By Karl R. Popper<\/em><\/p>\n The following excerpt was originally published in Conjectures and Refutations<\/span> London: Routledge and Keagan Paul, (1963), pp. 33-39; from Theodore Schick, ed., Readings in the Philosophy of Science, <\/em>Mountain View, California, Mayfield Publishing Company, 2000, \u03c3\u03c3. 9-13<\/em><\/em>.<\/em><\/p>\n 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, \u201cWhen should a theory be ranked as scientific?\u201d <\/em>or \u201cIs there a criterion for the scientific character or status of a theory?\u201d<\/em><\/p>\n The problem which troubled me at the time was neither, \u201cWhen is a theory true?\u201d nor \u201cWhen is a theory acceptable?\u201d my problem was different. I wished to distinguish between science and pseudo-science<\/em>; knowing very well that science often errs, and that pseudoscience may happen to stumble on the truth.<\/p>\n I knew, of course, the most widely accepted answer to my problem that science is distinguished from pseudoscience\u2014or from \u201cmetaphysics\u201d\u2014by its empirical method<\/em>, which is essentially inductive<\/em>, 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\u2014that 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\u2014on horoscopes and on biographies.<\/p>\n But as it was not the example of astrology which leads 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\u2019s theory of relativity was no doubt by far the most important. The three others were Marx\u2019s theory of history, Freud\u2019s psycho-analysis, and Alfred Adler\u2019s so-called \u201cindividual psychology.\u201d<\/p>\n 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\u2014the small circle of students to which I belong\u2014were thrilled with the result of Eddington\u2019s eclipse observations which in 1919 brought the first important confirmation of Einstein\u2019s theory of gravitation. TI was a great experience for us, and one which had a lasting influence on my intellectual development.<\/p>\n 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 district of Vienna where he had established social guidance clinics.<\/p>\n It was the summer of 1919 that I began to feel more and more dissatisfied with these three theories\u2014the 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, \u201cWhat is wrong with Marxism, psycho-analysis and individual psychology? Why are they so different from physical theories, from Newton\u2019s theory and especially from the theory of relativity?<\/p>\n To make this contrast clear I should explain few of us at the time would have said that we believed in the truth<\/em> of Einstein\u2019s theory of gravitation. This shows that it was not my doubting the truth<\/em> 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<\/em> 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.<\/p>\n 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.<\/em> 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<\/em> 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 \u201cun-analyzed\u201d and crying aloud for treatment.<\/p>\n The most characteristic element in this situation seemed to me the incessant stream of confirmations, of observations which \u201cverified\u201d the theories in question; and this point was constantly emphasized by their adherents. A Marxist could not open a newspaper without finding one every page confirming evidence for his interpretation of history; not only in the news, but also in its presentation\u2014which revealed the class bias of the paper\u2014and especially of course what the paper did not<\/em> say. The Freudian analysts emphasized that their theories were constantly verified by their \u201cclinical observations.\u201d 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. \u201cBecause of my thousandfold experience,\u201d he replied; whereupon I could not help saying: \u201cAnd with this new case, I suppose, your experience has become thousand-and-one-fold.\u201d<\/p>\n 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 \u201cprevious experience,\u201d 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\u2019s theory, or equally of Freud\u2019s, 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\u2014that they always fitted, that they were always confirmed\u2014which 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.<\/p>\n With Einstein\u2019s theory the situation was strikingly different. Take one typical instance\u2014Einstein\u2019s prediction, just then confirmed by the finding of Eddington\u2019s expedition. Einstein\u2019s 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\u2019s 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.<\/p>\n Now the impressive thing about this case is the risk<\/em> 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<\/em>\u2014in fact with results which everybody before Einstein would have expected.[1]<\/a> 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 fo these theories.<\/p>\n These considerations led me in the winter of 1919-20 to conclusions which I may now reformulate as follows.<\/p>\n 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.<\/em><\/p>\n II<\/p>\n I may perhaps exemplify this with the help of the various theories so far mentioned. Einstein\u2019s 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.<\/p>\n Astrology did not pass the test. Astrologers were greatly impressed, and misled, by what they believed to be confirming evidence\u2014so 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\u2019s trick to predict things so vaguely that the predictions can hardly fail: that they become irrefutable.<\/p>\n 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\u2019s analysis of the character of the \u201ccoming social revolution\u201d) their predictions were testable, and in fact falsified.[2]<\/a> 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 \u201cconventionalist twist\u201d to the theory; and by this stratagem they destroyed its much advertised claim to scientific status.<\/p>\n The two psycho-analytic theories were in a different class. They were simply nontestable, 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 \u201cclinical observations\u201d which analysts naively believe confirm their theory cannot do this anymore than the daily confirmations which astrologers find in their practice.[3]<\/a> And as for Freud\u2019s 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\u2019s 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.<\/p>\n At the same time I realized that such myths may be developed, and become testable, that historically speaking all\u2014or very nearly all\u2014scientific theories originate from myths, and that a myth may contain important anticipations of scientific theories. Examples are Empedocles\u2019 theory of evolution by trial and error, or Parmenides\u2019 myth of the unchanging block universe in which nothing ever happens and which, if we add another dimension, becomes Einstein\u2019s 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 \u201cmetaphysical\u201d (as we might say), it is not thereby found to be unimportant, or insignificant, or \u201cmeaningless\u201d, or \u201cnonsensical.\u201d[4]<\/a> But it cannot claim to be backed by empirical evidence in the scientific sense\u2014although it may easily be, in some genetic sense, the \u201cresult of observation.\u201d<\/p>\n (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\u2014another of those impressive and all-explanatory theories which act upon weak minds like revelations.)<\/p>\n 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\u2014whether they are of a religious or of a metaphysical character, or simply pseudo-scientific. Years later\u2014it must have been in 1928 or 1929\u2014I called this first problem of mine the \u201cproblem of demarcation<\/em>.\u201d 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.<\/p>\n\n
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