Re-Assessing Kuhn’s The Structure of Scientific Revolutions

Victor Suchar

BRLSI Member

7 September 2004


Is Natural Science cumulative and progressive as traditionally conceived? In the Structure of Scientific Revolutions, Thomas Kuhn (1922-1996) argued that the history of science is not gradual and cumulative, but punctuated by a series of radical paradigm shifts. The book was originally published in 1962 as part of the International Encyclopedia of Unified Science edited by the leading positivist philosopher Rudolf Carnap (Suchar 1999), and revised in 1969 to a 2nd edition, which included a Post- Script intended as a response to critics. Indeed, Structure stimulated considerable debate immediately after its first publication, and became subject of the now famous symposium of 13 July 1965 organised by Imre Lakatos and chaired by Karl Popper (Suchar 2000), that included the leading philosophers of science. The papers presented at the symposium were published in 1970 as Criticism and the Growth of Knowledge (Lakatos & Musgrave, 1979), now a seminal book. This was the period when the Received View came under strong attack and when Kuhn became a leading contender in the debate. The Received View (Suchar 2003 Part I), was basically the synthesis effected by the logical positivists in the late 1920s between the analytical methods of Russell and Wittgenstein and the empiricist epistemology of Mach, and it considered philosophy as the logical arm of natural sciences.

Structure is now a landmark in the philosophy and history of science - in the last 40 years it has sold more than one million copies and it was translated in 20 languages. The literature, including commentary on this work, is by itself extensive, and it is for this reason that in the next part of this talk I will go to the primary source to give an account of Kuhn's ideas highlighting the essence of his arguments, then giving an assessment of these ideas.

Kuhn’s Ideas (Kuhn, 1969)

Kuhn's principal categories. One can say that Kuhn's Structure is built on three fundamental notions: Normal Science, Paradigm, Incomensurability. The first two are of an historical and social character, the last has considerable philosophical implications. This is why Kuhn's work straddles the history, sociology and the philosophy of science - he is in effect the father of sociology of science and a writer of philosophical history.

Normal Science and Paradigm. In Structure, Normal Science means research firmly based upon one or more past scientific achievements, achievements that some particular scientific community acknowledges for a time as supplying the foundation for its further practice. Today such achievements are recounted, though seldom in their original form, by science textbooks, elementary and advanced. These textbooks expound the body of accepted theory, illustrate many of its successful applications, and compare these applications with observations and experiments. Before such books became popular in the early 19th century, many of the famous classics fulfilled a similar function: Aristotle's Physica, Ptolomey's Almagest, Newton's Principia and Optiks, Franklin's Electricity, Lavoiser's Chemistry and Lyell's Geology. They were able to do so because they shared two essential characteristics. Their achievement was sufficiently unprecedented to attract an enduring group of adherents away from competing modes of activity. Simultaneously, it was sufficiently open ended to leave all sorts of problems for the redefined group of practitioners to resolve. Achievements that have these two characteristics are referred by Kuhn as ‘Paradigms’, a term which relates closely to ‘Normal Science’ Incidentally, the origin of the word is Greek, it meant ‘standard’ and it was used by Plato in his Dialogues (standard of piety for example). By choosing it, Kuhn meant to suggest that some accepted examples of scientific research - examples which include law, theory application and instrumentation together - provide models from which spring particular coherent traditions of scientific research, such as Ptolomaic Astronomy, Copernician Astronomy, Aristotelian Dynamics, Newtonian Dynamics, Corpuscular Optics, Wave Optics, The study of paradigms, is what prepares the student for membership in the particular scientific community in which he will later practice. Because he there joins men who learned the bases of their field from the same concrete models, his subsequent practice will seldom evoke overt disagreement with the fundamentals. Men whose research is based on shared paradigms are committed to the same rules and standards for scientific practice. That commitment and the apparent consensus it produces are pre requisites of normal science, i.e. for the continuation of a research tradition. But the road to a research consensus is extraordinarily arduous. In absence of a paradigm, or a candidate for paradigm, all facts that could possibly pertain to the development of a given science are likely to seem equally relevant. As a result, early fact gathering is a more random activity than the one that scientific development makes familiar. No natural history can be interpreted in the absence of at least some implicit body of intertwined theoretical and methodological belief that permits selection, evaluation and criticism. If that body of belief is not already implicit in the collection of facts- in which case more than mere facts are on hand- it must be externally supplied by a current metaphysic, by another science, or by personal and historical accident. No wonder that in the early stages of the development of any science, different men confronting the same range of phenomena describe them in different ways. What is surprising is that such initial differences should largely disappear. Their disappearance is usually caused by the triumph of one of the pre- paradigm schools, which because of its own characteristic beliefs and pre conceptions emphasised only some part of the too sizeable pool of information. To be accepted as a paradigm, a theory must seem better than its competitors, but it need not, and in fact never does, explain all the facts with which it can be confronted. There is some evidence for Bacon's dictum that "Truth emerges more readily from error than from confusion". But there are always men who cling to the one or another of the older views, and they are simply read out of the profession, which thereafter ignores their work. The new paradigm implies a new and more rigid definition of the field. Those unwilling or unable to accommodate their work to it must proceed in isolation or attach themselves to another group. Historically, they have simply stayed in the departments of philosophy from which so many of the special sciences have been spawned. In the sciences, the formation of specialised journals, the foundation of specialist societies and the claim for a special place in the curriculum, have usually been associated with a group's first reception of a single paradigm. When an individual scientist can take a paradigm for granted, he need no longer, in his major works attempt to build his field anew, starting from first principles and justifying the use of each concept introduced. That can be left to the writer of textbooks. Given a textbook, however, the creative scientist can begin research where it leaves off and thus concentrate exclusively upon the subtlest and more esoteric aspects of natural phenomena that concerns the group. No longer will his researches be embodied in books like Darwin's Origins of the Species , addressed to anyone who might be interested in the field. Instead, they will usually appear as brief articles addressed to professional colleagues, the men whose knowledge of a shared paradigm can be assumed, and who prove to be the only ones able to read the papers addressed to them.

The priority of paradigm

In science, a paradigm is like an accepted judicial decision in the common law, it is an object for further articulation and specification under new, more stringent conditions. Closely examined, whether historically or in the laboratory, that enterprise seems an attempt to force nature into the performed and relatively inflexible box that the paradigm supplies. No part of the aim of normal science is to call forth new sorts of phenomena; indeed those that will not fit the box are often not seen at all. Nor do scientists normally aim to invent new theories, and they are often intolerant of those invented by others. Instead, normal scientific research is directed to the articulation of those phenomena and theories that the paradigm already supplies. Perhaps those are defects. The areas investigated by normal science are, of course, minuscule- the enterprise under discussion has drastically restricted vision. But those restrictions, born from confidence in a paradigm, turn out to be essential to the development of science. By focusing attention on a small range of esoteric problems, the paradigm forces scientists to investigate some part of nature in a detail and depth that would be otherwise unimaginable. And normal science possesses a built in mechanism that ensures the relaxation of the restrictions that bound research whenever the paradigm from which they derive ceases to function properly. At that point, scientists begin to behave differently and the nature of their research problems changes. In the interim, however, during the period that the paradigm is successful, the profession will have solved problems that its members could scarcely have imagined and would have never undertaken without commitment to the paradigm. And at least part of that achievement proves to be permanent.

Normal Science as Puzzle-solving

So, one of the things a scientific community acquires with a paradigm is a criterion for choosing problems that, while the paradigm is taken for granted, can be assumed to have solutions. A paradigm, can, for the matter, even insulate the community from those socially important problems that are not reducible to the puzzle form, because they cannot be stated in terms of conceptual and instrumental tools the paradigm supplies. While the scientific enterprise as a whole, from time to time, tests long accepted belief and opens new territory, the individual engaged in a normal research problem is almost never doing any of these things. Once engaged, his motivation is of a different sort. What then challenges him is the conviction that if he is skilful enough, he will succeed in solving a puzzle that no one before has solved or solved so well. Many of the greatest scientific minds have devoted all their professional attention to demanding puzzles of this sort. But if it is to be classified as a puzzle, a problem must be characterised by more than an assured solution. There must also be rules that limit both the nature of acceptable solutions and the steps by which they are to be obtained. The man who builds an instrument to determine optical wave- lengths must not be satisfied with a piece of equipment that merely attributes particular numbers to particular spectral lines. He is not just an explorer or measurer. On the contrary, he must show, by analysing his apparatus in terms of the established body of optical theory, that the numbers his instrument produces are the ones that enter theory as wavelengths. Until those conditions had been satisfied, no problem has been solved. If, for example, some residual vagueness in the theory or some unanalysed component of his apparatus prevents his completing that demonstration, his colleagues may well conclude (although not always) that he has measured nothing at all. The existence of a strong network of commitments- conceptual, theoretical, instrumental and methodological- is a principal source of the metaphor that relates normal science to puzzle solving. Because it provides rules that tell the practitioner of a mature speciality what both the world and his science are like, he can concentrate with assurance upon the esoteric problems that these rules and existing knowledge define for him.

Paradigm Change: Anomaly and the Emergence of Scientific Discoveries

Normal science, the puzzle solving activity we have just examined, is a highly cumulative enterprise, eminently successful in its aim- the steady extension of the scope and precision of scientific knowledge. Yet one standard product of the scientific enterprise is missing. Normal science does not aim at novelties of fact or theory and, when successful finds none. New and unsuspected phenomena are, however, repeatedly uncovered by scientific research, and radical new theories have again and again been invented by scientists. History even suggests that the scientific enterprise has developed a uniquely powerful technique for producing surprises of this sort. If this is to be reconciled with what was said about normal science, then research under a paradigm must be inducing to paradigm change. That is what novelties of fact and theory do. Produced inadvertently by a game under one set of rules, their assimilation requires the elaboration of another set of rules. After they have become parts of that science, the enterprise, at least of these specialists in whose particular field the novelties lie, is never the same again. How are changes of this sort coming about, considering discoveries, or novelties of fact and inventions or novelties of theory? Fundamental in Kuhn is that the distinction between discovery and invention or between fact and theory is artificial. Discoveries are not isolated events but extended episodes with a regularly recurrent structure. Discovery commences with the awareness of anomaly i.e. the recognition that nature has somehow violated the paradigm-induced expectations that govern normal science. It then continues with a more or less extended exploration of the area of anomaly. And it closes when the paradigm theory has been adjusted so that the anomalous has become expected. Assimilating a new sort of fact demands a more than additive adjustment of theory, and until the adjustment is completed- until the scientist has learned to see nature in a different way- the new fact is not quite scientific fact at all. In the development of any science, the first received paradigm is usually felt to account quite successfully for most of the observations and experiments easily accessible to the science's practitioners. Further development, ordinarily calls for the construction of elaborate equipment, the development of an esoteric vocabulary and skills, and a refinement of concepts that increasingly lessens their resemblance to their usual common sense prototypes. This professionalisation leads, on one hand to an immense restriction of the scientist's vision and to considerable resistance to paradigm change. On the other hand, within those areas to which the paradigm directed the attention of the group, normal science leads to a detail of information and to a precision of the observation- theory match that can be achieved in no other way. Further, the detail and precision of match have a value, which surpasses their not always very high intrinsic interest. Without the special apparatus that is constructed mainly for anticipated functions, the results that lead ultimately to novelty would not occur. And even when the apparatus exists, novelty ordinarily emerges only for the man who, knowing with precision what to expect, is able to recognise that something has gone wrong. Anomaly appears only against the background provided by the paradigm. The more precise and far reaching a paradigm is the more sensitive an indicator it provides of anomaly and hence of an occasion for paradigm change. By ensuring that the paradigm will not be too easily surrendered, resistance guarantees that scientists will not be highly distracted and that the anomalies that lead to paradigm change will penetrate existing knowledge to the core. The very fact that a significant novelty so oft emerges from several laboratories is an index both to the strongly traditional nature of normal science and to the completeness with which that traditional pursuit prepares the way for its own change.


Once again, this is the notion that has profound philosophical implications. The term itself was borrowed from mathematics to describe the relationship between successive scientific theories. In geometry, the hypotenuse of an isosceles right angle triangle is incommensurable to the side, or the circumference of the circle with the radius, in the sense that there is no unit of length contained without residue an integral number of times in each member of the pair. There is thus no common measure. But lack of a common measure does not make comparison impossible. On the contrary, incommensurable magnitudes can be compared to any degree of approximation. On the question of how scientists can debate the choice between successive theories, Kuhn argued that the parties in such debates inevitably see differently certain of the experimental or observational situations to which both have recourse. (For example ‘force’ and ‘mass’ or ‘element’ or ‘compound’ often changed with the theory in which they were employed). Since the vocabularies in which they discuss such situations consist, however, predominantly of the same terms, they must be attaching some of those terms to nature differently, and the communication is inevitably partial. As a result, the superiority of one theory over another is something that cannot be proved in debate. Instead, each party must try by persuasion to convert the other. In other words, debates over theory choice cannot be cast in a form that fully resembles logical or mathematical proof. In the case of logical or mathematical proof, premises and rules of inference are stipulated from the start. If there is disagreement about conclusions, the parties to the ensuing debate can retrace their steps one by one, checking each against prior stipulation. At the end of the process one or the other must concede that he has made a mistake, violated a previously accepted rule. After that concession he has no recourse, and his opponents proof is then compelling. Only if the two discover that they differ about meaning or application of stipulated rules, that their prior agreement provides no sufficient basis for proof, that the debate continues in the form it inevitably takes during scientific revolutions. The debate is about premises, and its recourse to persuasion as a prelude to the possibility of proof. Nothing about this thesis implies that there are no good reasons for being persuaded or that those reasons are not ultimately decisive for the group. Nor does it imply that the reasons for choice are different than those listed by philosophers of science: accuracy, simplicity, fruitfulness and the like. What it suggests, however, is that such reasons function as values, and that they can be differently applied individually and collectively, by men who concur in honouring them. The concept of incommensurability was strongly criticised as violating the fundamental principle of intelligibility in science, and Kuhn charged with irrationality by philosophers of science. Kuhn's 1969 ‘PostScript’ to Structure and most of his essays in The Road since Structure attempt to deal with these criticisms. And the key appears to be once again a difference in interpretation- analytical philosophers, particularly Quine in ‘Word and Object’, equate translation with interpretation, so in their view proponents of incommensurability cannot communicate with each other. As a result, in a debate over theory choice there can be no recourse to good reasons, instead theory must be chosen for reasons that are ultimately personal and subjective. Historians, however, claim to be able to produce successful interpretations or hermeneutics.

An Assessment

This brings us to the first of the fundamental questions, which in my view were brought forward by Kuhn's work:

[1] What is History of Science?

From a methodological point of view, is it more akin to the Natural Science that it studies? Or is it a particular domain of Social (historically based) Sciences? The debate is not new: it started in the most general and conceptual way with Dilthey in the 1860-1870s (Dilthey 1989; Suchar, 2002), and continued in the 1930s and 40s with the ‘Covering Law’ controversy between Popper and Hempel on one side and the historians such as Collingwood, Dray and Donegan (Suchar, 2002). Dilthey's starting point was the positivist neo- Kantianism of the 1860s and 70s and most important, with his reaction to the Comtean positivism.of the period and its ideology of scientism- the only methods that lead to truth are the methods of natural science. He challenged the view that the methods of social sciences are identical in essence with those of natural sciences, that natural sciences by their very success are an exemplar of what science should be: empirically grounded, universally binding and value free. He argued that there is a distinction (elevated to a radical discontinuity by his followers) between natural and social sciences. According to his view, the social world consists of speaking and acting subjects who consistently make sense of themselves and others and whose meaningful and wilful activities cannot be comprehended by the methods of natural sciences. The study of social sciences requires a fundamentally different approach. Dilthey adhered to a scientific standpoint but his type of scientific method- hermeneutics- was different than that of natural sciences. In analysing the natural system of the 17th and 18th centuries he concluded that ‘the human type melts away in the process of history’ and that ‘for us, the question as to whether man of different periods may be regarded as the same within certain limits in respect to strength of motives cannot be answered at all at present’.(Dilthey, 1989). Indeed, this is the prototype for incommensurability, but as in the later case of Kuhn, it does not preclude understanding or hermeneutics, but according to different norms than the universal norms of natural science. The conception of historical understanding illustrated by the work of Dilthey and later Collingwood, entered in a period of heavy criticism in the 1930s and 40s. His critics considered his claim that such understanding is radically different in form from scientific kinds of explanation is unwarranted.The gist of this criticism was expressed by the ‘Covering Law’ model, developed by the logical positivists and by Popper in the 30s and 40s and formulated by Carl Hempel, a leading exponent of positivism in his article ‘The Function of General Laws in History’ published in 1942. ‘The Covering Law’ model rested on the contention that any adequate explanation of a causal type must necessarily exhibit the event to be explained as instantiating some general laws. When strictly interpreted, this was held to imply that the explanandum (what is to be explained) should be deduced from a set of premises consisting, on the one hand, of statements descriptive of initial or boundary conditions, and on the other, of further statements expressing well confirmed universal premises. The proponents of the claim suggested that it conformed to the account of causality given by Hume. They argued that historical explanation when the basic structure is fully revealed, displayed no significant divergences from those used in other fields of inquiry. Indeed, it was only on an analysis along the lines that the historians manner of making the past understandable could be appreciated as implying a rational procedure subject to the check and verification that was a precondition of respectability in any empirical discipline. Talk of empathetic projection into the minds of historical agents, as indulged by Dilthey and Collingwood, may have heuristic value in the historian's work, but as far as enunciating the fundamental logic of historical explanation, it was just a mystifying irrelevance. Despite its seeming plausibility, ‘The Covering Law’ analysis encountered in turn considerable criticism from more recent philosophers of history such as Wm. Dray and Allan Donegan, resting on Dilthey and Collingwood. First, the explanations of the historians do not measure up to the stipulations of the ‘Covering Law’ model. The historian would be hard put to cite universal hypotheses upon which the meaning or validity of his causal propositions allegedly depend. Second, the presentation of events as rationally intelligible in the light of the motives, aims and beliefs of agents involved constitutes an intrinsic feature of historical understanding and that feature cannot be accommodated within the limits of the covering law theory. Third, the actions of the agents stem from their internationality, and are not occurrences expected or predicted on the basis of inductively established uniformities. In my view, Kuhn's ideas belong to a re- enactment and further refinement of this debate; one focused in a particular domain- the history of science. And in this debate, Kuhn, is a philosopher and historian, whose work stems from Dilthey and Collingwood, is influenced by the French historian Alexandre Koyre, and is nearer the Continental philosophical hermeneutic tradition of Gadamer and Ricoeur than to analytic philosophy. His clash with the Putnam, Quine, Davidson, and with philosophers of science working within the analytic tradition was unavoidable. He proposed a different paradigm, one that makes history of science responsive to the methodology of social sciences.

[2] There is the second fundamental question: What is the import of Kuhn's idea on Natural Science and on its practitioners?

Kuhn was principally a student of the social context of natural science, the father not of a new history of science but of the Sociology of Science. He produced, not only a sociology of science, but also a normative sociology. When he describes ‘normal science’, he describes for the first time how things should be done in the world of large laboratories such as the Fermi Lab, Los Alamos, Livermore, Jet Propulsion, Cern, not how things were done in Plank, Einstein and Bohr's time when 50 odd scientists meeting at the Solvay Congresses debated and decided new physical theories. His scientist is more of an assembly line research worker, and on this view, Structure is the first manual to provide justification and establish criteria for such activities. This is the main import of his work.

Victor Suchar


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Dilthey, Wilhelm, Introduction to the Human Sciences. Selected Works vol.i (Priceton UP, Eng. ed 1989).

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Suchar, Victor, ‘Pierre Duhem's The Aim and Structure of Physical Theory’, BRLSI Proceedings 2003