with exceptional ability to recognize a theory's potential will be the first whose preference may shift to the challenging paradigm. A period follows in which there are adherents to both paradigms. In time, if solidification and unification of the challenging paradigm is achieved, it will replace the old, and a paradigm shift has occurred.

Three phases

Chronologically, Kuhn distinguishes three phases. The first phase, which is undergone only once, is the pre-scientific phase, in which there is no consensus on any theory. This phase is characterized by several incompatible and incomplete theories. If this pre-scientific community eventually gravitates to one of these frames of thought, leading to wide-spread consensus on choice of methods, terminology, recognition of what kind of experiment is likely to contribute to sharpening the insights, then the second phase, normal science begins. From time to time, a science may go through a phase of revolutionary science.

Transition period

The transition period between paradigms is neither quick nor calm. Sometimes, as Max Planck observed, and Kuhn quoted (SSR, p. 151):

"a new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it."

According to Kuhn, the scientific paradigms before and after a paradigm shift are so different that their theories are incomparable. The paradigm shift does not just change a single theory, it changes the way that words are defined, the way that the scientists look at their subject and, perhaps most importantly, the questions that are considered valid and the rules used to determine the truth of a particular theory. Kuhn observes that they are incommensurable — literally, lacking comparison, untranslatable. New theories were not, as they had thought of before, simply extensions of old theories, but radically new worldviews. This incommensurability applies not just before and after a paradigm shift, but 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 belong within the paradigm and are therefore different in different paradigms. Advocates of mutually exclusive paradigms are in an insidious 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 proof." (SSR, p. 148).

Kuhn (SSR, section XII) points out that the probabilistic tools used by verificationists are in themselves inadequate to the task of deciding between conflicting theories, since they are a component of the very paradigms they seek to compare. Similarly, observations intended to falsify a statement will be part of one of the paradigms they seek to compare, and so inadequate to the task. According to Kuhn, the concept of falsifiability does not help in understanding why and how science has developed the way it did. In the actual practice of science, scientists will only consider the possibility that a theory is falsified if an alternative that they judge as credible is available. If there isn't, the scientist will trust the established frame of thought. If a paradigm shift has taken place, the schoolbooks are rewritten, stating that the previous theory is falsified.

Kuhn's opinion on scientific progress

In the postscript in the 3rd edition, in section 6, Kuhn writes about his opinion on the matter of scientific progress. He describes the thought experiment of an observer, who gets to inspect a collection of theories that have been stages in a succession of theories. What if the observer is presented with these theories without explicit indications of their chronological order? Kuhn expects that it will be possible to reconstruct the original chronology on the basis of the content and scope of the theories, because the more recent theories will be better instruments for solving the kind of puzzles that scientists aim to solve. Kuhn writes: That is not a relativist's position, and it displays the sense in which I am a convinced believer in scientific progress.

Relevance of SSR

SSR is interpreted by postmodern and post-structuralist thinkers as having undermined the enterprise of science by showing that scientific knowledge is dependent on the culture of groups of scientists rather than on adherence to a specific, definable method. In this regard, Kuhn is considered a precursor to the more radical thinking of Paul Feyerabend. Kuhn's work has also been interpreted as blurring the demarcation between scientific and non-scientific enterprises because it describes scientific progress without reference to an idealized scientific method that can be used to distinguish science from non-science. In the years after the publication of The Structure of Scientific Revolutions, debate raged with adherents of Popper's falsificationism such as Imre Lakatos.

On the one hand, logical positivists and many scientists criticize Kuhn's "humanizing" of the scientific process going too far, while the postmodernists in line with Feyerabend have criticized Kuhn for not going far enough. SSR was also embraced by those wishing to discredit or attack the authority of science, such as creationists and radical environmentalists, and the changing national attitudes about science which occurred at the same time of the book's publication (Rachel Carson's Silent Spring was released in the same year), and modern scholars have wondered whether Kuhn himself would have made more explicit that he meant not to create a tool with which to undermine science had he seen what was coming down the pipe.

Outside of the history and philosophy of science, the book's basic tenets have been adopted and co-opted by a variety of fields and disciplines.

Changes in politics, society, and business are often expressed in Kuhnian terms, however poor their analog to science may seem to scientists and historians. The terms paradigm and "paradigm shift" have become such notorious buzzwords that in many circles they are considered hollow and empty, and rarely have any strong connection to Kuhn's original text.

In 1987, Kuhn's work was reported as the most heavily cited book of the 20th century, and the Times Literary Supplement labeled it as one of "The Hundred Most Influential Books Since the Second World War."

Criticisms of Kuhn and SSR

C.R. Kordig, in a series of texts published in the early 1970's, asserted a position somewhere between that of Kuhn and the older philosophy of science. The crucial point of Kordig's analysis centered around the existence of observational invariance and his criticism of the Kuhnian position was that the incommensurability thesis was too radical, rendering it impossible to explain the confrontation of scientific theories which actually takes place. According to Kordig, it is possible to admit the existence of revolutions and paradigm shifts in science while still recognizing that theories which belong to different paradigms can be compared and confronted on the plane of observation. Those who accept the incommensurability thesis do not do so because they admit the discontinuity of paradigms, but because they attribute as an effect of such shifts a radical change in meanings.

Kordig maintains, in the first place, that there is a common observational plane. Kepler and Tycho Brahe, for example, when trying to explain the relative variation of the distance of the sun from the horizon at sunrise, both see the same thing (the same configuration is designed on the retina of each individual). This is but one example of the fact that "rival scientific theories share some observations, and therefore some meanings." Kordig suggests that, with this approach, he is not reintroduing the distinction between observations and theory, where the former is assigned a privileged and neutral status, but that one can affirm more simply that, even if there is no sharp distinction between theory and observations, this does not imply that at the two extremes of this polarity there are no comprehensible differences.

On a second level, there is, for Kordig, a common plane of inter-paradigmatic standards or shared norms which permit the effective confrontation of rival theories.

In 1973, Hartry Field published an article which also sharply criticized Kuhn's idea of incommensurability. In particular, he took issue with this passage from Kuhn:

"Newtonian mass is immutably conserved; that of Einstein is convertible into energy. Only at very low relative velocities can the two masses be measured in the same way, and even then they must not be conceived as if they were the same thing." (Kuhn 1970).

Field takes this idea of incommensurbality between the same terms in different theories one step further, transforming the entire nature of the discussion. Instead of attempting to identify a persistence of the reference of terms in different theories, Field's analysis results in the recognition of the indeterminacy of reference even within single theories. Field takes up the example of the term "mass" and asks what exactly does "mass" mean in modern post-relativistic physics. He finds that there are at least two different definitions:

1) Relativistic mass: the mass of a particle is equal to the total energy of the particle divided by the speed of light squared. Since the total energy of a particle in relation to one system of reference differs from the total energy in relation to other systems of reference, while the speed of light remains constant in all systems, it follows that the mass of a particle has different values in different systems of reference.

2) "Real" mass: the mass of a particle is equal to the non-kinetic energy of a particle divided by the speed of light squared. Since non-kinetic energy is the same in all systems of reference, and the same is true of light, it follows that the mass of a particle has the same value in all systems of reference.

Now projecting this distinction backward in time onto Newtonian dynamics, we can formulate the following two hypotheses:

HR: the term "mass" in Newton denotes relativistic mass.
Hp: the term "mass" in Newton denotes "real" mass.

And, according to Field, it is impossible to decide which of these two affermations is true. Before the discovery of the theory of relativity, the term "mass" was referentially indeterminate. But this doesn't mean that the term mass did not have a different meaning than that which it now has. The problem is not with meaning but with reference. The reference of such terms as mass is only partially determined: we don't really know how Newton intended his use of this term. On this basis, we can reformulate the two hypotheses above:

HR*: the term "mass" in Newton partially denotes the relativistic mass.
HP*: the term "mass" in Newton partially denotes the "real" mass.

As a consequence, neither of the two terms fully denotes (refers). The conseqeunce of this result is that it is improper to maintain that a term has changed its reference during the course of a scientific revolution. It is more appropiate to describe the process of development of such terms as "mass" as "having undergone a denotional refinement" during the course of a scientific revolution.

In his 1970, Steven Toulmin argued that a more realistic picture of science that that presented in SSR would admit the fact that revisions in science take place much more frequently and are much less dramatic than can be explained by the revolution/normal science model. Such revisions occur, in Toulmin's view, quite often during periods of what Kuhn would call "normal science. In order for Kuhn to explain such revisions in terms of the non-paradigmatic puzzle-solutions of normal science, he would need to delineate a, perhaps implausibly, sharp distintion between paradigmatic and non-paradigmatic science.

The tight relation between the interpretationalist hypothesis and a holistic conception of beliefs is at the base of the idea of the dependence of perception on theory, a central concept in Kuhn's Structure of Scientific Revolutions. Kuhn (1962), Norwood Hanson (1958) and Nelson Goodman (1968) have all maintained that the perception of the world depends on how the percipient conceives the world: two individuals (two scientists) who witness the same phenomenon and are steeped in two radically different theories will see two radically different things. It is our interpretation of the world, in this view, which determines that which we see.

Jerry Fodor attempts to establish that this theoretical paradigm is fallacious and misleading by demonstrating the impenetrability of perception to the background knowledge of subjects. The strongest case can be based on the evidence from experimental cognitive psychology itself: the persistence of perceptual illusions. Just knowing that the two horizontal lines in the Muller-Lyer illusion are equal does not prevent one from continuing to see them as one being longer than the other. It is this impenetrability of the information elaborated by the mental modules (informationally encapsulated) which limits the extent of interpretationalism.

In epistemology, for example, the criticism of, what Fodor calls, the interpretationalist hypothesis accounts for the common sense intuition (at the base of naïve physics) of the independence of reality from the conceptual categories of the epistemic subject. If the processes of elaboration of the mental modules are independent of the background theories, in fact, then it is possible to maintain the realist view that two scientists who embrace two radically diverse theories see the world exactly in the same manner even if they interpret it differently. The point is that is necessary to distinguish between observations and the perceptual fixation of beliefs. While it is beyond doubt that the second process involves the holistic relation between beliefs, the first is largely independent of the background beliefs of individuals.

Other critics, such as I. Sheffler, Hilary Putnam and Saul Kripke, have focused on the Fregean distinction between sense and reference in order to defend a position of scientific realism. Sheffler contends that Kuhn confuses the meanings of terms such as "mass" with their reference. While their meanings may very well differ, their reference (the object or entity to which they correspond in the external world) remains fixed. Putnam and Kripke developed a causal theory of reference which does away with the idea of meaning altogether. These ideas of reference anchor theories to the external world and thus make it possible to measure their progress toward the truth about the external world, contrary to the view of Kuhn.

See also

External links

fr:Structure des révolutions scientifiques {

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