I
When I received the list of participants in
this course and realized that I had been asked to speak to
philosophical colleagues I thought, after some hesitation and
consolation, that you would probably prefer me to speak about
those problems which interests me most, and about those
developments with which I am most intimately acquainted.
I therefore decided to do what I have never
done before:
to give you a report on my own work in the
philosophy of science, since the autumn 1919 when I first begin
to grapple with the problem,
"When should a theory be ranked as
scientific?" or "Is there a criterion for the scientific
character or status of a theory?"
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 pseudoscience may happen
to stumble on the truth.
I knew, of course, the most widely accepted answer to my
problem: that science is distinguished from pseudoscience - 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 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 lead 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 by far the most
important.
The three others were,
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 belong - 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 the 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 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 three other theories which bothered me, but
something else.
Yet neither was it that I nearly felt
mathematical physics to be more exact than 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 science, 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 appear 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, open your
eyes to a new truth hidden from those not yet initiated. Once
your eyes were thus opened you saw confirmed 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 refuse to see it, either because it was
against their class interest, or because of their repressions
which were still "un-analyzed" 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 emphasize 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 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 analyzing 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 a theory. But this meant very little, I
reflected, since every conceivable case could be interpreted in
the light Adler's theory, or equally of Freud's.
I may illustrate this by two very different
examples of human behavior: 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 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 behavior 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 favor 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 finding 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 photographs of them. If the same constellation
is photographed at night one can measure the distance 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
behavior, so that it was practically impossible to describe any
human behavior 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 non-scientific. 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 testability:
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.
(I now speak in such cases of "corroborating evidence.")
-
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 reinterpreting 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
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.
II
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
unfavorable 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. [2]
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 behavior 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 naively believe confirm their
theory cannot do this any more than the daily confirmations
which astrologers find in their practice. [3]
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." [4]
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.
