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 pseudoscienceóor from "metaphysics"óby 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 ó 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 ó 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 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 allóthe small circle of students to which I belongówere

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 theoriesóthe 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 ó which revealed the class bias of the paper ó 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 factóthat they always

fitted, that they were always confirmedówhich 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 ó 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 observationóin

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 ó 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 ó 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

ó 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 ó 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 naÔvely 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 ó or very nearly all ó 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 ó although it may