The Structure of Scientific Revolutions

The Structure of Scientific Revolutions by Thomas S. Kuhn, is an analysis of the history of science, published in 1962. Its publication was a landmark event in the history, philosophy, and sociology of scientific knowledge and it triggered an ongoing worldwide assessment and reaction in—and beyond—those scholarly communities. In this work, Kuhn challenged the then prevailing view of progress in ‘normal science’ (the routine work of scientists experimenting within a paradigm).

Scientific progress had been seen primarily 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. During revolutions in science the discovery of anomalies leads to a whole new paradigm that changes the rules of the game and the ‘map’ directing new research, asks new questions of old data, and moves beyond the puzzle-solving of normal science.

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. Kuhn addressed concerns in the 1969 postscript to the second edition. For some commentators it 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.

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 concluded that Aristotle’s concepts were not ‘bad Newton,’ just different. Kuhn’s approach to the history and philosophy of science has been described as focusing on conceptual issues: what sorts of ideas were thinkable at a particular time? 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 modern modes of thought to historical actors, 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 explains his ideas using examples taken from the history of science. For instance, at a particular stage in the history of chemistry, some chemists began to explore the idea of atomism. When many substances are heated they have a tendency to decompose into their constituent elements, and often (though not invariably) these elements can be observed to combine only in set proportions. At one time, 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, but they could be separated when heated. Water and alcohol can be combined in any proportion.

A chemist favoring atomic theory would have viewed all compounds whose elements combine in fixed proportions as exhibiting normal behavior, and all known exceptions to this pattern would be regarded as anomalies whose behavior would probably be explained at some time in the future. On the other hand, if a chemist believed that theories of the atomicity of matter were erroneous, then all compounds whose elements combined in fixed proportions would be regarded as anomalies whose behavior would probably be explained at some time in the future, and all those compounds whose elements are capable of combining in any ratio would be seen as exhibiting the normal behavior of compounds. Nowadays the consensus is that the atomists’ view was correct. But if one were to restrict oneself to thinking about chemistry using only the knowledge available at the time, either point of view would be defensible.

What is arguably the most 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. But Copernicus’ model needed more cycles and epicycles than existed in the then-current Ptolemaic model, and due to a lack of accuracy in calculations, Copernicus’s 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.

Thomas 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.

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 for as long as no credible alternative is available; to lose faith in the solubility 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 Thomas 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. Whether the anomalies of a candidate for a new paradigm will be resolvable is almost impossible to predict. 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.

Chronologically, Kuhn distinguishes between three phases. The first phase, which exists only once, is the pre-paradigm phase, in which there is no consensus on any particular theory, though the research being carried out can be considered scientific in nature. This phase is characterized by several incompatible and incomplete theories. 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, then the second phase, 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. Kuhn refers to this as a crisis. 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 third phase, that of revolutionary science, in which the underlying assumptions of the field are reexamined and a new paradigm is established. After the new paradigm’s dominance is established, 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.

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. 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.

The first edition of SSR ended with a chapter entitled ‘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 for 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.’ And overall Kuhn maintained that the new paradigm must also solve more problems than its predecessor, which therefore entailed that the number of newly solved problems must be greater than those solved in the old paradigm.

In the second edition of SSR, 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.’

SSR is viewed by postmodern and post-structuralist thinkers as having called into question the enterprise of science by demonstrating that scientific knowledge is dependent on the culture and historical circumstances of groups of scientists rather than on their adherence to a specific, definable method. In this regard, Kuhn is considered a precursor to the more radical thinking of Austrian-American philosopher of science Paul Feyerabend. Kuhn’s work has also been regarded as blurring the demarcation between scientific and non-scientific enterprises, because it describes the mechanism of scientific progress without invoking any idealized scientific method that is capable of distinguishing science from non-science. In the years following the publication of SSR, debate raged with adherents of Karl Popper’s doctrine of falsificationism, such as Imre Lakatos.

On the one hand, logical positivists and many scientists have criticized Kuhn’s ‘humanizing’ of the scientific process for going too far, while the postmodernists, together with Feyerabend, have criticized Kuhn for not going far enough. SSR has also been embraced by creationists who see creationism as an incommensurate worldview in contrast to naturalism while holding science as a valuable tool. It was also in tune with a national change in attitudes towards science in the United States at the time of the book’s publication, influenced by the Cold War confrontation with the Soviet Union, beginning with the launching of the space satellite Sputnik in 1957. (Rachel Carson’s ‘Silent Spring’ was also published in 1962). The changes that occur in politics, society and business are often expressed in Kuhnian terms, however poor their parallel with the practice of science may seem to scientists and historians of science. The terms ‘paradigm’ and ‘paradigm shift’ have become such notorious clichés and buzzwords that they are viewed as effectively devoid of content. Misused and overused to the point of becoming meaningless, their use in these contexts rarely has any firm foundation in Kuhn’s original definitions.

Kuhn’s SSR was soon criticized by his colleagues in the history and philosophy of science. In 1965, a special symposium on SSR was held at an International Colloquium on the Philosophy of Science that took place at Bedford College, London, and was chaired by Karl Popper. The symposium led to the publication of the symposium’s presentations plus other essays, most of them critical, which eventually appeared in an influential volume of essays that by 1999 had gone through 21 printings. Kuhn expressed the opinion that his critics’ readings of his book were so inconsistent with his own understanding of it that he was ‘…tempted to posit the existence of two Thomas Kuhns,’ one the author of his book, the other the individual who had been criticized in the symposium by ‘Professors Popper, Feyerabend, Lakatos, Toulmin, and Watkins.’

In his 1972 work, ‘Human Understanding,’ British philosopher Stephen Toulmin argued that a more realistic picture of science than 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 model of revolution/normal science. In Toulmin’s view, such revisions occur quite often during periods of what Kuhn would call ‘normal science.’ For Kuhn to explain such revisions in terms of the non-paradigmatic puzzle solutions of normal science, he would need to delineate what is perhaps an implausibly sharp distinction between paradigmatic and non-paradigmatic science.

In a series of texts published in the early 1970s, philosopher C.R. Kordig asserted a position somewhere between that of Kuhn and the older philosophy of science. His criticism of the Kuhnian position was that the incommensurability thesis was too radical, and that this made it impossible to explain the confrontation of scientific theories that actually occurs. According to Kordig, it is in fact possible to admit the existence of revolutions and paradigm shifts in science while still recognizing that theories belonging 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 a radical change in meanings to such shifts.

Kordig maintains that there is a common observational plane. For example, when Kepler and Tycho Brahe are 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 focused on the retina of each individual). This is just 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 reintroducing the distinction between observations and theory in which the former is assigned a privileged and neutral status, but that it is possible to affirm more simply the fact that, even if no sharp distinction exists between theory and observations, this does not imply that there are no comprehensible differences at the two extremes of this polarity. At a secondary level, for Kordig there is a common plane of inter-paradigmatic standards or shared norms that permit the effective confrontation of rival theories.

In 1973, American philosopher Hartry Field published an article that 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.’ Field takes this idea of incommensurability between the same terms in different theories one step further. Instead of attempting to identify a persistence of the reference of terms in different theories, Field’s analysis emphasizes the indeterminacy of reference within individual theories.

Field takes the example of the term ‘mass,’ and asks what exactly ‘mass’ means 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. And 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.

Projecting this distinction backwards in time onto Newtonian dynamics, we can formulate the following two hypotheses: A) the term ‘mass’ in Newtonian theory denotes relativistic mass; Or B) the term ‘mass’ in Newtonian theory denotes ‘real’ mass. According to Field, it is impossible to decide which of these two affirmations is true. Prior to the theory of relativity, the term ‘mass’ was referentially indeterminate. But this does not mean that the term ‘mass’ did not have a different meaning than it now has. The problem is not one of meaning but of 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 to be applied. As a consequence, neither of the two terms fully denotes (refers). It follows that it is improper to maintain that a term has changed its reference during a scientific revolution; it is more appropriate to describe terms such as ‘mass’ as ‘having undergone a denotional refinement.’

The close connection between the interpretationalist hypothesis and a holistic conception of beliefs is at the root of the notion of the dependence of perception on theory, a central concept in SSR. Kuhn maintained that the perception of the world depends on how the percipient conceives the world: two scientists who witness the same phenomenon and are steeped in two radically different theories will see two different things. According to this view, our interpretation of the world determines what 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 evidence from experimental cognitive psychology, namely the persistence of perceptual illusions. Knowing that the lines in the Müller-Lyer optical illusion are equal does not prevent one from continuing to see one line as being longer than the other. This impenetrability of the information elaborated by the mental modules limits the scope of interpretationalism.

In epistemology, for example, the criticism of what Fodor calls the interpretationalist hypothesis accounts for the common-sense intuition (on which naïve physics [folk physics] is based) of the independence of reality from the conceptual categories of the experimenter. If the processes of elaboration of the mental modules are in fact independent of the background theories, 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 it is necessary to distinguish between observations and the perceptual fixation of beliefs. While it is beyond doubt that the second process involves the holistic relationship between beliefs, the first is largely independent of the background beliefs of individuals.

Other critics, such as Israel Sheffler, Hilary Putnam and Saul Kripke, have focused on the Fregean distinction between sense and reference in order to defend scientific realism. Sheffler contends that Kuhn confuses the meanings of terms such as ‘mass’ with their references. While their meanings may very well differ, their references (the objects or entities to which they correspond in the external world) remain fixed.

More recently, criticism from a different direction has been developed by Arun Bala in his study ‘The Dialogue of Civilizations in the Birth of Modern Science’ (2006). He charges that The Structure of Scientific Revolutions is itself a profoundly Eurocentric work, although it is often perceived as opening the door to the multicultural turn in historical studies of science. Bala charges that Kuhn ignores the significant impact of Arabic and Chinese science when he writes: ‘Every civilization of which we have records has possessed a technology, an art, a religion, a political system, laws and so on. In many cases those facets of civilizations have been as developed as our own. But only the civilizations that descend from Hellenic Greece have possessed more than the most rudimentary science. The bulk of scientific knowledge is a product of Europe in the last four centuries. No other place and time has supported the very special communities from which scientific productivity comes.’ Bala argues that it is precisely Kuhn’s postmodern epistemological paradigm that obstructs recognition of non-Western influences on modern science. Bala argues that this leads Kuhn to treat different cultural scientific traditions as separate intellectual universes isolated from each other. Instead, Bala argues, we would have a different multicultural picture of science by including the contributions from Arabic, Chinese, ancient Egyptian and Indian traditions of philosophy, mathematics, astronomy and physics that went into shaping the birth of modern science.


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