IX. The Nature and Necessity of Scientific Revolutions
These remarks permit us at last to consider the problems that provide this essay with its title. What are scientific revolutions, and what is their function in scientific development?
Much of the answer to these questions has been anticipated in earlier sections. In particular, the preceding discussion has indicated that scientific revolutions are here taken to be those non-cumulative developmental episodes in which an older paradigm is replaced in whole or in part by an incompatible new one. There is more to be said, however, and an essential part of it can be introduced by asking one further question. Why should a change of
paradigm be called a revolution? In the face of the vast and essential differences between political and scientific development, what parallelism can justify the metaphor that finds revolutions in both? One aspect of the parallelism mustalready be apparent. Political revolutions are inaugurated by a growing sense, often restricted to a segment of the political community, that existing institutions have ceased adequately to meet the problems posed by an environment that they have in part created. In much the same way, scientific revolutions are inaugurated by a growing sense, again often restricted to a narrow subdivision of the scientific community, that an existing paradigm has ceased to function adequately in the exploration of an aspect of nature to which that paradigm itself had previously led the way. In both political and scientific development the sense of malfunction that can lead to crisis is prerequisite to revolution. Furthermore, though it admittedly strains the metaphor, that parallelism holds not only for the major paradigm changes, like those attributable to Copernicus and Lavoisier, but also for the far smaller ones associated with the assimilation of a new sort of phenomenon, like oxygen or X-rays. Scientific revolutions, as we noted at the end of Section V, need seem
revolutionary only to
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those whose paradigms are affected by them. To outsiders they may, like
the Balkan revolutions of the early twentieth century, seem normal
parts of the developmental process. Astronomers, for example, could
accept X-rays as a mere addition to knowledge, for their paradigms were
unaffected by the existence of th
e new radiation. But for men like
Kelvin, Crookes, and Roentgen, whose research dealt with radiation
theory or with cathode ray tubes,
the emergence of X-rays necessarily
violated one paradigm as it created another. That is why these rays
could be discovered only through something’s first going wrong with
normal research.
This genetic aspect of the parallel between political and scientific
development should no longer be open to doubt. The parallel has,
however, a second and more profound aspect upon which the
significance of the first depends. P
olitical revolutions aim to change
political institutions in ways that th
ose institutions themselves prohibit.
Their success therefore necessitates the partial relinquishment of one
set of institutions in favor of another,
and in the interim, society is not
fully governed by institutions at all. Initially it is crisis alone that
attenuates the role of political instit
utions as we have already seen it
attenuate the role of paradigms. In increasing numbers individuals
become increasingly estranged from
political life and behave more and
more eccentrically within it. Then, as
the crisis deepens, many of these
individuals commit themselves to some concrete proposal for the
reconstruction of society in a new in
stitutional framework. At that point
the society is divided into competing camps or parties, one seeking to
defend the old institutional constellation, the others seeking to institute
some new one. And, once that polarization has occurred,
political
recourse fails.
Because they differ about the institutional matrix within
which political change is to be achieved and evaluated, because they
acknowledge no supra-institutional framework for the adjudication of
revolutionary difference, the partie
s to a revolutionary conflict must
finally resort to the techniques of mass persuasion, often including
force. Though revolutions have had a vital role in the evolution of
political institutions, that role depends upon
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their being partially extrapolitical or extrainstitutional events.
The remainder of this essay aims to demonstrate that the historical
study of paradigm change reveals very similar characteristics in the
evolution of the sciences. Like th
e choice between competing political
institutions, that between competing paradigms proves to be a choice
between incompatible modes of community life. Because it has that
character, the choice is not and cannot be determined merely by the
evaluative procedures characteristic of normal science, for these depend
in part upon a particular paradigm, and that paradigm is at issue. When
paradigms enter, as they must, into a debate about paradigm choice,
their role is necessarily circular. Ea
ch group uses its own paradigm to
argue in that paradigm’s defense.
The resulting circularity does not, of course, make the arguments
wrong or even ineffectual. The man who premises a paradigm when
arguing in its defense can nonetheles
s provide a clear exhibit of what
scientific practice will be like for those who adopt the new view of
nature. That exhibit can be immensely persuasive, often compellingly
so. Yet, whatever its force, the status of the circular argument is only
that of persuasion. It cannot be made
logically or even probabilistically
compelling for those who refuse to step into the circle. The premises
and values shared by the two parties to a debate over paradigms are not
sufficiently extensive for that. As in p
olitical revolutions,
so in paradigm
choice—there is no standard higher than the assent of the relevant
community. To discover how scientific revolutions are effected, we shall
therefore have to examine not only
the impact of nature and of logic,
but also the techniques of persuasiv
e argumentation effective within the
quite special groups that constitu
te the community of scientists.
To discover why this issue of paradigm choice can never be
unequivocally settled by logic and experiment alone, we must shortly
examine the nature of the differences
that separate the proponents of a
traditional paradigm from their revolutionary successors. That
examination is the principal object of this section and the next. We have,
however, already noted numerous examples of such differences, and no
one will doubt that history
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resulting discovery will itself be proportional to the extent and
stubbornness of the anomaly that foreshadowed it. Obviously, then,
there must be a conflict between the paradigm that discloses anomaly
and the one that later renders the anomaly lawlike. The examples of
discovery through paradigm destruction
examined in Section VI did not
confront us with mere historical a
ccident. There is no other effective
way in which discoveries might be generated.
The same argument applies even more
clearly to the invention of new
theories. There are, in principle, only three types of phenomena about
which a new theory might be developed. The first consists of
phenomena already well explained by existing paradigms, and these
seldom provide either motive or point of departure for theory
construction. When they do, as with the three famous anticipations
discussed at the end of Section VII,
the theories that result are seldom
accepted, because nature provides
no ground for discrimination. A
second class of phenomena consists
of those whose nature is indicated
by existing paradigms but whose details can be understood only through
further theory articulation. These are the phenomena to which
scientists direct their research much
of the time, but that research aims
at the articulation of existing paradigms rather than at the invention of
new ones. Only when these attempts at articulation fail do scientists
encounter the third type of phenomena, the recognized anomalies
whose characteristic feature is their stubborn refusal to be assimilated to
existing paradigms. This type alone gi
ves rise to new theories. Paradigms
provide all phenomena except anomalies with a theory-determined
place in the scientist’s field of vision.
But if new theories are called fo
rth to resolve anomalies in the
relation of an existing theory to na
ture, then the successful new theory
must somewhere permit predictions that are different from those
derived from its predecessor. That di
fference could not occur if the two
were logically compatible. In the process of being assimilated, the
second must displace the first. Even a theory like energy conservation,
which today seems a logical superstructure that relates to nature only
through independent-
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ly established theories, did not develop historically without paradigm
destruction. Instead, it emerged from a crisis in which an essential
ingredient was the incompatibility between Newtonian dynamics and
some recently formulated consequences of the caloric theory of heat.
Only after the caloric theory had been rejected could energy
conservation become part of science.
1
And only after it had been part of
science for some time could it come
to seem a theory of a logically
higher type, one not in conflict with its predecessors. It is hard to see
how new theories could arise with
out these destructive changes in
beliefs about nature. Though logical inclusiveness remains a permissible
view of the relation between