Science, Pseudo-Science, and Falsifiability
Karl Popper, 1962
The problem which troubled me at the time was neither, "When is a theory
true?" nor, "When is a theory acceptable?" My problem was
different. I wished to distinguish between science and pseudo-science;
knowing very well that science often errs, and that pseudo-science may happen to
stumble on the truth.
I knew, of course, the most widely accepted answer to my problem: that
science is distinguished from pseudo-science—or from
"metaphysics"—by its empirical method, which is essentially inductive,
proceeding from observation or experiment. But this did not satisfy me. On the
contrary, I often formulated my problem as one of distinguishing between a
genuinely empirical method and a non-empirical or even a pseudo-empirical
method—that is to say, a method which, although it appeals to observation and
experiment, nevertheless does not come up to scientific standards. The latter
method may be exemplified by astrology, with its stupendous mass of empirical
evidence based on observation—on horoscopes and on biographies.
But as it was not the example of astrology which led me to my problem I
should perhaps briefly describe the atmosphere in which my problem arose and the
examples by which it was stimulated. After the collapse of the Austrian Empire
there had been a revolution in Austria: the air was full of revolutionary
slogans and ideas, and new and often wild theories. Among the theories which
interested me Einstein’s theory of relativity was no doubt the most important.
Three others were Marx’s theory of history, Freud’s psycho-analysis, and
Alfred Adler’s so-called "individual psychology."
There was a lot of popular nonsense talked about these theories, and
especially about relativity (as still happens even today), but I was fortunate
in those who introduced me to the study of this theory. We all—the small
circle of students to which I belonged—were thrilled with the result of
Eddington’s eclipse observations which in 1919 brought the first important
confirmation of Einstein’s theory of gravitation. It was a great experience
for us, and one which had a lasting influence on my intellectual development.
The three other theories I have mentioned were also widely discussed among
students at that time. I myself happened to come into personal contact with
Alfred Adler, and even to cooperate with him in his social work among the
children and young people in the working-class districts of Vienna where he had
established social guidance clinics.
It was during the summer of 1919 that I began to feel more and more
dissatisfied with these three theories—the Marxist theory of history,
psycho-analysis, and individual psychology; and I began to feel dubious about
their claims to scientific status. My problem perhaps first took the simple
form, "What is wrong with Marxism, psycho-analysis, and individual
psychology? Why are they so different from physical theories, from Newton’s
theory, and especially from the theory of relativity?"
To make this contrast clear I should explain that few of us at the time would
have said that we believed in the truth of Einstein’s theory of
gravitation. This shows that it was not my doubting the truth of these
other three theories which bothered me, but something else. Yet neither was it
that I merely felt mathematical physics to be more exact than the
sociological or psychological type of theory. Thus what worried me was neither
the problem of truth, at that stage at least, nor the problem of exactness or
measurability. It was rather that I felt that these other three theories, though
posing as sciences, had in fact more in common with primitive myths than with
science; that they resembled astrology rather than astronomy.
I found that those of my friends who were admirers of Marx, Freud, and Adler,
were impressed by a number of points common to these theories, and especially by
their apparent explanatory power. These theories appeared to be able to
explain practically everything that happened within the fields to which they
referred. The study of any of them seemed to have the effect of an intellectual
conversion or revelation, opening your eyes to a new truth hidden from those not
yet initiated. Once your eyes were thus opened you saw confirming instances
everywhere: the world was full of verifications of the theory. Whatever
happened always confirmed it. This its truth appeared manifest; and unbelievers
were clearly people who did not want to see the manifest truth; who refused to
see it, either because it was against their class interest, or because of their
repressions which were still "un-analysed" and crying aloud for
treatment.
The most characteristic element in this situation seemed to me the incessant
stream of confirmation, of observations which "verified" the theories
in question; and this point was constantly emphasized by their adherents. A
Marxist could not open a newspaper without finding on every page confirming
evidence for his interpretation of history; not only in the news, but also in
its presentation—which revealed the class bias of the paper—and especially
of course in what the paper did not say. The Freudian analysts emphasized
that their theories were constantly verified by their "clinical
observations." As for Adler, I was much impressed by a personal experience.
Once, in 1919, I reported to him a case which to me did not seem particularly
Adlerian, but which he found no difficulty in analysing in terms of his theory
of inferiority feelings, although he had not even seen the child. Slightly
shocked, I asked him how he could be so sure. "Because of my thousandfold
experience," he replied; whereupon I could not help saying: "And with
this new case, I suppose, your experience has become
thousand-and-one-fold."
What I had in mind was that his previous observations may not have been much
sounder than this new one; that each in its turn had been interpreted in the
light of "previous experience," and at the same time counted as
additional confirmation. What, I asked myself, did it confirm? No more than that
a case could be interpreted in the light of the theory. But this means very
little, I reflected, since every conceivable case could be interpreted in the
light of Adler’s theory, or equally of Freud’s. I may illustrate this by two
very different examples of human behaviour: that of a man who pushes a child
into the water with the intention of drowning him; and that of a man who
sacrifices his life in an attempt to save the child. Each of these two cases can
be explained with equal ease in Freudian and Adlerian terms. According to Freud
the first man suffered from repression (say, of some component of his Oedipus
complex), while the second man had achieved sublimation. According to Adler the
first man suffered from feelings of inferiority (producing perhaps the need to
prove to himself that he dared to commit some crime), and so did the second man
(whose need was to prove to himself that he dared to rescue the child). I could
not think of any human behaviour which could not be interpreted in terms of
either theory. It was precisely this fact—that they always fitted, that they
were always confirmed—which in the eyes of their admirers constituted the
strongest argument in favour of these theories. It began to dawn on me that this
apparent strength was in fact their weakness.
With Einstein’s theory the situation was strikingly different. Take one
typical instance—Einstein’s prediction, just then confirmed by the findings
of Eddington’s expedition. Einstein’s gravitational theory had led to the
result that light must be attracted by heavy bodies (such as the sun), precisely
as material bodies were attracted. As a consequence it could be calculated that
light from a distant fixed star whose apparent position was close to the sun
would reach the earth from such a direction that the star would seem to be
slightly shifted away from the sun; or, in other words, that stars close to the
sun would look as if they had moved a little away from the sun, and from one
another. This is a thing which cannot normally be observed since such stars are
rendered invisible in daytime by the sun’s overwhelming brightness; but during
an eclipse it is possible to take pictures of them. If the same constellation is
photographed at night one can measure the distances on the two photographs, and
check the predicted effect.
Now the impressive thing about this case is the risk involved in a
prediction of this kind. If observation shows that the predicted effect is
definitely absent, then the theory is simply refuted. The theory is incompatible
with certain possible results of observation—in fact with results which
everybody before Einstein would have expected. This is quite different from the
situation I have previously described, when it turned out that the theories in
question were compatible with the most divergent human behaviour, so that it was
practically impossible to describe any human behaviour that might not be claimed
to be a verification of these theories.
These considerations led me in the winter of 1919–20 to conclusions which I
may now reformulate as follows.
I may perhaps exemplify this with the help of the various theories so far
mentioned. Einstein’s theory of gravitation clearly satisfied the criterion of
falsifiability. Even if our measuring instruments at the time did not allow us
to pronounce on the results of the tests with complete assurance, there was
clearly a possibility of refuting the theory.
Astrology did not pass the test. Astrologers were greatly impressed, and
misled, by what they believed to be confirming evidence—so much so that they
were quite unimpressed by any unfavourable evidence. Moreover, by making their
interpretations and prophesies sufficiently vague they were able to explain away
anything that might have been a refutation of the theory had the theory and the
prophesies been more precise. In order to escape falsification they destroyed
the testability of their theory. It is a typical soothsayer’s trick to predict
things so vaguely that the predictions can hardly fail: that they become
irrefutable.
The Marxist theory of history, in spite of the serious efforts of some of its
founders and followers, ultimately adopted this soothsaying practice. In some of
its earlier formulations (for example in Marx’s analysis of the character of
the "coming social revolution") their predictions were testable, and
in fact falsified. Yet instead of accepting the refutations the followers of
Marx re-interpreted both the theory and the evidence in order to make them
agree. In this way they rescued the theory from refutation; but they did so at
the price of adopting a device which made it irrefutable. They thus gave a
"conventionalist twist" to the theory; and by this stratagem they
destroyed its much advertised claim to scientific status.
The two psycho-analytic theories were in a different class. They were simply
non-testable, irrefutable. There was no conceivable human behaviour which could
contradict them. This does not mean that Freud and Adler were not seeing certain
things correctly; I personally do not doubt that much of what they say is of
considerable importance, and may well play its part one day in a psychological
science which is testable. But it does mean that those "clinical
observations" which analysts naïvely believe confirm their theory cannot
do this any more than the daily confirmations which astrologers find in their
practice. And as for Freud’s epic of the Ego, the Super-ego, and the Id, no
substantially stronger claim to scientific status can be made for it than for
Homer’s collected stories from Olympus. These theories describe some facts,
but in the manner of myths. They contain most interesting psychological
suggestions, but not in a testable form.
At the same time I realized that such myths may be developed, and become
testable; that historically speaking all—or very nearly all—scientific
theories originate from myths, and that a myth may contain important
anticipations of scientific theories. Examples are Empedocles’ theory of
evolution by trial and error, or Parmenides’ myth of the unchanging block
universe in which nothing ever happens and which, if we add another dimension,
becomes Einstein’s block universe (in which, too, nothing ever happens, since
everything is, four-dimensionally speaking, determined and laid down from the
beginning). I thus felt that if a theory is found to be non-scientific, or
"metaphysical" (as we might say), it is not thereby found to be
unimportant, or insignificant, or "meaningless," or
"nonsensical." But it cannot claim to be backed by empirical evidence
in the scientific sense—although it may easily be, in some genetic sense, the
"result of observation."
(There were a great many other theories of this pre-scientific or
pseudo-scientific character, some of them, unfortunately, as influential as the
Marxist interpretation of history; for example, the racialist interpretation of
history—another of those impressive and all-explanatory theories which act
upon weak minds like revelations.)
Thus the problem which I tried to solve by proposing the criterion of
falsifiability was neither a problem of meaningfulness or significance, nor a
problem of truth or acceptability. It was the problem of drawing a line (as well
as this can be done) between the statements, or systems of statements, of the
empirical sciences, and all other statements—whether they are of a religious
or of a metaphysical character, or simply pseudo-scientific. Years later—it
must have been in 1928 or 1929—I called this first problem of mine the "problem
of demarcation." The criterion of falsifiability is a solution to this
problem of demarcation, for it says that statements or systems of statements, in
order to be ranked as scientific, must be capable of conflicting with possible,
or conceivable, observations.