Reproduced, with two additional footnotes, by
Norman Swartz,
Simon Fraser University. |
Popper’s Account of Scientific Theories
Selection from Karl Popper’s Conjectures and
Refutations (NY: Basic Books), 1962, pp. 34-37.
… 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 by far
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 co-operate 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 those 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. Thus 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 confirmations, 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 meant 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 it;
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 in 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 star[s] are rendered invisible
in daytime by the sun’s overwhelming brightness; but during an
eclipse it is possible to take photographs 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.[1]
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.
- It is easy to obtain confirmations, or verifications, for nearly
every theory—if we look for confirmations.
- Confirmations should count only if they are the result of risky
predictions; that is to say, if, unenlightened by the theory in
question, we should have expected an event which was incompatible with
the theory—an event which would have refuted the theory.
- Every ‘good’ scientific theory is a prohibition: it
forbids certain things to happen. The more a theory forbids, the better
it is. [†]
- A theory which is not refutable by any conceivable event is
nonscientific. Irrefutability is not a virtue of a theory (as people
often think) but a vice.
- Every genuine test of a theory is an attempt to falsify
it, or to refute it. Testability is falsifiability; but there are
degrees of testabilty: some theories are more testable, more exposed to
refutation, than others; they take, as it were, greater risks.
- Confirming evidence should not count except when it is the
result of a genuine test of the theory; and this means that it can
be presented as a serious but unsuccessful attempt to falsify the
theory. …
- Some genuinely testable theories, when found to
be false, are still upheld by their admirers—for example by
introducing ad hoc some auxiliary assumption, or by
re-interpreting the theory ad hoc in such a way that it
escapes refutation. Such a procedure is always possible, but it rescues
the theory from refutation only at the price of destroying, or at least
lowering, its scientific status. (I later [after 1920 –NS]
described such a rescuing operation as a ‘conventionalist
twist’ or a ‘conventionalist
stratagem’.) [††]
One can sum up all this by saying that the criterion of the
scientific status of a theory is its falsifiability, or refutability,
or testability.
| Notes |
| 1. |
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This is a slight oversimplification, for about half of
the Einstein effect may be derived from the classical theory, provided
we assume a ballistic theory of light. [Resume]
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| Notes added by Norman Swartz |
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† |
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When Popper speaks of a theory ‘forbidding
certain things to happen’ it sounds as if he is subscribing to a
prescriptivist view of theories. (And indeed, later in his
life he did adopt a view that is somewhat similar to a prescriptivist
view.) But in this instance we can regard Popper as indulging in a bit
of exaggeration in order to stress a philosophical point. What he is
trying to say is that good (scientific) theories are
incompatible with great numbers of possible (or
conceivable) occurrences. [Resume]
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†† |
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Some would regard Freud’s theory of repression as
just such a maneuver to ‘save’ the theory from
falsification. The theory (very roughly!) claims that adult neuroses
are caused by childhood trauma. But what if the patient cannot recall a
childhood trauma? Does that refute the theory, has it been falsified?
Not according to some adherents of the theory. The failure to recall a
childhood trauma is evidence of repression (the memory has
been repressed). But invoking this ‘rescue’, of an appeal
to repression, clearly lessens the falsifiability of the (original)
theory. [Resume] |
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