Construction of scientific revolution

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The essay that follows is the first full published report on a project originally conceived almost fifteen years ago. At that time I was a graduate student in theoretical physics already within sight of the end of my dissertation.
The Structure of Scientific
Second Edition, Enlarged
Thomas S. Kuhn
2. International Encyclopedia of Unified Science
Editor-in-Chief Otto Neurath
Associate Editors Rudolf Carnap Charles Morris
Foundations of the Unity of Science
(Volumes I—II of the Encyclopedia)
Committee of Organization
Advisory Committee
© 1962, 1970 by The University of Chicago.
All rights reserved. Published 1962.
Second Edition, enlarged, 1970
Printed in the United States of America
81 80 79 78 11 10 9 8
ISBN: 0-226-45803-2 (clothbound); 0-226-45804-0 (paperbound)
Library of Congress Catalog Card Number: 79-107472
3. International Encyclopedia of Unified Science
Volume 2 • Number 2
The Structure of Scientific Revolutions
Thomas S. Kuhn
PREFACE ...................................................... v
II. THE ROUTE TO NORMAL SCIENCE .................... 10
III. THE NATURE OF NORMAL SCIENCE ................. 23
V. THE PRIORITY OF PARADIGMS .......................... 43
VIII. THE RESPONSE TO CRISIS ................................. 77
Postscript-1969 ................................................ 174
5. The essay that follows is the first full published report on a project
originally conceived almost fifteen years ago. At that time I was a
graduate student in theoretical physics already within sight of the end
of my dissertation. A fortunate involvement with an experimental
college course treating physical science for the non-scientist provided
my first exposure to the history of science. To my complete surprise, that
exposure to out-of-date scientific theory and practice radically
undermined some of my basic conceptions about the nature of science
and the reasons for its special success.
Those conceptions were ones I had previously drawn partly from
scientific training itself and partly from a long-standing avocational
interest in the philosophy of science. Somehow, whatever their
pedagogic utility and their abstract plausibility, those notions did not at
all fit the enterprise that historical study displayed. Yet they were and
are fundamental to many discussions of science, and their failures of
verisimilitude therefore seemed thoroughly worth pursuing. The result
was a drastic shift in my career plans, a shift from physics to history of
science and then, gradually, from relatively straightforward historical
problems back to the more philosophical concerns that had initially led
me to history. Except for a few articles, this essay is the first of my
published works in which these early concerns are dominant. In some
part it is an attempt to explain to myself and to friends how I happened
to be drawn from science to its history in the first place.
My first opportunity to pursue in depth some of the ideas set forth
below was provided by three years as a Junior Fellow of the Society of
Fellows of Harvard University. Without that period of freedom the
transition to a new field of study would have been far more difficult and
might not have been achieved. Part of my time in those years was
devoted to history of science proper. In particular I continued to study
the writings of Alex-
Vol. II, No. 2
6. andre Koyré and first encountered those of Emile Meyerson, Hélène
Metzger, and Anneliese Maier.1 More clearly than most other recent
scholars, this group has shown what it was like to think scientifically in a
period when the canons of scientific thought were very different from
those current today. Though I increasingly question a few of their
particular historical interpretations, their works, together with A. O.
Lovejoy’s Great Chain of Being, have been second only to primary source
materials in shaping my conception of what the history of scientific
ideas can be.
Much of my time in those years, however, was spent exploring fields
without apparent relation to history of science but in which research
now discloses problems like the ones history was bringing to my
attention. A footnote encountered by chance led me to the experiments
by which Jean Piaget has illuminated both the various worlds of the
growing child and the process of transition from one to the next.2 One of
my colleagues set me to reading papers in the psychology of perception,
particularly the Gestalt psychologists; another introduced me to B. L.
Whorf’s speculations about the effect of language on world view; and W.
V. O. Quine opened for me the philosophical puzzles of the analytic-
synthetic distinction.3 That is the sort of random exploration that the
Society of Fellows permits, and only through it could I have encountered
Ludwik Fleck’s almost unknown monograph, Entstehung und
Entwicklung einer wis-
Particularly influential were Alexandre Koyré, Études Galiléennes (3 vols.;
Paris, 1939); Emile Meyerson, Identity and Reality, trans. Kate Loewenberg (New
York, 1930); Hélène Metzger, Les doctrines chimiques en France du début du XVIIe à
la fin du XVIIIe siècle (Paris, 1923), and Newton, Stahl, Boerhaave et la doctrine
chimique (Paris, 1930); and Anneliese Maier, Die Vorläufer Galileis im 14.
Jahrhundert (“Studien zur Naturphilosophie der Spätscholastik”; Rome, 1949).
Because they displayed concepts and processes that also emerge directly from
the history of science, two sets of Piaget s investigations proved particularly
important: The Child’s Conception of Causality, trans. Marjorie Gabain (London,
1930), and Les notions de mouvement et de vitesse chez l’enfant (Paris, 1946).
Whorf’s papers have since been collected by John B. Carroll, Language,
Thought, and Reality—Selected Writings of Benjamin Lee Whorf (New York, 1956).
Quine has presented his views in “Two Dogmas of Empiricism,” reprinted in his
From a Logical Point of View (Cambridge, Mass., 1953), pp. 20-46.
Vol. II, No. 2
7. senschaftlichen Tatsache (Basel, 1935), an essay that anticipates many of
my own ideas. Together with a remark from another Junior Fellow,
Francis X. Sutton, Fleck’s work made me realize that those ideas might
require to be set in the sociology of the scientific community. Though
readers will find few references to either these works or conversations
below, I am indebted to them in more ways than I can now reconstruct
or evaluate.
During my last year as a Junior Fellow, an invitation to lecture for the
Lowell Institute in Boston provided a first chance to try out my still
developing notion of science. The result was a series of eight public
lectures, delivered during March, 1951, on “The Quest for Physical
Theory.” In the next year I began to teach history of science proper, and
for almost a decade the problems of instructing in a field I had never
systematically studied left little time for explicit articulation of the ideas
that had first brought me to it. Fortunately, however, those ideas proved
a source of implicit orientation and of some problem-structure for much
of my more advanced teaching. I therefore have my students to thank
for invaluable lessons both about the viability of my views and about the
techniques appropriate to their effective communication. The same
problems and orientation give unity to most of the dominantly
historical, and apparently diverse, studies I have published since the end
of my fellowship. Several of them deal with the integral part played by
one or another metaphysic in creative scientific research. Others
examine the way in which the experimental bases of a new theory are
accumulated and assimilated by men committed to an incompatible
older theory. In the process they describe the type of development that I
have below called the “emergence” of a new theory or discovery. There
are other such ties besides.
The final stage in the development of this essay began with an
invitation to spend the year 1958-59 at the Center for Advanced Studies
in the Behavioral Sciences. Once again I was able to give undivided
attention to the problems discussed below. Even more important,
spending the year in a community
Vol. II, No. 2
8. composed predominantly of social scientists confronted me with
unanticipated problems about the differences between such
communities and those of the natural scientists among whom I had
been trained. Particularly, I was struck by the number and extent of the
overt disagreements between social scientists about the nature of
legitimate scientific problems and methods. Both history and
acquaintance made me doubt that practitioners of the natural sciences
possess firmer or more permanent answers to such questions than their
colleagues in social science. Yet, somehow, the practice of astronomy,
physics, chemistry, or biology normally fails to evoke the controversies
over fundamentals that today often seem endemic among, say,
psychologists or sociologists. Attempting to discover the source of that
difference led me to recognize the role in scientific research of what I
have since called “paradigms.” These I take to be universally recognized
scientific achievements that for a time provide model problems and
solutions to a community of practitioners. Once that piece of my puzzle
fell into place, a draft of this essay emerged rapidly.
The subsequent history of that draft need not be recounted here, but
a few words must be said about the form that it has preserved through
revisions. Until a first version had been completed and largely revised, I
anticipated that the manuscript would appear exclusively as a volume in
the Encyclopedia of Unified Science. The editors of that pioneering work
had first solicited it, then held me firmly to a commitment, and finally
waited with extraordinary tact and patience for a result. I am much
indebted to them, particularly to Charles Morris, for wielding the
essential goad and for advising me about the manuscript that resulted.
Space limits of the Encyclopedia made it necessary, however, to present
my views in an extremely condensed and schematic form. Though
subsequent events have somewhat relaxed those restrictions and have
made possible simultaneous independent publication, this work remains
an essay rather than the full-scale book my subject will ultimately
Since my most fundamental objective is to urge a change in
Vol. II, No. 2
9. the perception and evaluation of familiar data, the schematic character
of this first presentation need be no drawback. On the contrary, readers
whose own research has prepared them for the sort of reorientation
here advocated may find the essay form both more suggestive and easier
to assimilate. But it has disadvantages as well, and these may justify my
illustrating at the very start the sorts of extension in both scope and
depth that I hope ultimately to include in a longer version. Far more
historical evidence is available than I have had space to exploit below.
Furthermore, that evidence comes from the history of biological as well
as of physical science. My decision to deal here exclusively with the
latter was made partly to increase this essay’s coherence and partly on
grounds of present competence. In addition, the view of science to be
developed here suggests the potential fruitfulness of a number of new
sorts of research, both historical and sociological. For example, the
manner in which anomalies, or violations of expectation, attract the
increasing attention of a scientific community needs detailed study, as
does the emergence of the crises that may be induced by repeated
failure to make an anomaly conform. Or again, if I am right that each
scientific revolution alters the historical perspective of the community
that experiences it, then that change of perspective should affect the
structure of postrevolutionary textbooks and research publications. One
such effect—a shift in the distribution of the technical literature cited in
the footnotes to research reports—ought to be studied as a possible
index to the occurrence of revolutions.
The need for drastic condensation has also forced me to forego
discussion of a number of major problems. My distinction between the
pre- and the post-paradigm periods in the development of a science is,
for example, much too schematic. Each of the schools whose
competition characterizes the earlier period is guided by something
much like a paradigm; there are circumstances, though I think them
rare, under which two paradigms can coexist peacefully in the later
period. Mere possession of a paradigm is not quite a sufficient criterion
for the developmental transition discussed in Section II. More
important, ex-
Vol. II, No. 2
10. cept in occasional brief asides, I have said nothing about the role of
technological advance or of external social, economic, and intellectual
conditions in the development of the sciences. One need, however, look
no further than Copernicus and the calendar to discover that external
conditions may help to transform a mere anomaly into a source of acute
crisis. The same example would illustrate the way in which conditions
outside the sciences may influence the range of alternatives available to
the man who seeks to end a crisis by proposing one or another
revolutionary reform.4 Explicit consideration of effects like these would
not, I think, modify the main theses developed in this essay, but it would
surely add an analytic dimension of first-rate importance for the
understanding of scientific advance.
Finally, and perhaps most important of all, limitations of space have
drastically affected my treatment of the philosophical implications of
this essay’s historically oriented view of science. Clearly, there are such
implications, and I have tried both to point out and to document the
main ones. But in doing so I have usually refrained from detailed
discussion of the various positions taken by contemporary philosophers
on the corresponding issues. Where I have indicated skepticism, it has
more often been directed to a philosophical attitude than to any one of
its fully articulated expressions. As a result, some of those who know
and work within one of those articulated positions may feel that I have
missed their point. I think they will be wrong, but this essay is not
calculated to convince them. To attempt that would have required a far
longer and very different sort of book.
The autobiographical fragments with which this preface
These factors are discussed in T. S. Kuhn, The Copernican Revolution: Planetary
Astronomy in the Development of Western Thought (Cambridge, Mass., 1957), pp.
122-32, 270-71. Other effects of external intellectual and economic conditions
upon substantive scientific development are illustrated in my papers,
“Conservation of Energy as an Example of Simultaneous Discovery,” Critical
Problems in the History of Science, ed. Marshall Clagett (Madison, Wis., 1959), pp.
321-56; “Engineering Precedent for the Work of Sadi Carnot,” Archives
internationales d’histoire des sciences, XIII (1960), 247-51; and “Sadi Carnot and the
Cagnard Engine,” Isis, LII (1961), 567-74. It is, therefore, only with respect to the
problems discussed in this essay that I take the role of external factors to be
Vol. II, No. 2
11. opens will serve to acknowledge what I can recognize of my main debt
both to the works of scholarship and to the institutions that have helped
give form to my thought. The remainder of that debt I shall try to
discharge by citation in the pages that follow. Nothing said above or
below, however, will more than hint at the number and nature of my
personal obligations to the many individuals whose suggestions and
criticisms have at one time or another sustained and directed my
intellectual development. Too much time has elapsed since the ideas in
this essay began to take shape; a list of all those who may properly find
some signs of their influence in its pages would be almost coextensive
with a list of my friends and acquaintances. Under the circumstances, I
must restrict myself to the few most significant influences that even a
faulty memory will never entirely suppress.
It was James B. Conant, then president of Harvard University, who
first introduced me to the history of science and thus initiated the
transformation in my conception of the nature of scientific advance.
Ever since that process began, he has been generous of his ideas,
criticisms, and time—including the time required to read and suggest
important changes in the draft of my manuscript. Leonard K. Nash, with
whom for five years I taught the historically oriented course that Dr.
Conant had started, was an even more active collaborator during the
years when my ideas first began to take shape, and he has been much
missed during the later stages of their development. Fortunately,
however, after my departure from Cambridge, his place as creative
sounding board and more was assumed by my Berkeley colleague,
Stanley Cavell. That Cavell, a philosopher mainly concerned with ethics
and aesthetics, should have reached conclusions quite so congruent to
my own has been a constant source of stimulation and encouragement
to me. He is, furthermore, the only person with whom I have ever been
able to explore my ideas in incomplete sentences. That mode of
communication attests an understanding that has enabled him to point
me the way through or around several major barriers encountered while
preparing my first manuscript.
Vol. II, No. 2
12. Since that version was drafted, many other friends have helped with
its reformulation. They will, I think, forgive me if I name only the four
whose contributions proved most far-reaching and decisive: Paul K.
Feyerabend of Berkeley, Ernest Nagel of Columbia, H. Pierre Noyes of
the Lawrence Radiation Laboratory, and my student, John L. Heilbron,
who has often worked closely with me in preparing a final version for
the press. I have found all their reservations and suggestions extremely
helpful, but I have no reason to believe (and some reason to doubt) that
either they or the others mentioned above approve in its entirety the
manuscript that results.
My final acknowledgments, to my parents, wife, and children, must
be of a rather different sort. In ways which I shall probably be the last to
recognize, each of them, too, has contributed intellectual ingredients to
my work. But they have also, in varying degrees, done something more
important. They have, that is, let it go on and even encouraged my
devotion to it. Anyone who has wrestled with a project like mine will
recognize what it has occasionally cost them. I do not know how to give
them thanks.
T. S. K.
February 1962
Vol. II, No. 2
13. I. Introduction: A Role for History
History, if viewed as a repository for more than anecdote or
chronology, could produce a decisive transformation in the image of
science by which we are now possessed. That image has previously been
drawn, even by scientists themselves, mainly from the study of finished
scientific achievements as these are recorded in the classics and, more
recently, in the textbooks from which each new scientific generation
learns to practice its trade. Inevitably, however, the aim of such books is
persuasive and pedagogic; a concept of science drawn from them is no
more likely to fit the enterprise that produced them than an image of a
national culture drawn from a tourist brochure or a language text. This
essay attempts to show that we have been misled by them in
fundamental ways. Its aim is a sketch of the quite different concept of
science that can emerge from the historical record of the research
activity itself.
Even from history, however, that new concept will not be forthcoming
if historical data continue to be sought and scrutinized mainly to answer
questions posed by the unhistorical stereotype drawn from science texts.
Those texts have, for example, often seemed to imply that the content of
science is uniquely exemplified by the observations, laws, and theories
described in their pages. Almost as regularly, the same books have been
read as saying that scientific methods are simply the ones illustrated by
the manipulative techniques used in gathering textbook data, together
with the logical operations employed when relating those data to the
textbook’s theoretical generalizations. The result has been a concept of
science with profound implications about its nature and development.
If science is the constellation of facts, theories, and methods collected
in current texts, then scientists are the men who, successfully or not,
have striven to contribute one or another element to that particular
constellation. Scientific development becomes the piecemeal process by
which these items have been
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14. The Structure of Scientific Revolutions
added, singly and in combination, to the ever growing stockpile that
constitutes scientific technique and knowledge. And history of science
becomes the discipline that chronicles both these successive increments
and the obstacles that have inhibited their accumulation. Concerned
with scientific development, the historian then appears to have two
main tasks. On the one hand, he must determine by what man and at
what point in time each contemporary scientific fact, law, and theory
was discovered or invented. On the other, he must describe and explain
the congeries of error, myth and superstition that have inhibited the
more rapid accumulation of the constituents of the modern science text.
Much research has been directed to these ends, and some still is.
In recent years, however, a few historians of science have been finding
it more and more difficult to fulfil the functions that the concept of
development-by-accumulation assigns to them. As chroniclers of an
incremental process, they discover that additional research makes it
harder, not easier, to answer questions like: When was oxygen
discovered? Who first conceived of energy conservation? Increasingly, a
few of them suspect that these are simply the wrong sorts of questions
to ask. Perhaps science does not develop by the accumulation of
individual discoveries and inventions. Simultaneously, these same
historians confront growing difficulties in distinguishing the “scientific”
component of past observation and belief from what their predecessors
had readily labeled “error” and “superstition.” The more carefully they
study, say, Aristotelian dynamics, phlogistic chemistry, or caloric
thermodynamics, the more certain they feel that those once current
views of nature were, as a whole, neither less scientific nor more the
product of human idiosyncrasy than those current today. If these out-of-
date beliefs are to be called myths, then myths can be produced by the
same sorts of methods and held for the same sorts of reasons that now
lead to scientific knowledge. If, on the other hand, they are to be called
science, then science has included bodies of belief quite incompatible
with the ones we hold today. Given these alternatives, the historian must
choose the latter. Out-of-
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15. Introduction: A Role for History
date theories are not in principle unscientific because they have been
discarded. That choice, however, makes it difficult to see scientific
development as a process of accretion. The same historical research that
displays the difficulties in isolating individual inventions and discoveries
gives ground for profound doubts about the cumulative process through
which these individual contributions to science were thought to have
been compounded.
The result of all these doubts and difficulties is a historiographic
revolution in the study of science, though one that is still in its early
stages. Gradually, and often without entirely realizing they are doing so,
historians of science have begun to ask new sorts of questions and to
trace different, and often less than cumulative, developmental lines for
the sciences. Rather than seeking the permanent contributions of an
older science to our present vantage, they attempt to display the
historical integrity of that science in its own time. They ask, for example,
not about the relation of Galileo’s views to those of modern science, but
rather about the relationship between his views and those of his group,
i.e., his teachers, contemporaries, and immediate successors in the
sciences. Furthermore, they insist upon studying the opinions of that
group and other similar ones from the viewpoint—usually very different
from that of modern science—that gives those opinions the maximum
internal coherence and the closest possible fit to nature. Seen through
the works that result, works perhaps best exemplified in the writings of
Alexandre Koyré, science does not seem altogether the same enterprise
as the one discussed by writers in the older historiographic tradition. By
implication, at least, these historical studies suggest the possibility of a
new image of science. This essay aims to delineate that image by making
explicit some of the new historiography’s implications.
What aspects of science will emerge to prominence in the course of
this effort? First, at least in order of presentation, is the insufficiency of
methodological directives, by themselves, to dictate a unique
substantive conclusion to many sorts of scientific questions. Instructed
to examine electrical or chemical phe-
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16. The Structure of Scientific Revolutions
nomena, the man who is ignorant of these fields but who knows what it
is to be scientific may legitimately reach any one of a number of
incompatible conclusions. Among those legitimate possibilities, the
particular conclusions he does arrive at are probably determined by his
prior experience in other fields, by the accidents of his investigation, and
by his own individual makeup. What beliefs about the stars, for
example, does he bring to the study of chemistry or electricity? Which of
the many conceivable experiments relevant to the new field does he
elect to perform first? And what aspects of the complex phenomenon
that then results strike him as particularly relevant to an elucidation of
the nature of chemical change or of electrical affinity? For the
individual, at least, and sometimes for the scientific community as well,
answers to questions like these are often essential determinants of
scientific development. We shall note, for example, in Section II that the
early developmental stages of most sciences have been characterized by
continual competition between a number of distinct views of nature,
each partially derived from, and all roughly compatible with, the
dictates of scientific observation and method. What differentiated these
various schools was not one or another failure of method— they were all
“scientific”—but what we shall come to call their incommensurable ways
of seeing the world and of practicing science in it. Observation and
experience can and must drastically restrict the range of admissible
scientific belief, else there would be no science. But they cannot alone
determine a particular body of such belief. An apparently arbitrary
element, compounded of personal and historical accident, is always a
formative ingredient of the beliefs espoused by a given scientific
community at a given time.
That element of arbitrariness does not, however, indicate that any
scientific group could practice its trade without some set of received
beliefs. Nor does it make less consequential the particular constellation
to which the group, at a given time, is in fact committed. Effective
research scarcely begins before a scientific community thinks it has
acquired firm answers to questions like the following: What are the
fundamental entities
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17. Introduction: A Role for History
of which the universe is composed? How do these interact with each
other and with the senses? What questions may legitimately be asked
about such entities and what techniques employed in seeking solutions?
At least in the mature sciences, answers (or full substitutes for answers)
to questions like these are firmly embedded in the educational initiation
that prepares and licenses the student for professional practice. Because
that education is both rigorous and rigid, these answers come to exert a
deep hold on the scientific mind. That they can do so does much to
account both for the peculiar efficiency of the normal research activity
and for the direction in which it proceeds at any given time. When
examining normal science in Sections III, IV, and V, we shall want finally
to describe that research as a strenuous and devoted attempt to force
nature into the conceptual boxes supplied by professional education.
Simultaneously, we shall wonder whether research could proceed
without such boxes, whatever the element of arbitrariness in their
historic origins and, occasionally, in their subsequent development.
Yet that element of arbitrariness is present, and it too has an
important effect on scientific development, one which will be examined
in detail in Sections VI, VII, and VIII. Normal science, the activity in
which most scientists inevitably spend almost all their time, is
predicated on the assumption that the scientific community knows what
the world is like. Much of the success of the enterprise derives from the
community’s willingness to defend that assumption, if necessary at
considerable cost. Normal science, for example, often suppresses
fundamental novelties because they are necessarily subversive of its
basic commitments. Nevertheless, so long as those commitments retain
an element of the arbitrary, the very nature of normal research ensures
that novelty shall not be suppressed for very long. Sometimes a normal
problem, one that ought to be solvable by known rules and procedures,
resists the reiterated onslaught of the ablest members of the group
within whose competence it falls. On other occasions a piece of
equipment designed and constructed for the purpose of normal
research fails
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18. The Structure of Scientific Revolutions
to perform in the anticipated manner, revealing an anomaly that cannot,
despite repeated effort, be aligned with professional expectation. In
these and other ways besides, normal science repeatedly goes astray.
And when it does—when, that is, the profession can no longer evade
anomalies that subvert the existing tradition of scientific practice—then
begin the extraordinary investigations that lead the profession at last to
a new set of commitments, a new basis for the practice of science. The
extraordinary episodes in which that shift of professional commitments
occurs are the ones known in this essay as scientific revolutions. They
are the tradition-shattering complements to the tradition-bound activity
of normal science.
The most obvious examples of scientific revolutions are those famous
episodes in scientific development that have often been labeled
revolutions before. Therefore, in Sections IX and X, where the nature of
scientific revolutions is first directly scrutinized, we shall deal repeatedly
with the major turning points in scientific development associated with
the names of Copernicus, Newton, Lavoisier, and Einstein. More clearly
than most other episodes in the history of at least the physical sciences,
these display what all scientific revolutions are about. Each of them
necessitated the community’s rejection of one time-honored scientific
theory in favor of another incompatible with it. Each produced a
consequent shift in the problems available for scientific scrutiny and in
the standards by which the profession determined what should count as
an admissible problem or as a legitimate problem-solution. And each
transformed the scientific imagination in ways that we shall ultimately
need to describe as a transformation of the world within which scientific
work was done. Such changes, together with the controversies that
almost always accompany them, are the defining characteristics of
scientific revolutions.
These characteristics emerge with particular clarity from a study of,
say, the Newtonian or the chemical revolution. It is, however, a
fundamental thesis of this essay that they can also be retrieved from the
study of many other episodes that were not so obviously revolutionary.
For the far smaller professional
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19. Introduction: A Role for History
group affected by them, Maxwell’s equations were as revolutionary as
Einstein’s, and they were resisted accordingly. The invention of other
new theories regularly, and appropriately, evokes the same response
from some of the specialists on whose area of special competence they
impinge. For these men the new theory implies a change in the rules
governing the prior practice of normal science. Inevitably, therefore, it
reflects upon much scientific work they have already successfully
completed. That is why a new theory, however special its range of
application, is seldom or never just an increment to what is already
known. Its assimilation requires the reconstruction of prior theory and
the re-evaluation of prior fact, an intrinsically revolutionary process that
is seldom completed by a single man and never overnight. No wonder
historians have had difficulty in dating precisely this extended process
that their vocabulary impels them to view as an isolated event.
Nor are new inventions of theory the only scientific events that have
revolutionary impact upon the specialists in whose domain they occur.
The commitments that govern normal science specify not only what
sorts of entities the universe does contain, but also, by implication, those
that it does not. It follows, though the point will require extended
discussion, that a discovery like that of oxygen or X-rays does not simply
add one more item to the population of the scientist’s world. Ultimately
it has that effect, but not until the professional community has re-
evaluated traditional experimental procedures, altered its conception of
entities with which it has long been familiar, and, in the process, shifted
the network of theory through which it deals with the world. Scientific
fact and theory are not categorically separable, except perhaps within a
single tradition of normal-scientific practice. That is why the unexpected
discovery is not simply factual in its import and why the scientist’s world
is qualitatively transformed as well as quantitatively enriched by
fundamental novelties of either fact or theory.
This extended conception of the nature of scientific revolutions is the
one delineated in the pages that follow. Admittedly the extension strains
customary usage. Nevertheless, I shall con-
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20. The Structure of Scientific Revolutions
tinue to speak even of discoveries as revolutionary, because it is just the
possibility of relating their structure to that of, say, the Copernican
revolution that makes the extended conception seem to me so
important. The preceding discussion indicates how the complementary
notions of normal science and of scientific revolutions will be developed
in the nine sections immediately to follow. The rest of the essay attempts
to dispose of three remaining central questions. Section XI, by
discussing the textbook tradition, considers why scientific revolutions
have previously been so difficult to see. Section XII describes the
revolutionary competition between the proponents of the old normal-
scientific tradition and the adherents of the new one. It thus considers
the process that should somehow, in a theory of scientific inquiry,
replace the confirmation or falsification procedures made familiar by
our usual image of science. Competition between segments of the
scientific community is the only historical process that ever actually
results in the rejection of one previously accepted theory or in the
adoption of another. Finally, Section XIII will ask how development
through revolutions can be compatible with the apparently unique
character of scientific progress. For that question, however, this essay
will provide no more than the main outlines of an answer, one which
depends upon characteristics of the scientific community that require
much additional exploration and study.
Undoubtedly, some readers will already have wondered whether
historical study can possibly effect the sort of conceptual transformation
aimed at here. An entire arsenal of dichotomies is available to suggest
that it cannot properly do so. History, we too often say, is a purely
descriptive discipline. The theses suggested above are, however, often
interpretive and sometimes normative. Again, many of my
generalizations are about the sociology or social psychology of scientists;
yet at least a few of my conclusions belong traditionally to logic or
epistemology. In the preceding paragraph I may even seem to have
violated the very influential contemporary distinction between “the
context of discovery” and “the context of justifica-
Vol. II, No. 2
21. Introduction: A Role for History
tion.” Can anything more than profound confusion be indicated by this
admixture of diverse fields and concerns?
Having been weaned intellectually on these distinctions and others
like them, I could scarcely be more aware of their import and force. For
many years I took them to be about the nature of knowledge, and I still
suppose that, appropriately recast, they have something important to
tell us. Yet my attempts to apply them, even grosso modo, to the actual
situations in which knowledge is gained, accepted, and assimilated have
made them seem extraordinarily problematic. Rather than being
elementary logical or methodological distinctions, which would thus be
prior to the analysis of scientific knowledge, they now seem integral
parts of a traditional set of substantive answers to the very questions
upon which they have been deployed. That circularity does not at all
invalidate them. But it does make them parts of a theory and, by doing
so, subjects them to the same scrutiny regularly applied to theories in
other fields. If they are to have more than pure abstraction as their
content, then that content must be discovered by observing them in
application to the data they are meant to elucidate. How could history
of science fail to be a source of phenomena to which theories about
knowledge may legitimately be asked to apply?
Vol. II, No. 2
22. II. The Route to Normal Science
In this essay, ‘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 or all of its successful applications, and
compare these applications with exemplary observations and
experiments. Before such books became popular early in the nineteenth
century (and until even more recently in the newly matured sciences),
many of the famous classics of science fulfilled a similar function.
Aristotle’s Physica, Ptolemy’s Almagest, Newton’s Principia and Opticks,
Franklin’s Electricity, Lavoisier’s Chemistry, and Lyell’s Geology—these and
many other works served for a time implicitly to define the legitimate
problems and methods of a research field for succeeding generations of
practitioners. 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
scientific activity. Simultaneously, it was sufficiently open-ended to leave
all sorts of problems for the redefined group of practitioners to resolve.
Achievements that share these two characteristics I shall henceforth
refer to as ‘paradigms,’ a term that relates closely to ‘normal science.’ By
choosing it, I mean to suggest that some accepted examples of actual
scientific practice—examples which include law, theory, application, and
instrumentation together— provide models from which spring particular
coherent traditions of scientific research. These are the traditions which
the historian describes under such rubrics as ‘Ptolemaic astronomy’ (or
‘Copernican’), ‘Aristotelian dynamics’ (or ‘Newtonian’), ‘corpuscular
optics’ (or ‘wave optics’), and so on. The study of
Vol. II, No. 2
23. The Route to Normal Science
paradigms, including many that are far more specialized than those
named illustratively above, is what mainly prepares the student for
membership in the particular scientific community with 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 over 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 prerequisites for normal science,
i.e., for the genesis and continuation of a particular research tradition.
Because in this essay the concept of a paradigm will often substitute
for a variety of familiar notions, more will need to be said about the
reasons for its introduction. Why is the concrete scientific achievement,
as a locus of professional commitment, prior to the various concepts,
laws, theories, and points of view that may be abstracted from it? In
what sense is the shared paradigm a fundamental unit for the student of
scientific development, a unit that cannot be fully reduced to logically
atomic components which might function in its stead? When we
encounter them in Section V, 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 abstract
discussion will depend, however, upon a previous exposure to examples
of normal science or of paradigms in operation. In particular, both these
related concepts will be clarified by noting that there can be a sort of
scientific research without paradigms, or at least without any so
unequivocal and so binding as the ones named above. Acquisition of a
paradigm and of the more esoteric type of research it permits is a sign of
maturity in the development of any given scientific field.
If the historian traces the scientific knowledge of any selected group
of related phenomena backward in time, he is likely to encounter some
minor variant of a pattern here illustrated from the history of physical
optics. Today’s physics textbooks tell the
Vol. II, No. 2
24. The Structure of Scientific Revolutions
student that light is photons, i.e., quantum-mechanical entities that
exhibit some characteristics of waves and some of particles. Research
proceeds accordingly, or rather according to the more elaborate and
mathematical characterization from which this usual verbalization is
derived. That characterization of light is, however, scarcely half a
century old. Before it was developed by Planck, Einstein, and others
early in this century, physics texts taught that light was transverse wave
motion, a conception rooted in a paradigm that derived ultimately from
the optical writings of Young and Fresnel in the early nineteenth
century. Nor was the wave theory the first to be embraced by almost all
practitioners of optical science. During the eighteenth century the
paradigm for this field was provided by Newton’s Opticks, which taught
that light was material corpuscles. At that time physicists sought
evidence, as the early wave theorists had not, of the pressure exerted by
light particles impinging on solid bodies.1
These transformations of the paradigms of physical optics are
scientific revolutions, and the successive transition from one paradigm
to another via revolution is the usual developmental pattern of mature
science. It is not, however, the pattern characteristic of the period before
Newton’s work, and that is the contrast that concerns us here. No period
between remote antiquity and the end of the seventeenth century
exhibited a single generally accepted view about the nature of light.
Instead there were a number of competing schools and sub-schools,
most of them espousing one variant or another of Epicurean,
Aristotelian, or Platonic theory. One group took light to be particles
emanating from material bodies; for another it was a modification of the
medium that intervened between tie body and the eye; still another
explained light in terms of an interaction of the medium with an
emanation from the eye; and there were other combinations and
modifications besides. Each of the corresponding schools derived
strength from its relation to some particular metaphysic, and each
emphasized, as para-
Joseph Priestley, The History and Present State of Discoveries Relating to Vision, Light, and
Colours (London, 1772), pp. 385-90.
Vol. II, No. 2
25. The Route to Normal Science
digmatic observations, the particular cluster of optical phenomena that
its own theory could do most to explain. Other observations were dealt
with by ad hoc elaborations, or they remained as outstanding problems
for further research.2
At various times all these schools made significant contributions to
the body of concepts, phenomena, and techniques from which Newton
drew the first nearly uniformly accepted paradigm for physical optics.
Any definition of the scientist that excludes at least the more creative
members of these various schools will exclude their modern successors
as well. Those men were scientists. Yet anyone examining a survey of
physical optics before Newton may well conclude that, though the field’s
practitioners were scientists, the net result of their activity was
something less than science. Being able to take no common body of
belief for granted, each writer on physical optics felt forced to build his
field anew from its foundations. In doing so, his choice of supporting
observation and experiment was relatively free, for there was no
standard set of methods or of phenomena that every optical writer felt
forced to employ and explain. Under these circumstances, the dialogue
of the resulting books was often directed as much to the members of
other schools as it was to nature. That pattern is not unfamiliar in a
number of creative fields today, nor is it incompatible with significant
discovery and invention. It is not, however, the pattern of development
that physical optics acquired after Newton and that other natural
sciences make familiar today.
The history of electrical research in the first half of the eighteenth
century provides a more concrete and better known example of the way
a science develops before it acquires its first universally received
paradigm. During that period there were almost as many views about
the nature of electricity as there were important electrical
experimenters, men like Hauksbee, Gray, Desaguliers, Du Fay, Nollett,
Watson, Franklin, and others. All their numerous concepts of electricity
had something in common—they were partially derived from one or an-
Vasco Ronchi, Histoire de la lumière, trans. Jean Taton (Paris, 1956), chaps. i-iv.
Vol. II, No. 2
26. The Structure of Scientific Revolutions
other version of the mechanico-corpuscular philosophy that guided all
scientific research of the day. In addition, all were components of real
scientific theories, of theories that had been drawn in part from
experiment and observation and that partially determined the choice
and interpretation of additional problems undertaken in research. Yet
though all the experiments were electrical and though most of the
experimenters read each other’s works, their theories had no more than
a family resemblance.3
One early group of theories, following seventeenth-century practice,
regarded attraction and factional generation as the fundamental
electrical phenomena. This group tended to treat repulsion as a
secondary effect due to some sort of mechanical rebounding and also to
postpone for as long as possible both discussion and systematic research
on Gray’s newly discovered effect, electrical conduction. Other
“electricians” (the term is their own) took attraction and repulsion to be
equally elementary manifestations of electricity and modified their
theories and research accordingly. (Actually, this group is remarkably
small—even Franklin’s theory never quite accounted for the mutual
repulsion of two negatively charged bodies.) But they had as much
difficulty as the first group in accounting simultaneously for any but the
simplest conduction effects. Those effects, however, provided the
starting point for still a third group, one which tended to speak of
electricity as a “fluid” that could run through conductors rather than as
an “effluvium” that emanated from non-conductors. This group, in its
turn, had difficulty reconciling its theory with a number of attractive
Duane Roller and Duane H. D. Roller, The Development of the Concept of Electric Charge:
Electricity from the Greeks to Coulomb (“Harvard Case Histories in Experimental Science,”
Case 8; Cambridge, Mass., 1954); and I. B. Cohen, Franklin and Newton: An Inquiry into
Speculative Newtonian Experimental Science and Franklin’s Work in Electricity as an
Example Thereof (Philadelphia, 1956), chaps, vii-xii. For some of the analytic detail in the
paragraph that follows in the text, I am indebted to a still unpublished paper by my
student John L. Heilbron. Pending its publication, a somewhat more extended and more
precise account of the emergence of Franklin’s paradigm is included in T. S. Kuhn, “The
Function of Dogma in Scientific Research,” in A. C. Crombie (ed.), “Symposium on the
History of Science, University of Oxford, July 9-15, 1961,” to be published by Heinemann
Educational Books, Ltd.
Vol. II, No. 2
27. The Route to Normal Science
repulsive effects. Only through the work of Franklin and his immediate
successors did a theory arise that could account with something like
equal facility for very nearly all these effects and that therefore could
and did provide a subsequent generation of “electricians” with a
common paradigm for its research.
Excluding those fields, like mathematics and astronomy, in which the
first firm paradigms date from prehistory and also those, like
biochemistry, that arose by division and recombination of specialties
already matured, the situations outlined above are historically typical.
Though it involves my continuing to employ the unfortunate
simplification that tags an extended historical episode with a single and
somewhat arbitrarily chosen name (e.g., Newton or Franklin), I suggest
that similar fundamental disagreements characterized, for example, the
study of motion before Aristotle and of statics before Archimedes, the
study of heat before Black, of chemistry before Boyle and Boerhaave,
and of historical geology before Hutton. In parts of biology—the study of
heredity, for example—the first universally received paradigms are still
more recent; and it remains an open question what parts of social
science have yet acquired such paradigms at all. History suggests that
the road to a firm research consensus is extraordinarily arduous.
History also suggests, however, some reasons for the difficulties
encountered on that road. In the absence of a paradigm or some
candidate for paradigm, all of the 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 far more nearly random activity than the
one that subsequent scientific development makes familiar.
Furthermore, in the absence of a reason for seeking some particular
form of more recondite information, early fact-gathering is usually
restricted to the wealth of data that lie ready to hand. The resulting pool
of facts contains those accessible to casual observation and experiment
together with some of the more esoteric data retrievable from
established crafts like medicine, calendar making, and metallurgy.
Because the crafts are one readily accessible source of facts that could
not have been casually discovered, technology
Vol. II, No. 2
28. The Structure of Scientific Revolutions
has often played a vital role in the emergence of new sciences.
But though this sort of fact-collecting has been essential to the origin
of many significant sciences, anyone who examines, for example, Pliny’s
encyclopedic writings or the Baconian natural histories of the
seventeenth century will discover that it produces a morass. One
somehow hesitates to call the literature that results scientific. The
Baconian “histories” of heat, color, wind, mining, and so on, are filled
with information, some of it recondite. But they juxtapose facts that will
later prove revealing (e.g., heating by mixture) with others (e.g., the
warmth of dung heaps) that will for some time remain too complex to
be integrated with theory at all.4 In addition, since any description must
be partial, the typical natural history often omits from its immensely
circumstantial accounts just those details that later scientists will find
sources of important illumination. Almost none of the early “histories”
of electricity, for example, mention that chaff, attracted to a rubbed
glass rod, bounces off again. That effect seemed mechanical, not
electrical.5 Moreover, since the casual fact-gatherer seldom possesses the
time or the tools to be critical, the natural histories often juxtapose
descriptions like the above with others, say, heating by antiperistasis (or
by cooling), that we are now quite unable to confirm.8 Only very
occasionally, as in the cases of ancient statics, dynamics, and geometrical
optics, do facts collected with so little guidance from pre-established
theory speak with sufficient clarity to permit the emergence of a first
This is the situation that creates the schools characteristic of the early
stages of a science’s development. No natural history can be interpreted
in the absence of at least some implicit body
Compare the sketch for a natural history of heat in Bacon’s Novum Organum, Vol. VIII
of The Works of Francis Bacon, ed. J. Spedding, R. L. Ellis, and D. D. Heath (New York,
1869), pp. 179-203.
Roller and Roller, op. cit., pp. 14, 22, 28, 43. Only after the work recorded in the last of
these citations do repulsive effects gain general recognition as unequivocally electrical.
Bacon, op. cit., pp. 235, 337, says, “Water slightly warm is more easily frozen than quite
cold.” For a partial account of the earlier history of this strange observation, see Marshall
Clagett, Giovanni Marliani and Late Medieval Physics (New York, 1941), chap. iv.
Vol. II, No. 2
29. The Route to Normal Science
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 at hand—it must be externally supplied, perhaps by a current
metaphysic, by another science, or by personal and historical accident.
No wonder, then, that in the early stages of the development of any
science different men confronting the same range of phenomena, but
not usually all the same particular phenomena, describe and interpret
them in different ways. What is surprising, and perhaps also unique in
its degree to the fields we call science, is that such initial divergences
should ever largely disappear.
For they do disappear to a very considerable extent and then
apparently once and for all. Furthermore, their disappearance is usually
caused by the triumph of one of the pre-paradigm schools, which,
because of its own characteristic beliefs and preconceptions, emphasized
only some special part of the too sizable and inchoate pool of
information. Those electricians who thought electricity a fluid and
therefore gave particular emphasis to conduction provide an excellent
case in point. Led by this belief, which could scarcely cope with the
known multiplicity of attractive and repulsive effects, several of them
conceived the idea of bottling the electrical fluid. The immediate fruit of
their efforts was the Leyden jar, a device which might never have been
discovered by a man exploring nature casually or at random, but which
was in fact independently developed by at least two investigators in the
early 1740’s.7 Almost from the start of his electrical researches, Franklin
was particularly concerned to explain that strange and, in the event,
particularly revealing piece of special apparatus. His success in doing so
provided the most effective of the arguments that made his theory a
paradigm, though one that was still unable to account for quite all the
known cases of electrical repulsion.8 To be accepted as a paradigm, a
theory must seem better than its competitors, but
Roller and Roller, op. cit., pp. 51-54.
The troublesome case was the mutual repulsion of negatively charged bodies, for
which see Cohen, op. cit., pp. 491-94, 531-43.
Vol. II, No. 2
30. The Structure of Scientific Revolutions
it need not, and in fact never does, explain all the facts with which it can
be confronted.
What the fluid theory of electricity did for the subgroup that held it,
the Franklinian paradigm later did for the entire group of electricians. It
suggested which experiments would be worth performing and which,
because directed to secondary or to overly complex manifestations of
electricity, would not. Only the paradigm did the job far more
effectively, partly because the end of interschool debate ended the
constant reiteration of fundamentals and partly because the confidence
that they were on the right track encouraged scientists to undertake
more precise, esoteric, and consuming sorts of work.9 Freed from the
concern with any and all electrical phenomena, the united group of
electricians could pursue selected phenomena in far more detail,
designing much special equipment for the task and employing it more
stubbornly and systematically than electricians had ever done before.
Both fact collection and theory articulation became highly directed
activities. The effectiveness and efficiency of electrical research
increased accordingly, providing evidence for a societal version of
Francis Bacon’s acute methodological dictum: “Truth emerges more
readily from error than from confusion.”10
We shall be examining the nature of this highly directed or paradigm-
based research in the next section, but must first note briefly how the
emergence of a paradigm affects the structure of the group that
practices the field. When, in the development of a natural science, an
individual or group first produces a synthesis able to attract most of the
next generation’s practitioners, the older schools gradually disappear. In
part their disappear-
It should be noted that the acceptance of Franklin’s theory did not end quite all debate.
In 1759 Robert Symmer proposed a two-fluid version of that theory, and for many years
thereafter electricians were divided about whether electricity was a single fluid or two.
But the debates on this subject only confirm what has been said above about the manner
in which a universally recognized achievement unites the profession. Electricians,
though they continued divided on this point, rapidly concluded that no experimental
tests could distinguish the two versions of the theory and that they were therefore
equivalent. After that, both schools could and did exploit all the benefits that the
Franklinian theory provided (ibid., pp. 543-46,548-54).
Bacon, op. cit., p. 210.
Vol. II, No. 2
31. The Route to Normal Science
ance is caused by their members’ conversion to the new paradigm. But
there are always some men who cling to 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 some other
group.11 Historically, they have often simply stayed in the departments
of philosophy from which so many of the special sciences have been
spawned. As these indications hint, it is sometimes just its reception of a
paradigm that transforms a group previously interested merely in the
study of nature into a profession or, at least, a discipline. In the sciences
(though not in fields like medicine, technology, and law, of which the
principal raison d’être is an external social need), the formation of
specialized journals, the foundation of specialists’ 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. At least this was the
case between the time, a century and a half ago, when the institutional
pattern of scientific specialization first developed and the very recent
time when the paraphernalia of specialization acquired a prestige of
their own.
The more rigid definition of the scientific group has other
consequences. When the 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 justify-
The history of electricity provides an excellent example which could be duplicated
from the careers of Priestley, Kelvin, and others. Franklin reports that Nollet, who at mid-
century was the most influential of the Continental electricians, “lived to see himself the
last of his Sect, except Mr. B.—his Élève and immediate Disciple” (Max Farrand [ed.],
Benjamin Franklin’s Memoirs [Berkeley, Calif., 1949], pp. 384-86). More interesting,
however, is the endurance of whole schools in increasing isolation from professional
science. Consider, for example, the case of astrology, which was once an integral part of
astronomy. Or consider the continuation in the late eighteenth and early nineteenth
centuries of a previously respected tradition of “romantic” chemistry. This is the
tradition discussed by Charles C. Gillispie in “The Encyclopédie and the Jacobin
Philosophy of Science: A Study in Ideas and Consequences,” Critical Problems in the
History of Science, ed. Marshall Clagett (Madison, Wis., 1959), pp. 255-89; and “The
Formation of Lamarck’s Evolutionary Theory,” Archives internationales d’histoire des
sciences, XXXVII (1956), 323-38.
Vol. II, No. 2
32. The Structure of Scientific Revolutions
ing the use of each concept introduced. That can be left to the writer of
textbooks. Given a textbook, however, the creative scientist can begin his
research where it leaves off and thus concentrate exclusively upon the
subtlest and most esoteric aspects of the natural phenomena that
concern his group. And as he does this, his research communiqués will
begin to change in ways whose evolution has been too little studied but
whose modern end products are obvious to all and oppressive to many.
No longer will his researches usually be embodied in books addressed,
like Franklin’s Experiments . . . on Electricity or Darwin’s Origin of Species,
to anyone who might be interested in the subject matter of the field.
Instead they will usually appear as brief articles addressed only 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.
Today in the sciences, books are usually either texts or retrospective
reflections upon one aspect or another of the scientific life. The scientist
who writes one is more likely to find his professional reputation
impaired than enhanced. Only in the earlier, pre-paradigm, stages of the
development of the various sciences did the book ordinarily possess the
same relation to professional achievement that it still retains in other
creative fields. And only in those fields that still retain the book, with or
without the article, as a vehicle for research communication are the
lines of professionalization still so loosely drawn that the layman may
hope to follow progress by reading the practitioners’ original reports.
Both in mathematics and astronomy, research reports had ceased
already in antiquity to be intelligible to a generally educated audience.
In dynamics, research became similarly esoteric in the later Middle
Ages, and it recaptured general intelligibility only briefly during the
early seventeenth century when a new paradigm replaced the one that
had guided medieval research. Electrical research began to require
translation for the layman before the end of the eighteenth century, and
most other fields of physical science ceased to be generally accessible in
the nineteenth. During the same two cen-
Vol. II, No. 2
33. The Route to Normal Science
turies similar transitions can be isolated in the various parts of the
biological sciences. In parts of the social sciences they may well be
occurring today. Although it has become customary, and is surely
proper, to deplore the widening gulf that separates the professional
scientist from his colleagues in other fields, too little attention is paid to
the essential relationship between that gulf and the mechanisms
intrinsic to scientific advance.
Ever since prehistoric antiquity one field of study after another has
crossed the divide between what the historian might call its prehistory
as a science and its history proper. These transitions to maturity have
seldom been so sudden or so unequivocal as my necessarily schematic
discussion may have implied. But neither have they been historically
gradual, coextensive, that is to say, with the entire development of the
fields within which they occurred. Writers on electricity during the first
four decades of the eighteenth century possessed far more information
about electrical phenomena than had their sixteenth-century
predecessors. During the half-century after 1740, few new sorts of
electrical phenomena were added to their lists. Nevertheless, in
important respects, the electrical writings of Cavendish, Coulomb, and
Volta in the last third of the eighteenth century seem further removed
from those of Gray, Du Fay, and even Franklin than are the writings of
these early eighteenth-century electrical discoverers from those of the
sixteenth century.12 Sometime between 1740 and 1780, electricians were
for the first time enabled to take the foundations of their field for
granted. From that point they pushed on to more concrete and
recondite problems, and increasingly they then reported their results in
articles addressed to other electricians rather than in books addressed to
the learned world at large. As a group they achieved what had been
gained by astronomers in antiquity
The post-Franklinian developments include an immense increase in the sensitivity of
charge detectors, the first reliable and generally diffused techniques for measuring
charge, the evolution of the concept of capacity and its relation to a newly refined notion
of electric tension, and the quantification of electrostatic force. On all of these see Roller
and Roller, op. cit., pp. 66-81; W. C. Walker, “The Detection and Estimation of Electric
Charges in the Eighteenth Century,” Annals of Science, I (1936), 66-100; and Edmund
Hoppe, Geschichte der Elektrizität (Leipzig, 1884), Part I, chaps, iii-iv.
Vol. II, No. 2
34. The Structure of Scientific Revolutions
and by students of motion in the Middle Ages, of physical optics in the
late seventeenth century, and of historical geology in the early
nineteenth. They had, that is, achieved a paradigm that proved able to
guide the whole group’s research. Except with the advantage of
hindsight, it is hard to find another criterion that so clearly proclaims a
field a science.
Vol. II, No. 2
35. III. The Nature of Normal Science
What then is the nature of the more professional and esoteric
research that a group’s reception of a single paradigm permits? If the
paradigm represents work that has been done once and for all, what
further problems does it leave the united group to resolve? Those
questions will seem even more urgent if we now note one respect in
which the terms used so far may be misleading. In its established usage,
a paradigm is an accepted model or pattern, and that aspect of its
meaning has enabled me, lacking a better word, to appropriate
‘paradigm’ here. But it will shortly be clear that the sense of ‘model’ and
‘pattern’ that permits the appropriation is not quite the one usual in
defining ‘paradigm.’ In grammar, for example, ‘amo, amas, amat’ is a
paradigm because it displays the pattern to be used in conjugating a
large number of other Latin verbs, e.g., in producing ‘laudo, laudas,
laudat.’ In this standard application, the paradigm functions by
permitting the replication of examples any one of which could in
principle serve to replace it. In a science, on the other hand, a paradigm
is rarely an object for replication. Instead, like an accepted judicial
decision in the common law, it is an object for further articulation and
specification under new or more stringent conditions.
To see how this can be so, we must recognize how very limited in both
scope and precision a paradigm can be at the time of its first
appearance. Paradigms gain their status because they are more
successful than their competitors in solving a few problems that the
group of practitioners has come to recognize as acute. To be more
successful is not, however, to be either completely successful with a
single problem or notably successful with any large number. The success
of a paradigm—whether Aristotle’s analysis of motion, Ptolemy’s
computations of planetary position, Lavoisier’s application of the
balance, or Maxwell’s mathematization of the electromagnetic field—is
at the start largely a promise of success discoverable in selected and
Vol. II, No. 2
36. The Structure of Scientific Revolutions
still incomplete examples. Normal science consists in the actualization
of that promise, an actualization achieved by extending the knowledge
of those facts that the paradigm displays as particularly revealing, by
increasing the extent of the match between those facts and the
paradigm’s predictions, and by further articulation of the paradigm
Few people who are not actually practitioners of a mature science
realize how much mop-up work of this sort a paradigm leaves to be done
or quite how fascinating such work can prove in the execution. And
these points need to be understood. Mop-ping-up operations are what
engage most scientists throughout their careers. They constitute what I
am here calling normal science. Closely examined, whether historically
or in the contemporary laboratory, that enterprise seems an attempt to
force nature into the preformed 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.1 Instead,
normal-scientific research is directed to the articulation of those
phenomena and theories that the paradigm already supplies.
Perhaps these are defects. The areas investigated by normal science
are, of course, minuscule; the enterprise now 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 upon a small range of relatively esoteric problems,
the paradigm forces scientists to investigate some part of nature in a
detail and depth that would otherwise be 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 effectively. At that point scientists begin
to behave differently, and the nature of their research problems
changes. In the interim, however, during the
Bernard Barber, “Resistance by Scientists to Scientific Discovery,” Science, CXXXIV
(1961), 596-602.
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37. The Nature of Normal Science
period when the paradigm is successful, the profession will have solved
problems that its members could scarcely have imagined and would
never have undertaken without commitment to the paradigm. And at
least part of that achievement always proves to be permanent.
To display more clearly what is meant by normal or paradigm-based
research, let me now attempt to classify and illustrate the problems of
which normal science principally consists. For convenience I postpone
theoretical activity and begin with fact-gathering, that is, with the
experiments and observations described in the technical journals
through which scientists inform their professional colleagues of the
results of their continuing research. On what aspects of nature do
scientists ordinarily report? What determines their choice? And, since
most scientific observation consumes much time, equipment, and
money, what motivates the scientist to pursue that choice to a
There are, I think, only three normal foci for factual scientific
investigation, and they are neither always nor permanently distinct.
First is that class of facts that the paradigm has shown to be particularly
revealing of the nature of things. By employing them in solving
problems, the paradigm has made them worth determining both with
more precision and in a larger variety of situations. At one time or
another, these significant factual determinations have included: in
astronomy—stellar position and magnitude, the periods of eclipsing
binaries and of planets; in physics—the specific gravities and
compressibilities of materials, wave lengths and spectral intensities,
electrical conductivities and contact potentials; and in chemistry—
composition and combining weights, boiling points and acidity of
solutions, structural formulas and optical activities. Attempts to increase
the accuracy and scope with which facts like these are known occupy a
significant fraction of the literature of experimental and observational
science. Again and again complex special apparatus has been designed
for such purposes, and the invention, construction, and deployment of
that apparatus have demanded first-rate talent, much time, and
considerable financial
Vol. II, No. 2
38. The Structure of Scientific Revolutions
backing. Synchrotrons and radiotelescopes are only the most recent
examples of the lengths to which research workers will go if a paradigm
assures them that the facts they seek are important. From Tycho Brahe
to E. O. Lawrence, some scientists have acquired great reputations, not
from any novelty of their discoveries, but from the precision, reliability,
and scope of the methods they developed for the redetermination of a
previously known sort of fact.
A second usual but smaller class of factual determinations is directed
to those facts that, though often without much intrinsic interest, can be
compared directly with predictions from the paradigm theory. As we
shall see shortly, when I turn from the experimental to the theoretical
problems of normal science, there are seldom many areas in which a
scientific theory, particularly if it is cast in a predominantly
mathematical form, can be directly compared with nature. No more
than three such areas are even yet accessible to Einstein’s general theory
of relativity.2 Furthermore, even in those areas where application is
possible, it often demands theoretical and instrumental approximations
that severely limit the agreement to be expected. Improving that
agreement or finding new areas in which agreement can be
demonstrated at all presents a constant challenge to the skill and
imagination of the experimentalist and observer. Special telescopes to
demonstrate the Copernican prediction of annual parallax; Atwood’s
machine, first invented almost a century after the Principia, to give the
first unequivocal demonstration of Newton’s second law; Foucault’s
apparatus to show that the speed of light is greater in air than in water;
or the gigantic scintillation counter designed to demonstrate the
existence of
The only long-standing check point still generally recognized is the precession of
Mercury’s perihelion. The red shift in the spectrum of light from distant stars can be
derived from considerations more elementary than general relativity, and the same may
be possible for the bending of light around the sun, a point now in some dispute. In any
case, measurements of the latter phenomenon remain equivocal. One additional check
point may have been established very recently: the gravitational shift of Mossbauer
radiation. Perhaps there will soon be others in this now active but long dormant field.
For an up-to-date capsule account of the problem, see L. I. Schiff, “A Report on the NASA
Conference on Experimental Tests of Theories of Relativity,” Physics Today, XIV (1961),
Vol. II, No. 2
39. The Nature of Normal Science
the neutrino—these pieces of special apparatus and many others like
them illustrate the immense effort and ingenuity that have been
required to bring nature and theory into closer and closer agreement.3
That attempt to demonstrate agreement is a second type of normal
experimental work, and it is even more obviously dependent than the
first upon a paradigm. The existence of the paradigm sets the problem
to be solved; often the paradigm theory is implicated directly in the
design of apparatus able to solve the problem. Without the Principia, for
example, measurements made with the Atwood machine would have
meant nothing at all.
A third class of experiments and observations exhausts, I think, the
fact-gathering activities of normal science. It consists of empirical work
undertaken to articulate the paradigm theory, resolving some of its
residual ambiguities and permitting the solution of problems to which it
had previously only drawn attention. This class proves to be the most
important of all, and its description demands its subdivision. In the
more mathematical sciences, some of the experiments aimed at
articulation are directed to the determination of physical constants.
Newton’s work, for example, indicated that the force between two unit
masses at unit distance would be the same for all types of matter at all
positions in the universe. But his own problems could be solved without
even estimating the size of this attraction, the universal gravitational
constant; and no one else devised apparatus able to determine it for a
century after the Principia appeared. Nor was Cavendish’s famous
determination in the 1790’s the last. Because of its central position in
physical theory, improved values of the gravitational constant have been
the object of repeated efforts ever since by a number of outstanding
For two of the parallax telescopes, see Abraham Wolf, A History of Science, Technology,
and Philosophy in the Eighteenth Century (2d ed.; London, 1952), pp. 103-5. For the
Atwood machine, see N. R. Hanson, Patterns of Discovery (Cambridge, 1958), pp. 100-102,
207-8. For the last two pieces of special apparatus, see M. L. Foucault, “Méthode générale
pour mesurer la vitesse de la lumière dans l’air et les milieux transparants. Vitesses
relatives de la lumière dans l’air et dans l’eau . . . ,” Comptes rendus . . . de l’Académie des
sciences, XXX (1850), 551-60; and C. L. Cowan, Jr., et al., “Detection of the Free Neutrino:
A Confirmation,” Science, CXXIV (1956), 103-4.
Vol. II, No. 2
40. The Structure of Scientific Revolutions
experimentalists.4 Other examples of the same sort of continuing work
would include determinations of the astronomical unit, Avogadro’s
number, Joule’s coefficient, the electronic charge, and so on. Few of
these elaborate efforts would have been conceived and none would have
been carried out without a paradigm theory to define the problem and
to guarantee the existence of a stable solution.
Efforts to articulate a paradigm are not, however, restricted to the
determination of universal constants. They may, for example, also aim
at quantitative laws: Boyle’s Law relating gas pressure to volume,
Coulomb’s Law of electrical attraction, and Joule’s formula relating heat
generated to electrical resistance and current are all in this category.
Perhaps it is not apparent that a paradigm is prerequisite to the
discovery of laws like these. We often hear that they are found by
examining measurements undertaken for their own sake and without
theoretical commitment. But history offers no support for so excessively
Baconian a method. Boyle’s experiments were not conceivable (and if
conceived would have received another interpretation or none at all)
until air was recognized as an elastic fluid to which all the elaborate
concepts of hydrostatics could be applied.5 Coulomb’s success depended
upon his constructing special apparatus to measure the force between
point charges, (Those who had previously measured electrical forces
using ordinary pan balances, etc., had found no consistent or simple
regularity at all.) But that design, in turn, depended upon the previous
recognition that every particle of electric fluid acts upon every other at a
distance. It was for the force between such particles—the only force
which might safely be assumed
J. H. P[oynting] reviews some two dozen measurements of the gravitational constant
between 1741 and 1901 in “Gravitation Constant and Mean Density of the Earth,”
Encyclopaedia Britannica (11th ed.; Cambridge, 1910-11), XII, 385-89.
For the full transplantation of hydrostatic concepts into pneumatics, see The Physical
Treatises of Pascal, trans. I. H. B. Spiers and A. G. H. Spiers, with an introduction and
notes by F. Barry (New York, 1937). Torricelli’s original introduction of the parallelism
(“We live submerged at the bottom of an ocean of the element air”) occurs on p. 164. Its
rapid development is displayed by the two main treatises.
Vol. II, No. 2
41. The Nature of Normal Science
a simple function of distance—that Coulomb was looking.6 Joule’s
experiments could also be used to illustrate how quantitative laws
emerge through paradigm articulation. In fact, so general and close is
the relation between qualitative paradigm and quantitative law that,
since Galileo, such laws have often been correctly guessed with the aid
of a paradigm years before apparatus could be designed for their
experimental determination.7
Finally, there is a third sort of experiment which aims to articulate a
paradigm. More than the others this one can resemble exploration, and
it is particularly prevalent in those periods and sciences that deal more
with the qualitative than with the quantitative aspects of nature’s
regularity. Often a paradigm developed for one set of phenomena is
ambiguous in its application to other closely related ones. Then
experiments are necessary to choose among the alternative ways of
applying the paradigm to the new area of interest. For example, the
paradigm applications of the caloric theory were to heating and cooling
by mixtures and by change of state. But heat could be released or
absorbed in many other ways—e.g., by chemical combination, by
friction, and by compression or absorption of a gas—and to each of these
other phenomena the theory could be applied in several ways. If the
vacuum had a heat capacity, for example, heating by compression could
be explained as the result of mixing gas with void. Or it might be due to
a change in the specific heat of gases with changing pressure. And there
were several other explanations besides. Many experiments were
undertaken to elaborate these various possibilities and to distinguish
between them; all these experiments arose from the caloric theory as
paradigm, and all exploited it in the design of experiments and in the
interpretation of results.8 Once the phe-
Duane Roller and Duane H. D. Roller, The Development of the Concept of Electric
Charge: Electricity from the Greeks to Coulomb (“Harvard Case Histories in Experimental
Science,” Case 8; Cambridge, Mass., 1954), pp. 66-80.
For examples, see T. S. Kuhn, “The Function of Measurement in Modern Physical
Science,” Isis, LII (1961), 161-93.
T. S. Kuhn, “The Caloric Theory of Adiabatic Compression,” Isis, XLIX (1958), 132-40.
Vol. II, No. 2
42. The Structure of Scientific Revolutions
nomenon of heating by compression had been established, all further
experiments in the area were paradigm-dependent in this way. Given
the phenomenon, how else could an experiment to elucidate it have
been chosen?
Turn now to the theoretical problems of normal science, which fall
into very nearly the same classes as the experimental and observational.
A part of normal theoretical work, though only a small part, consists
simply in the use of existing theory to predict factual information of
intrinsic value. The manufacture of astronomical ephemerides, the
computation of lens characteristics, and the production of radio
propagation curves are examples of problems of this sort. Scientists,
however, generally regard them as hack work to be relegated to
engineers or technicians. At no time do very many of them appear in
significant scientific journals. But these journals do contain a great
many theoretical discussions of problems that, to the non-scientist, must
seem almost identical. These are the manipulations of theory
undertaken, not because the predictions in which they result are
intrinsically valuable, but because they can be confronted directly with
experiment. Their purpose is to display a new application of the
paradigm or to increase the precision of an application that has already
been made.
The need for work of this sort arises from the immense difficulties
often encountered in developing points of contact between a theory and
nature. These difficulties can be briefly illustrated by an examination of
the history of dynamics after Newton. By the early eighteenth century
those scientists who found a paradigm in the Principia took the
generality of its conclusions for granted, and they had every reason to
do so. No other work known to the history of science has simultaneously
permitted so large an increase in both the scope and precision of
research. For the heavens Newton had derived Kepler’s Laws of
planetary motion and also explained certain of the observed respects in
which the moon failed to obey them. For the earth he had derived the
results of some scattered observations on pendulums and the tides. With
the aid of additional but ad hoc assumptions, he had also been able to
derive Boyle’s Law
Vol. II, No. 2
43. The Nature of Normal Science
and an important formula for the speed of sound in air. Given the state
of science at the time, the success of the demonstrations was extremely
impressive. Yet given the presumptive generality of Newton’s Laws, the
number of these applications was not great, and Newton developed
almost no others. Furthermore, compared with what any graduate
student of physics can achieve with those same laws today, Newton’s few
applications were not even developed with precision. Finally, the
Principia had been designed for application chiefly to problems of
celestial mechanics. How to adapt it for terrestrial applications,
particularly for those of motion under constraint, was by no means
clear. Terrestrial problems were, in any case, already being attacked with
great success by a quite different set of techniques developed originally
by Galileo and Huyghens and extended on the Continent during the
eighteenth century by the Bernoullis, d’Alembert, and many others.
Presumably their techniques and those of the Principia could be shown
to be special cases of a more general formulation, but for some time no
one saw quite how.9
Restrict attention for the moment to the problem of precision. We
have already illustrated its empirical aspect. Special equipment—like
Cavendish’s apparatus, the Atwood machine, or improved telescopes—
was required in order to provide the special data that the concrete
applications of Newton’s paradigm demanded. Similar difficulties in
obtaining agreement existed on the side of theory. In applying his laws
to pendulums, for example, Newton was forced to treat the bob as a
mass point in order to provide a unique definition of pendulum length.
Most of his theorems, the few exceptions being hypothetical and
preliminary, also ignored the effect of air resistance. These were sound
physical approximations. Nevertheless, as approximations they
restricted the agreement to be expected
C. Truesdell, “A Program toward Rediscovering the Rational Mechanics of the Age of
Reason,” Archive for History of the Exact Sciences, I (1960), 3-36, and “Reactions of Late
Baroque Mechanics to Success, Conjecture, Error, and Failure in Newton’s Principia,”
Texas Quarterly, X (1967), 281-97. T. L. Hankins, “The Reception of Newton’s Second Law
of Motion in the Eighteenth Century.” Archives internationales d’histoire des sciences, XX
(1967), 42-65.
Vol. II, No. 2
44. The Structure of Scientific Revolutions
between Newton’s predictions and actual experiments. The same
difficulties appear even more clearly in the application of Newton’s
theory to the heavens. Simple quantitative telescopic observations
indicate that the planets do not quite obey Kepler’s Laws, and Newton’s
theory indicates that they should not. To derive those laws, Newton had
been forced to neglect all gravitational attraction except that between
individual planets and the sun. Since the planets also attract each other,
only approximate agreement between the applied theory and telescopic
observation could be expected.10
The agreement obtained was, of course, more than satisfactory to
those who obtained it. Excepting for some terrestrial problems, no other
theory could do nearly so well. None of those who questioned the
validity of Newton’s work did so because of its limited agreement with
experiment and observation. Nevertheless, these limitations of
agreement left many fascinating theoretical problems for Newton’s
successors. Theoretical techniques were, for example, required for
treating the motions of more than two simultaneously attracting bodies
and for investigating the stability of perturbed orbits. Problems like
these occupied many of Europe’s best mathematicians during the
eighteenth and early nineteenth century. Euler, Lagrange, Laplace, and
Gauss all did some of their most brilliant work on problems aimed to
improve the match between Newton’s paradigm and observation of the
heavens. Many of these figures worked simultaneously to develop the
mathematics required for applications that neither Newton nor the
contemporary Continental school of mechanics had even attempted.
They produced, for example, an immense literature and some very
powerful mathematical techniques for hydrodynamics and for the
problem of vibrating strings. These problems of application account for
what is probably the most brilliant and consuming scientific work of the
eighteenth century. Other examples could be discovered by an
examination of the post-paradigm period in the development of
thermodynamics, the wave theory of light, electromagnetic the-
Wolf, op. cit., pp. 75-81, 96-101; and William Whewell, History of the Inductive Sciences
(rev. ed.; London, 1847), II, 213-71.
Vol. II, No. 2
45. The Nature of Normal Science
ory, or any other branch of science whose fundamental laws are fully
quantitative. At least in the more mathematical sciences, most
theoretical work is of this sort.
But it is not all of this sort. Even in the mathematical sciences there
are also theoretical problems of paradigm articulation; and during
periods when scientific development is predominantly qualitative, these
problems dominate. Some of the problems, in both the more
quantitative and more qualitative sciences, aim simply at clarification by
reformulation. The Principia, for example, did not always prove an easy
work to apply, partly because it retained some of the clumsiness
inevitable in a first venture and partly because so much of its meaning
was only implicit in its applications. For many terrestrial applications, in
any case, an apparently unrelated set of Continental techniques seemed
vastly more powerful. Therefore, from Euler and Lagrange in the
eighteenth century to Hamilton, Jacobi, and Hertz in the nineteenth,
many of Europe’s most brilliant mathematical physicists repeatedly
endeavored to reformulate mechanical theory in an equivalent but
logically and aesthetically more satisfying form. They wished, that is, to
exhibit the explicit and implicit lessons of the Principia and of
Continental mechanics in a logically more coherent version, one that
would be at once more uniform and less equivocal in its application to
the newly elaborated problems of mechanics.11
Similar reformulations of a paradigm have occurred repeatedly in all
of the sciences, but most of them have produced more substantial
changes in the paradigm than the reformulations of the Principia cited
above. Such changes result from the empirical work previously
described as aimed at paradigm articulation. Indeed, to classify that sort
of work as empirical was arbitrary. More than any other sort of normal
research, the problems of paradigm articulation are simultaneously
theoretical and experimental; the examples given previously will serve
equally well here. Before he could construct his equipment and make
measurements with it, Coulomb had to employ electrical theory to
determine how his equipment should be built. The
René Dugas, Histoire de la mécanique (Neuchatel, 1950), Books IV-V.
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46. The Structure of Scientific Revolutions
consequence of his measurements was a refinement in that theory. Or
again, the men who designed the experiments that were to distinguish
between the various theories of heating by compression were generally
the same men who had made up the versions being compared. They
were working both with fact and with theory, and their work produced
not simply new information but a more precise paradigm, obtained by
the elimination of ambiguities that the original from which they worked
had retained. In many sciences, most normal work is of this sort. These
three classes of problems—determination of significant fact, matching of
facts with theory, and articulation of theory-exhaust, I think, the
literature of normal science, both empirical and theoretical. They do
not, of course, quite exhaust the entire literature of science. There are
also extraordinary problems, and it may well be their resolution that
makes the scientific enterprise as a whole so particularly worthwhile.
But extraordinary problems are not to be had for the asking. They
emerge only on special occasions prepared by the advance of normal
research. Inevitably, therefore, the overwhelming majority of the
problems undertaken by even the very best scientists usually fall into
one of the three categories outlined above. Work under the paradigm
can be conducted in no other way, and to desert the paradigm is to cease
practicing the science it defines. We shall shortly discover that such
desertions do occur. They are the pivots about which scientific
revolutions turn. But before beginning the study of such revolutions, we
require a more panoramic view of the normal-scientific pursuits that
prepare the way.
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