THOMAS S. KUHN. shall henceforth refer to as paradigms, a term that re. lates closely to normal science. By choosing it, I mean

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1 achievements that some particular scientific commu nity acknowledges for a time as supplying the founda tion for its further practice. Today such achievements tions and experiments. Before such books became even more recently in the newly matured sciences), are recounted, though seldom in their original form, ilar function. Aristotle s Phvsica, Ptolemy s Almagest, essential characteristics. Their achievement was suffi Newton s Principia and Opticks, Franklin s Electricity, ciently unprecedented to attract an enduring group of textbooks expound the body of accepted theory, illus trate many or all of its successful applications, and many of the famous classics of science fulfilled a sim many other works served for a time implicitly to defme the legitimate problems and methods of a re search field for succeeding generations of practition 183 Chicago Press, 1970). Reprinted with permission from The University of Chicago Press. The Structure of Scientific Revolutions (Chicago: University of Lavoisier s Chemistry, and Lyell s Geology these and by science textbooks, elementary and advanced.these in this essay, normal science means research firmly compare these applications with exemplary observa adherents away from competing modes of scientific activity. Simultaneously, it was sufficiently openended to leave all sorts of problems for the redefmed group of practitioners to resolve. ers. They were able to do so because they shared two based upon one or more past scientific achievements, popular early in the nineteenth century (and until NORMAL SCIENCE THE ROUTE TO Achievements that share these two characteristics I to suggest that some accepted examples of actual sci application, and instrumentation together provide models from which spring particular coherent tradi which the historian describes under such rubrics as cluding many that are far more specialized than those from the same concrete models, his subsequent prac named illustratively above, is what mainly prepares the student for membership in the particular scientific community with which he will later practice. Because paradigms are committed to the same rules and stan dards for scientific practice. That commitment and the apparent consensus it produces are prerequisites wave optics ), and so on. The study of paradigms, in lates closely to normal science. By choosing it, I mean entific practice examples which include law, theory, tions of scientific research. These are the traditions Ptolemaic astronomy (or Copernican ), Aristotelian dynamics (or Newtonian ), corpuscular optics (or he there joins men who learned the bases of their field mentals. Men whose research is based on shared for normal science, i.e., for the genesis and continua to the various concepts, laws, theories, and points of be fully reduced to logically atomic components tice will seldom evoke overt disagreement over funda view that may be abstracted from it? In what sense more will need to be said about the reasons for its tion of a particular research tradition. Because in this essay the concept of a paradigm shall henceforth refer to as paradigms, a term that re will often substitute for a variety of familiar notions, The Structure of Scientific Revolutions THOMAS S. KUHN is the shared paradigm a fundamental unit for the 25 introduction. Why is the concrete scientific achieve ment, as a locus of professional commitment, prior student of scientific development, a unit that cannot

2 paradigm and of the more esoteric type of research If the historian traces the scientific knowledge variant of a pattern here illustrated from the his which might function in its stead? Answers to these questions and to others like them will prove basic to an understanding both of normal science and of the associated concept of paradigms. That more ab stract discussion will depend, however, upon a pre paradigms in operation. In particular, both these re binding as the ones named above. Acquisition of a ward in time, he is likely to encounter some minor tory of physical optics. Today s physics textbooks tell the student that light is photons, i.e., quantummechanical entities that exhibit some characteris proceeds accordingly, or rather according to the more elaborate and mathematical characterization from which the usual verbalization is derived. That Einstein, and others early in this century, physics Fresnel in the early nineteenth century. Nor was characterization of light is, however, scarcely half a eighteenth century the paradigm for this field was had not, of the pressure exerted by light particles impinging on solid bodies. 1 of any selected group of related phenomena back texts taught that light was transverse wave motion, timately from the optical writings of Young and ture science. It is not, however, the pattern charac sive transition from one paradigm to another via a conception rooted in a paradigm that derived ul century old. Before it was developed by Planck, provided by Newton s Op ticks, which taught that all practitioners.of optical science. During the the wave theory the first to be embraced by almost any given scientific field. tics of waves and some of particles. Research it permits is a sign of maturity in the development of ical optics are scientific revolutions, and the succes These transformations of the paradigms of phys 184 PART 5 /Theory and Observation: Is Seeing Believing? vious exposure to examples of normal science or of lated concepts wifi be clarified by noting that there can be a sort of scientific research without para cists sought evidence, as the early wave theorists digms, or at least without any so unequivocal and so light was material corpuscles. At that time physi revolution is the usual developmental pattern of ma teristic of the period before Newton s work, and that eye; still another explained light in terms of an in eye; and there were other combinations and modifi number of competing schools and subschools, most view about the nature of light. Instead there were a teenth century exhibited a single generally accepted of them espousing one variant or another of Epi curean, Aristotelian, or Platonic theory. One group medium that intervened between the body and the teraction of the medium with an emanation from the is the contrast that concerns us here. No period between remote antiquity and the end of the seven took light to be particles emanating from material bodies; for another it was a modification of the derived strength from its relation to some particular metaphysic, and each emphasized, as paradigmatic observations, the particular cluster of optical phe elaborations, or they remained as outstanding prob At various times all these schools made signifi nomena that its own theory could do most to ex cant contributions to the body of concepts, phe nomena, and techniques from which Newton drew examining a survey of physical optics before New ton may well conclude that, though the field s prac titioners were scientists, the net result of their writer on physical optics felt forced to build his field rected as much to the members of other schools as lems for further research. 2 cations besides. Each of the corresponding schools plain. Other observations were dealt with by ad hoc physical optics. Any definition of the scientist that excludes at least the more creative members of these various schools will exclude their modern succes not, however, the pattern of development that phys to take no common body of belief for granted, each tively free, for there was no standard set of methods ble with significant discovery and invention. It is supporting observation and experiment was rela number of creative fields today, nor is it incompati it was to nature. That pattern is not unfamiliar in a the first nearly uniformly accepted paradigm for activity was something less than science. Being able or of phenomena that every optical writer felt forced sors as well. Those men were scientists. Yet anyone anew from its foundations. In doing so, his choice of the dialogue of the resulting books was often di ural sciences make familiar today.. ical optics acquired after Newton and that other nat to employ and explain. Under these circumstances,.

3 cal latitude of expectation is only somewhat wider. to the continuing progress of electrical research. gave a result assimilable by paradigm articulation. why several of Coulomb s contemporaries had been 1 But it is also why that result surprised no one and tual or phenomenal. Sometimes, as in a wave-length little they aim to produce major novelties, concep tail of the result is known in advance, and the typi why Coulomb was able to design apparatus that Coulomb s measurements need not, perhaps, have one of several results.yet even in cases like these the come does not fall in that narrower range is usually just a research failure, one which reflects not on na is always small compared with the range that imag fitted an inverse square law; the men who worked on Perhaps the most striking feature of the normal re search problems we have just encountered is how measurement, every-thing but the most esoteric de heating by compression were often prepared for any range of anticipated, and thus of assimilable, results ination can conceive. And the project whose out they remained mere facts, unrelated and unrelatable poraries, of course, also possessed this later para attraction, yielded the same expectations. That is phenomena they display. Coulomb and his contem electrical attraction with devices like the pan bal ance. Because they yielded neither consistent nor simple results, they could not be used to articulate the paradigm from which they derived. Therefore, Only in retrospect, possessed of a subsequent para digm, can we see what characteristics of electrical digm or one that, when applied to the problem of itself uninteresting, the way to achieve that outcome PUZZLE-SOLVING NORMAL SCIENCE AS are significant because they add to the scope and lem of vibrating strings simpiv because of the im proves himself an expert puzzle-solver, and the challenge of the puzzle is an important part of what precision with which the paradigm can be applied. remains very much in doubt. Bringing a normal re search problem to a conclusion is achieving the an usually drives him on. KUHN / The Structure of Scientific Revolutions 185 thusiasm and devotion that scientists display for the problems of normal research. No one devotes years the production of an improved solution to the prob carried through before. That rejection provides a ticipated in a new way, and it requires the solution and mathematical puzzles. The man who succeeds the entirely standard meaning here employed, that ingenuity or skifi in solution. Dictionary illustrations the characteristics that these share with the prob rion of goodness in a puzzle that its outcome be in of all sorts of complex instrumental, conceptual, (though it might not) even the most ingenious of One of them has just been mentioned. It is no crite trinsically interesting or important. On the contrary, lution. Consider the jigsaw puzzle whose pieces are men, it cannot serve as a test of skill in solution. In so largely repetitions of procedures that have been zle boxes. Since that problem is likely to defy The data to be gained by computing ephemerides or portance of the information that will be obtained. ment are often just as significant, but those activities The terms puzzle and puzzle-solver highlight prominent in the preceding pages. Puzzles are, in to, say, the development of a better spectrometer or are regularly spurned by scientists because they are by further measurements with an existing instru in detail so great that what remains to be known is clue to the fascination of the normal research prob lem. Though its outcome can be anticipated, often several of the themes that have become increasingly special category of problems that can serve to test the really pressing problems, e.g., a cure for cancer or the design of a lasting peace, are often not puz selected at random from each of two different puz zles at all, largely because they may not have any so are jigsaw puzzle and crossword puzzle, and it is lems of normal science that we now need to isolate. That answer, however, cannot account for the en stantive novelties if failure to come near the antic why are these problems undertaken at all? Part of the answer has already been developed. To scien tists, at least, the results gained in normal research But if the aim of normal science is not major sub ipated result is usually failure as a scientist then able to predict it in advance. Even the project whose goal is paradigm articulation does not aim at the un expected novelty. ture but on the scientist. In the eighteenth century, for example, little at tention was paid to the experiments that measured

4 standards are rejected as metaphysical, as the con problems including many that had previously been or enc0u11ge its members to undertake. Other to have solutions.to a great extent these are the only We have akeady seen, however, that one of the an usual sense it is not a puzzle at all. Though in 186 PART 5 / TheorY and Observation: Is Seeing Believing? trinsic value is no criterion for a puzzle, the assured existence of a solution is. things a scientific community acquires with a para digm is a criterion for choosing problems that, rhjle the problems that the community will admit as scientific those socially important problems that are not re tools the paradigm supplies. Such problems can be facets of seventeenth-century Baconianism and by a distraction, a lesson brilliantly illustrated by several for that matter, even insulate the community from cern of another discipline, or sometimes as just too paradigm is taken for granted, can be assumed problematic to be worth the time. A paradigm can, ducible to the puzzle form, because they cannot be some of the contemporary social sciences. One of the reasons why normal science seems to progress these questions has been anticipated in earlier sec These remarks permit us at last to consider the OF SCIENTIFIC REVOLUTIONS THE NATURE AND NECESSITY problems that only their own lack of ingenuity so rapidly is that its practitioners concentrate on stated in terms of the conceptual and instrumental which an older paradigm is replaced in whole or in tions. In particular, the preceding discussion has in In the face of the vast and essential differences be tween political and scientific development, what parallelism can justify the metaphor that finds revo introduced by asking one further question. Why are scientific revolutions, and what is their function be those non-cumulative developmental episodes in be said, however, and an essential part of it can be part by an incompatible new one. There is more to in scientific development? Much of the answer to should keep them from solving. problems that provide this essay with its title. What dicated that scientific revolutions are here taken to should a change of paradigm be called a revolution? lutions in both? parent. Political revolutions are inaugurated by a environment that they have in part created. In much political community, that existing institutions have One aspect of the parallelism must already be ap growing sense, often restricted to a segment of the ceased adequately to meet the problems posed by an that paradigm itself had previously led the way. In of malfunction that can lead to crisis is prerequisite both political and scientific development the sense in the exploration of an aspect of nature to which row subdivision of the scientific community, that an by a growing sense, again often restricted to a nar the same way, scientific revolutions are inaugurated strains the metaphor, that parallelism holds not only existing paradigm has ceased to function adequately the developmental process. Astronomers, for exam able to Copernicus and Lavoisier, but also for the to revolution. Furthermore, though it admittedly for the major paradigm changes, like those attribut far smaller ones associated with the assimilation of a the early twentieth century, seem normal parts of knowledge, for their paradigms were unaffected by one paradigm as it created another. That is why the existence of the new radiation. But for men like be open to doubt. The parallel has, however, a sec Kelvin, Crookes, and Roentgen, whose research these rays could be discovered only through some dealt with radiation theory or with cathode ray tubes, the emergence of X-rays necessarily violated To outsiders they may, like the Balkan revolutions of ple, could accept X-rays as a mere addition to alone that attenuates the role of political institu of paradigms. In increasing numbers individuals tions as we have already seen it attenuate the role other, and in the interim, society is not fully governed by institutions at all. Initially it is crisis that those institutions themselves prohibit. Their quishment of one set of institutions in favor of an tions aim to change political institutions in ways ond and more profound aspect upon which the significance of the first depends. Political revolu success therefore necessitates the partial relin litical and scientific development should no longer This genetic aspect Of the parallel between po thing s first going wrong with normal research. Scientific revolutions. seem revolutionary.. need new sort of phenomenon, like oxygen or X-rays. only to those whose paradigms are affected by them.

5 seeking to defend the old institutional constella mentation effective within the quite special groups Then, as the crisis deepens, many of these indi become increasingly estranged from political life posal for the reconstruction of society in a new and behave more and more eccentrically within it. institutional framework. At that point the society is divided into competing camps or parties, one viduals commit themselves to some concrete pro debate over paradigms are not sufficiently extensive Examining the record of past research from the van that constitute the community of scientists. for that. As in political revolutions, so in paradigm choice there is no standard higher than the assent of the relevant community. To discover how scien I... - sciences. Like the choice between competing politi choice is not and cannot he determined merely by science, for these depend in part upon a particular very similar characteristics in the evolution of the to a revolutionary conflict must finally resort to proves to be a choice between incompatible modes adigms enter, as they must, into a debate about par adigm choice, their role is necessarily circular. Each what scientific practice will be like for those who make the arguments wrong or even ineffectual. The defense can nonetheless provide a clear exhibit of immensely persuasive, often compellingly so. Yet, whatever its force, the status of the circular argu those who refuse to step into the circle. The premises and values shared by the two parties to a i digm s defense. the evaluative procedures characteristic of normal achieved and evaluated, because they acknowl edge no supra-institutional framework for the ad force. Though revolutions have had a vital role in the evolution of political institutions, that role de of community life. Because it has that character, the paradigm, and that paradigm is at issue. When par group uses its own paradigm to argue in that para man who premises a paradigm when arguing in its adopt the new view of nature. That exhibit can be ment is only that of persuasion. It cannot be made tional matrix within which political change is to be judication of revolutionary difference, the parties the techniques of mass persuasion, often including extrainstitutional events. recourse fails. Because they differ about the institu. cal institutions, that between competing paradigms logically or even probabilistically compelling for tion, the others seeking to institute some new one. And, once that polarization has occurred, political pends upon their being partially extrapolitical or The remainder of this essay aims to demonstrate that the historical study of paradigm change reveals The resulting circularity does not, of course, KUHN /The Structure of Scientific Revolutions 187 dent sees confused and broken lines, the physicist a miliar ones as well. Of course, nothing of quite that tation; outside the laboratory everyday affairs usu scientists are responding to a different world. mations of the scientist s world that the familiar changes do cause scientists to see the world of their demonstrations of a switch in visual gestalt prove so Looking at a bubble-chamber photograph, the stu lines on paper, the cartographer a picture of a terrain. are seen in a different light and are joined by unfa only recourse to that world is through what they see before the revolution are rabbits afterwards. The man who first saw the exterior of the box from above later these, though usually more gradual and almost always irreversible, are common concomitants of scientific training. Looking at a contour map, the student sees sort does occur: there is no geographical transplan research-engagement differently. In so far as their and do, we may want to say that after a revolution transported to another planet where familiar objects as if the professional community had been suddenly ally continue as before. Nevertheless, paradigm sees its interior from below. Transformations like It is as elementary prototypes for these transfor suggestive. What were ducks in the scientist s world portant, during revolutions scientists see new and instruments and look in new places. Even more im paradigms change, the world itself changes with of science may be tempted to exclaim that when them. Led by a new paradigm, scientists adopt new different things when looking with familiar instru ments in places they have looked before. It is rather tage of contemporary historiography, the historian OF WORLD VIEW REVOLUTIONS AS CHANGES to examine not only the impact of nature and of logic, but also the techniques of persuasive argu tific revolutions are effected, we shall therefore have

6 188 PART 5 /Theory and Observation: Is Seeing Believing? 3 Literally as well as metaphorically, the 4 Surveying the rich exper record of familiar subnuclear events. Only after a number of such transformations of vision does the student become an inhabitant of the scientist s world, seeing what the scientist sees and responding as the scientist does. The world that the student then enters is not, however, fixed once and for all by the nature of the environment, on the one hand, and of science, on the other. Rather, it is determined jointly by the envi ronment and the particular normal-scientific tradi tion that the student has been trained to pursue. Therefore, at times of revolution, when the normalscientific tradition changes, the scientist s perception of his environment must be re-educated in some fa rniliar situations he must learn to see a new gestalt. After he has done so the world of his research will seem, here and there, incommensurable with the one he had inhabited before. That is another reason why schools guided by different paradigms are always slightly at cross-purposes. In their most usual form, of course, gestalt ex periments illustrate only the nature of perceptual transformations. They tell us nothing about the role of paradigms or of previously assimilated experi ence in the process of perception. But on that point there is a rich body of psychological literature, much of it stemming from the pioneering work of the Hanover Institute. An experimental subject who puts on goggles fitted with inverting lenses initially sees the entire world upside down. At the start his perceptual apparatus functions as it had been trained to function in the absence of the goggles, and the result is extreme disorientation, an acute personal crisis. But after the subject has begun to learn to deal with his new world, his entire visual field flips over, usually after an intervening period in which vision is simply confused. Thereafter, objects are again seen as they had been before the goggles were put on. The assimilation of a previously anom alous visual field has reacted upon and changed the field itself. man accustomed to inverting lenses has undergone a revolutionary transformation of vision... Still other experiments demonstrate that the per ceived size, color, and so on, of experimentally dis played objects also varies with the subject s previous training and experience. imental literature from which these examples are drawn makes one suspect that something like a paradigm is prerequisite to perception itself. What a man sees depends both upon what he looks at and also upon what his previous visual-conceptual expe rience has taught him to see. In the absence of such training there can only be, in William James s phrase, a bloomin buzzin confusion. In recent years several of those concerned with the history of science have found the sorts of exper iments described above immensely suggestive. N. R. Hansoh, in particular, has used gestalt demonstra tions to elaborate some of the same consequences of scientific belief that concern me here. colleagues have repeatedly noted that history of sci ence would make better and more coherent sense if one could suppose that scientists occasionally expe rienced shifts of perception like those described above. Yet, though psychological experiments are suggestive, they cannot, in the nature of the case, be more than that. They do display characteristics of perception that could be central to scientific devel opment, but they do not demonstrate that the care ful and controlled observation exercised by the research scientist at all partakes of those character istics. Furthermore, the very nature of these experi ments makes any direct demonstration of that point impossible. If historical example is to make these psychological experiments seem relevant, we must first notice the sorts of evidence that we may and may not expect history to provide. The subject of a gestalt demonstration knows that his perception has shifted because he can make it shift back and forth repeatedly while he holds the same book or piece of paper in his hands. Aware that nothing in his environment has changed, he directs his attention increasingly not to the figure (duck or rabbit) but to the lines on the paper he is looking at. Ultimately he may even learn to see those lines with out seeing either of the figures, and he may then say (what he could not legitimately have said earlier) that it is these lines that he really sees but that he sees them alternately as a duck and as a rabbit.. Unless there were an external standard with respect to which a switch of vision could be demonstrated, no conclusion about alternate perceptual possibili ties could be drawn. With scientific observation, however, the situation is exactly reversed. The scientist can have no re course above or beyond what he sees with his eyes 5 Other

7 KUHN /The Structure of Scientific Revolutions 189 and instruments. If there were some higher authority by recourse to which his vision might be shown to have shifted, then that authority would itself become the source of his data, and the behavior of his vision would become a source of problems (as that of the experimental subject is for the psychologist). The same sorts of problems would arise if the scientist could switch back and forth like the subject of the gestalt experiments. The period during which light was sometimes a wave and sometimes a particle was a period of crisis a period when something was wrong and it ended only with the development of wave mechanics and the realization that light was a self-consistent entity different from both waves and particles. In the sciences, therefore, if perceptual switches accompany paradigm changes, we may not expect scientists to attest to these changes directly. Looking at the moon, the convert to Copernicanism does not say, I used to see a planet, but now I see a satellite. That locution would imply a sense in which the Ptolemaic system had once been correct. Instead, a convert to the new astronomy says, I once took the moon to be (or saw the moon as) a planet, but I was mistaken. That sort of statement does recur in the aftermath of scientific revolutions. If it ordinarily disguises a shift of scientific vision or some other mental transformation with the same effect, we may not expect direct testimony about that shift. Rather we must look for indirect and behavioral evidence that the scientist with a new paradigm sees differ ently from the way he had seen before. Let us then return to the data and ask what sorts of transformations in the scientist s world the histo rian who believes in such changes can discover. Sir William Herschel s discovery of Uranus provides a first example.... On at least seventeen different oc casions between 1690 and 1781, a number of as tronomers, including several of Europe s most eminent observers, had seen a star in positions that we now suppose must have been occupied at the time by Uranus. One of the best observers in this group had actually seen the star on four successive nights in 1769 without noting the motion that could have suggested another identification. Herschel, when he first observed the same object twelve years later, did so with a much improved telescope of his own manufacture. As a result, he was able to notice an apparent disk-size that was at least unusual for stars. Something was awry, and he therefore post poned identification pending further scrutiny. That scrutiny disclosed Uranus motion among the stars, and Herschel therefore announced that he had seen a new comet! Only several months later, after fruit less attempts to fit the observed motion to a com etary orbit, did Lexell suggest that the orbit was probably planetary. accepted, there were several fewer stars and one more planet in the world of the professional as tronomer. A celestial body that had been observed off and on for almost a century was seen differently after 1781 because it could no longer be fitted to the perceptual categories (star or comet) provided by the paradigm that had previously prevailed. The shift of vision that enabled astronomers to see Uranus, the planet, does not, however, seem to have affected only the perception of that previously ob served object. Its consequences were more far-reach ing. Probably, though the evidence is equivocal, the minor paradigm change forced by Herschel helped to prepare astronomers for the rapid discovery, after 1801, of the numerous minor planets or asteroids. Because of their small size, these did not display the anomalous magnification that had alerted Herschel. Nevertheless, astronomers prepared to find addi tional planets were able, with standard instruments, to identify twentr of them in the first fifty years of the nineteenth century. vides many other examples of paradigm-induced changes in scientific perception, some of them even less equivocal. Can it conceivably be an accident, for example, that Western astronomers first saw change in the previously immutable heavens during the halfcentury after Copernicus new paradigm was first proposed? The Chinese, whose cosmological beliefs did not preclude celestial change, had recorded the appearance of many new stars in the heavens at a much earlier date. Also, even without the aid of a tele scope, the Chinese had systematically recorded the appearance of sunspots centuries before these were seen by Galileo and his contemporaries. sunspots and a new star the only examples of celestial change to emerge in the heavens of Western astron omy immediately after Copernicus. Using traditional instruments, some as simple as a piece of thread, late sixteenth-century astronomers repeatedly discovered that comets wandered at will through the space 6 When that suggestion was 7 The history of astronomy pro 8 Nor were I

8 pp 190 PART 5 I Theory and Observation: Is Seeing Believing? previously reserved for the immutable planets and stars. The very ease and rapidity with which tronomers saw new things when looking at old jects with old instruments may make us wish to say that, after Copernicus, astronomers lived in a differ ent world. In any case, their research responded as though that were the case. 9 NOTES as ob 1. Joseph Priestlev, The History and Present State of Discoveries Relating to Vision, Light, and Colours (London, 1772), pp Vasco Ronchi, Histoire de la lumière, trans. Jean Taton (Paris, 1956), chaps i iv. 3. The original experiments were by George M. Strat ton, Vision without Inversion of the Retinal Image, Psychological ReviezL IV (1897), , A more up-to-date review is provided by Harvey A. Carr, An Introduction to Space Perception (NewYork, 1935), pp For examples, see Albert H. Hastorf, The Influ ence of Suggestion on the Relationship between Stimulus Size and Perceived Distance, Journal of Psychology, XXIX (1950), ; and Jerome S. Bruner, Leo Postman, and John Rodrigues, Expec tations and the Perception of Color, American Journal of Psychology, LXIV (1951), N. R. Hanson, Patterns of Discovery (Cambridge, 1958), chap. i. 6. Peter Doig, A Concise History of Astronomy (Lon don, 1950), pp Rudolph Wolf, Geschichte der Astronomie (Munich, 1877), pp , Notice particularly how difficult Wolf s account makes it to explain these discoveries as a consequence of Bode s Law. 8. Joseph Needham, Science and Civilization in China, III (Cambridge, 1959), , T. S. Kuhn, The Copernican Revolution (Cambridge, Mass., 1957), pp LARRY LAUDAN A Problem-Solving Approach to Scientific Progress DESIDERATA Studies of the historical development of science have made it clear that any normative model of entific rationality which is to have the resources to show that science has been largely a rational prise must come to terms with certain persistent sci enter Beyond Positivism and Relativism (Boulder, CO: Westview Press, 1996), PP Reprinted with permission from the publisher. features of scientific change. To be specific, we may conclude from the existing historical evidence that: 1. Theory transitions are generally noncumula tive, i.e., neither the logical nor empirical content (nor even the confirmed consequences ) of earlier theories is wholly preserved when those theories are supplanted by newer ones. 2. Theories are generally not rejected simply because they have anomalies nor are they