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Karl R. Popper, ìConjectures and Refutations (The Growth of Scientific Knowledge)î published by Routledge, London and New York, 1963. This edition published 1992. ISBN 0 415 04318 2. Other books by the same author: ìThe Open Society and its Enemies ñ Vol 1. The Spell of Plato, Vol II: The High Tide of Prophecy: Hegel, Marx, and the Aftermathî; ìThe Poverty of Historicismî; ìThe Logic of Scientific Discoveryî; ìRealism and the Aim of Scienceî; ìThe Open Universeî; ìQuantum Theory and the Schism in Physicsî; ìThe Self and Its Brain (With John C. Eccles)î; ìUnended Questî; ìIn Search of a Better Worldî.

This excerpt is reprinted for its news value, since it was the subject ofÝ a U.S. Supreme Court decision which established a new standard for determining what evidence is admissible in court as being ìscientificî or ìexpert testimonyî. This article was not merely the subject of the decision; its logic was made the law of the land. This article was cited as the place one might go to understand the three new standards of ìscientificî, taken from this article, which are now U.S. law.








(Ed. the title describes the process of learning, which is the ìscientific methodî. We learn by making tentative assumptions, or theories, or ìconjecturesî, about the world around us, and then we test them, or watch for evidence which may confirm or ìrefuteî them. Popper explains in this article that this is what psychiatry fails to do, so it is a ìpseudoî (imitation) science. However, he draws for support upon animal experiments performed by early psychiatrists. So although he exposes virtually all psychoanalysis, and he exposes the devices that skew the results of much psychiatric research, he does not condemn absolutely all psychiatry; only that which does not test itself, and which is so vague, like the general predictions of fortune tellers, that they are untestable.

( Text in blue is editorial comment or explanation.)



Mr. Turnbull had predicted dire consequences,... and was now doing the best in his power to bring about the verification of his own prophecies.



A lecture given at Peterhouse, Cambridge. Summer 1953. as part of a course on developments and trends in contemporary British philosophy, organized by the British Council; originally published under the title Philosophy of Science: a Personal Reportí in British Philosophy in Mid-Century. ed. C. A. Mace. 1937.



The problem, ëWhen should a theory be ranked as scientific?í or ëIs there a criteria for the scientific character or status of a theory?í


The problem which troubled me at the time was neither, ëWhen is a theory true?î nor, ëWhen is a theory acceptable?í 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.


[ìScienceî is a process of verifying which ideas are true. It is a process of continual testing and double checking. When Popper says his concern is not ìwhen is a theory true?î he means that more important than finding some way of testing a particular theory, is having a procedure for testing all theories. Even if all you believe is absolutely true, if you have no means of testing anything you believe, how can you know the value of what you have? And how can you share your wisdom with others, without any means of proving what you say? Popper related none of his findings to the Bible, but when Jesus debated with Israelís top scholars at the age of 12, He set an example for us, subjecting his knowledge to the most rigorous test available on planet Earth. When Proverbs says ìin a multitude of counselors there is safetyî, the proverb is true because the effect of counsel is to test your assumptions.Ý


Why Psychiatry, like Astrology, or other Myths, are not ìScienceî in the same sense as Einsteinís Theory of Relativity


...The most widely accepted answer is is distinguished from pseudoscience [îpseudoî means it looks real but it isnít]óor from ëmetaphysicsíóby its empirical method, which is essentially Inductive, proceeding from observation or experiment. But [I wanted a way to distinguish] between a genuinely empirical method and a...pseudo-empirical method.... The latter method may be exemplified by astrology, with its stupendous mass of empirical evidence based on observationóon 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 Einsteinís theory of relativity was no doubt by far the most important. Three others were Marxís theory of history, Freudís psycho-analysis, and Alfred Adlerís so-called ëindividual psychologyí.


There was a lot of popular nonsense...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 belongedówere thrilled with the result of Eddingtonís eclipse observations [In which Eddington photographed the edge of a solar eclipse showing stars behind the sun, proving that the sunís gravity was bending the light of the stars, as Einstein predicted] which in 1919 brought the first important confirmation of Einsteinís 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 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 theoriesóthe Marxist theory of history, psychoanalysis, and individual psychology; and I begun to feel dubious about their claims to scientific status.Ý My problem perhaps first took the simple form, ëWhat is wrong with Marxism, psycho-analysis, and individual psychology? Why are they so different from physical theories, from Newtonís theory, and especially from the theory of relativity?í


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 Einsteinís theory of gravitation. This shows that it was not my doubting the truth of those other three theories which bothered mc, 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 (End of page 34) intellectual conversion or revelation, opening your eyes to a new truth bidden 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 ëun-analysedí and crying out for treatment.


The most characteristic element in this situation seemed to me the incessant stream of confirmations, of observations which ëverifiedí 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 presentationówhich revealed the class bias of the paperóand especially of course in what the paper did not say. The Freudian analysts emphasized that their theories were constantly verified by their ëclinical observationsí.


So youíve ìseen it a thousand timesî? Did prejudice blind you as much the first time as it does the thousandth?


ÝAs 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. ëBecause of my thousandfold experienceí, he replied; whereupon I could not help saying: And with this new case, I suppose, your experience has become thousand-and-one-fold.í [I meant] ...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 ëprevious experienceí, 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 Adlerís theory, or equally of Freudís.


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 factóthat they always fitted, that they were always confirmedówhich 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 Einsteinís theory the situation was strikingly different. Take one (End of page 35) typical instanceóEinsteinís prediction, just then confirmed by the findings of Eddingtonís expedition. Einsteinís 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 sunís 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 observationóin 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 impossible to describe an human behaviour that might not be claimed to be a verification of these theories.


TheseÝ considerations led me in the winter of 1919-20 to conclusions which I may now reformulate as follows.


(1) It is easy to obtain confirmations, or verifications, for nearly every theoryóif we look [uncritically] for confirmations.


(2) 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 theoryóan event which would have refuted the theory.


(3) Every ëgoodí scientific theory is a prohibition: it forbids certain things to happen. [Or, it predicts that certain things cannot happen.] The more a theory forbids, the better it is.


(4) 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.


(5) 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.


(6) Confirming evidence...(or ëcorroborating evidenceí)...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 [refute] the theory. (End of page 36)


(7) Some genuinely testable theories, when found to be false, are still upheld by their admirersófor example by introducing ad hoc [made up for the occasion] 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....

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.(This is the sentence quoted in the Daubert decision, 1993. These are not three separate criteria, but are three synonymsóthree words for the same criterion.)



I may perhaps exemplify this with the help of the various theories so far mentioned. Einsteinís 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 prophecies sufficiently vague they were able to explain away anything that might have been a refutation of the theory had the theory and the prophecies been more precise. In order to escape falsification they destroyed the testability of their theory. It is a typical soothsayerís trick to predict things so vaguely that the predictions can hardly fail: 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 Marxís analysis of the character of the ëcoming social revolutioní) their predictions were testable, and in fact falsified [disproved] Yet instead of accepting the refutations the followers of Marx reinterpreted 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...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 wall play its put one day in a psychological science which is testable. But it does mean that those ëclinical observationsí which analysts naively believe confirm their theory cannot do this any more than the daily confirmations which astrologers find (End of page 37) in their practice.


The devices by which psychiatric researchers make sure psychiatry cannot be disproved ñ or be scientific


The ëClinical observationsí [of psychiatrists], like all other observations, are Interpretations In the light of theories; and for this reason alone they are apt to seem to support those theories in the light of which they were interpreted. But real support can be obtained only from observations undertaken as tests (by ëattempted refutationsí); and for this purpose criteria of refutation have to be laid down beforehand: it must be agreed which observable situations, If actually observed, mean that the theory is refuted. But what kind of clinical responses would refute to the satisfaction of the analyst not merely a particular analytic diagnosis but psycho-analysis itself? And have such criteria ever been discussed or agreed upon by analysts? Is there not, on the contrary, a whole family of analytic concepts, such as ambivalenceí (I do not suggest that there is no such thing as ambivalence), which would make it difficult, if not impossible, to agree upon such criteria? Moreover, how much headway has been made in investigating the question of the extent to which the (conscious or unconscious) expectations and theories held by the analyst influence the ëclinical responsesí of the patient? (To say nothing about the conscious attempts to influence the patient by proposing interpretations to him, etc.) Years ago I Introduced the term ëOedipus effectí to describe the influence of a theory or expectation or prediction upon the event which it predicts or describes: it will be remembered that the causal chain loading to Oedipusí parricide was started by the oracleís prediction of this event. This is a characteristic and recurrent theme of such myths, but one which seems to have failed to attract the interest of the analysts, perhaps not accidentally. (The problem of confirmatory dreams suggested by the analyst is discussed by Freud, for example in Gesammelte Schriften, us, 1925, where he says on p. 314: ëIf anybody asserts that most of the dreams which can be utilized in an analysis... owe their origin to [the analystís] suggestion, then no objection can be made from the point of view of analytic theory. Yet there is nothing in this factí, he surprisingly adds, ëwhich would detract from the reliability of our results.í)


And as for Freudís epic of the Ego, the Super-ego, and the Id, no substantially stronger claim to scientific status can be made for it than for Homerís 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 allóor very nearly allóscientific theories originate from myths, and that a myth may contain important anticipations of scientific theories. ...I thus felt that if a theory is found to be non-scientific, or ëmetaphysicalí (as we might say), it is not thereby found to be unimportant, or insignificant, or ëmeaninglessí, or ënonsensicalí. But it cannot claim to be backed by empirical evidence in the scientific senseóalthough it may easily be, in some genetic sense, the ëresult of observationí.


The case of astrology, nowadays a typical pseudo-science, may illustrate this point. It was attacked, by Aristotelians and other rationalists, down to Newtonís day, for the wrong reasonófor its now accepted assertion that the planets had an ëinfluenceí upon terrestrial (ësublunarí) events. In fact Newtonís theory of gravity, and especially the lunar theory of the tides, was historically speaking an offspring of astrological lore. Newton, it seems, was most reluctant to adopt a theory which came from the same stable as for example the theory that ëinfluenzaí epidemics are due to an astral ëinfluenceí. And Galileo, no doubt for the same reason, actually rejected the lunar theory of the tides; and his misgivings about Kepler may easily be explained by his misgivings about astrology.


(There were a great many other theories of this pro-scientific or pseudo-scientific character, some of them, unfortunately, as influential as the Marxist interpretation of history; for example, the racialist interpretation of history [of Hitler and the Ku Klux Klan] ñ 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 [refutability, or testability] was neither a problem of meaningfulness [whether life has meaning] or significance [how to measure the value of a theory], nor a problem of truth or acceptability. It was the problem of drawing a line (as well as this can be done) between...the empirical sciences, and all other statements ñ whether they are of a religious or of a metaphysical character, or simply pseudo-scientific. Years later ñ it must have been in 1928 or 1929 ñ I called this first problem of mine the ëproblem of demarcationí [or, ìWhere is the line between real science and pretend science?î]. The criterion of falsifiability [testability, or refutability] is a solution to this problem of demarcation [where to draw the line], 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.



ÝÝÝÝÝÝÝÝÝÝÝ Today I know, of course, that this criterion of demarcation ñ the criterion of testability, or falsifiability, or refutability ñ is far from obvious; for even now its significance is seldom realized. At that time, in 1920, it seemed to me almost trivial, although it solved for me an intellectual problem which had worried me deeply, and one which also had obvious practical consequences (for example, political ones). But I did not yet realize its full implications, or its philosophical significance....



[Misunderstand about the scientific value of ìinductionî is heavily responsible for the widespread inability of even scientists to distinguish between pseudo sciences like psychology from real sciences. It is assumed that ìinductionî, the process where oneís conclusion is a generalization inferred from many observations, is the heart ofÝ the scientific method. Since ìinductionî is used by psychology it is imagined that therefore psychology must be scientific. But ìinductionî is used as much by astrology, the Ku Klux Klan, and witch doctors. As it turns out, ìinductionî isnít a scientific process at all. It is not what makes sciences scientific. We need to understand this word in order to understand how to test the truth.


[Hume was a famous psychiatrist who thought] that induction [where oneís conclusion is a generalization inferred from many observations] cannot be logically justified. He held that there can be no valid logical ...(he used the...misleading...word ìdemonstrativeî) arguments allowing us to establish ëthat those instances of which we had no experience, resemble those, of which we have had experience.í [In other words, it is illogical to assume that, just because a few things have happened within our experience, conditions in other places and times, outside our experience, will resemble conditions within our experience.]


Hume says, ëI would renew my question, why from this experience we form any conclusion beyond those past instances, of which we have had experienceí. ...I found Humeís refutation of inductive inference clear and conclusive. But I felt completely dissatisfied with his psychological explanation of induction in terms of custom or habit....óthe fact that we believe in [natural]... laws...Hume asserting that it is due to...custom or habit.... It is neither surprising nor enlightening to hear that such a custom or habit can be explained as due to custom or habit....

ÝÝÝÝÝÝÝÝÝÝÝ This ... theory is... hardly as revolutionary as Hume thought. It is no doubt an extremely popular psychological theoryópart of ëcommon senseí, one might say. But in spite of my love of both common sense and Hume, I felt convinced that this psychological theory was mistaken; and that it was in fact refutable on purely logical grounds.

ÝÝÝÝÝÝÝÝÝÝÝ Humeís psychology, which is the popular psychology, was mistaken, I felt, about at least three different things: (a) the [power] of repetition; (b) the [origin] of habits; and especially (c) the [nature of] ëbelieving in a law [of nature]í....

ÝÝÝÝÝÝÝÝÝÝÝ (a) The typical result of repetitionósay, of repeating a difficult passage on the pianoóis that movements which at first needed attention are in the end executed without attention... Such a development, far from creating a conscious expectation of law-like succession, or a belief in a law, may on the contrary begin with a conscious belief and destroy it by making it superfluous...

ÝÝÝÝÝÝÝÝÝÝÝ (b) Habits or customs do not, as a rule, originate in repetition. Even the habit of walking, or of speaking, or of feeding at certain hours, begins before repetition can play any part whatever. ... we must not say that the practices in question originated as the result of many repetitions.

ÝÝÝÝÝÝÝÝÝÝÝ ë(C) ...As Hume admits, even a single striking observation may be sufficient to create a belief or an expectationóa fact which he tries to explain as due to an inductive habit, formed as the result of a vast number of long repetitive sequences which had been experienced at an earlier period of life. ... But this, I contended, was merely his attempt to explain away unfavourable facts which threatened his theory; an unsuccessful attempt, since these unfavourable facts could be observed in very young animals and babiesóas early, indeed, as we like. ëA lighted cigarette was held near the noses of the young puppiesí, reports F. Bge. ëThey sniffed at it once, turned tail, and nothing would induce them to come back to the source of the smell and to sniff again. A few days later, they reacted to the mere sight of a cigarette or even of a rolled piece of white paper, by bounding away, and sneezing.í If we try to explain cases like this by postulating a vast number of long repetitive sequences at a still earlier age we are not only romancing, but forgetting that in the clever puppiesí short lives there must be room not only for repetition but also for a great deal of novelty, and consequently of non-repetition.

ÝÝÝÝÝÝÝÝÝÝÝ But it is not only that certain empirical facts do not support Hume; there are decisive arguments of a purely logical nature against his psychological theory.

ÝÝÝÝÝÝÝÝÝÝÝ The central idea of Humeís psychological theory is that of repetition, based upon similarity (or ëresemblanceí). This idea is used in a very uncritical way. [Hume imagines our minds are formed like] the water-drop that hollows the stone: of sequences of unquestionably like events slowly forcing themselves upon us...


The kind of repetition envisaged by Hume can never be perfect; the cases he has in mind cannot be cases of perfect sameness; they can only be cases of similarity. Thus they are repetitions only from a certain point of view. (What has the effect upon me of a repetition may not have this effect upon a spider.) But this means that, for logical reasons, there must always be a point of viewósuch as a system of expectations, anticipations, assumptions, or interestsóbefore there can be any repetition; which point of view, consequently, cannot be merely the result of repetition.


We must thus replace, for the purposes of a psychological theory of the origin of our beliefs, the naive idea of events which are similar by the idea of events to which we react by interpreting them as being similar. But if this is so (and I can see no escape from it)Ý then Humeís psychological theory of induction leads to an infinite regress.... [like trying to explain the existence of a chicken by describing an egg, which only leaves us wondering what produced the egg], precisely analogous to that other infinite regress which was discovered by Hume himself, and used by him to explode the logical theory of induction. ...In the example of the puppies we wish to explain behaviour which may be described as recognizing or interpreting a situation as a repetition of another. ...we cannot hope to explain this by an appeal to earlier repetitions....


We see that there is an infinite regress involved in Humeís psychological theory.


Hume, I felt, had never accepted the full force of his own logical analysis. Having refuted the logical idea of induction he was faced with the following problem: how do we actually obtain our knowledge, as a matter of psychological fact, if induction is a procedure which is logically invalid and rationally unjustifiable? ... This answer would imply that even scientific knowledge is irrational, so that rationalism is absurd, and must be given up. ...Having cast out the logical theory of induction by repetition he struck a bargain with common sense, meekly allowing the re-entry of induction by repetition, in the guise of a psychological fact....


Thus I was led by purely logical considerations to replace the psychological theory of induction by the following view. Without waiting, passively, for repetitions to impress or impose regularities [assumptions about how nature behaves] upon us, we actively try to impose regularities upon the world. We try to discover similarities in it, and to interpret it in terms of laws invented by us. Without waiting for premises we jump to conclusions. These may have to be discarded later, should observation show that they are wrong.


This was a theory of trial and error ñ of conjectures and refutations. It made it possible to understand why our attempts to force interpretations upon the world were logically prior to the observation of similarities. Since there were logical reasons behind this procedure, I thought that it would apply to the field of science also; that scientific theories were not the digest of observations, but that they were inventions ñ conjectures boldly put forward for trial, to be eliminated if they clashed with observations; with observations which were rarely accidental but as a rule undertaken with the definite intention of testing a theory by obtaining, if possible, a decisive refutation.



The belief that science proceeds from observation to theory is still so widely and so firmly held that my denial of it is often met with incredulity. I have even been suspected of being insincere ñ of denying what nobody in his senses can doubt.


But in fact the belief that we can start with pure observations alone, without anything in the nature of a theory, is absurd; as may be illustrated by the story of the man who dedicated his life to natural science, wrote down everything he could observe, and bequeathed his priceless collection of observations to the Royal Society to be used as inductive evidence. This story should show us that though beetles may profitably be collected, observations may not.


Twenty-five years ago I tried to bring home the same point to a group of physics students in Vienna by beginning a lecture with the following instructions: ëTake pencil and paper; carefully observe, and write down what you have observed!í They asked, of course, what I wanted them to observe. Clearly the instruction, ëObserve!í is absurd. ... Ovservation is always selective. It needs a chosen object, a definite task, an interest, a point of view, a problem. ... ëA hungry animalí, writes Katz, divides the environment into edible and inedible things. An animal in flight sees roads to escape and hiding places....Generally speaking, objects change....according to the needs of the animal.í We may add that objects can be classified, and can become similar or dissimilar, only in this way ñ by being related to needs and interests. This rule applies not only to animals but also to scientists. For the animal a point of view is provided by its needs, the task of the moment, and its expectations; for the scientist by his theoretical interests, the special problem under investigation, his conjectures and anticipations, and the theories which he accepts as a kind of background: his frame of reference, his ëhorizon of expectationsí.


... It is quite true that any particular hypothesis we choose will have been preceded by ...the observations... which it is designed to explain. But these observations, in their turn, presupposed the adoption of a frame of reference: a frame of expectations: a frame of theories. If they were significant, if they created a need for explanation and thus gave rise to the invention of a hypothesis, it was because they could not be explained within the old theoretical framework, the old horizon of expectations. There is no danger here of an infinite regress. Going back to more and more primitive theories and myths we shall in the end find unconscious, inborn expectations.


The idea of inborn ideas is absurd, I think; but every organism has inborn reactions or responses; and among them, responses adapted to impending events. These responses we may describe as ëexpectationsí without implying that these ëexpectationsí are conscious. The new-born baby ëexpectsí, in this sense, to be fed (and, one could even argue, to be protected and loved). In view of the close relation between expectation and knowledge we may even speak in quite a reasonable sense of ëinborn knowledgeí. This ëknowledgeí, however, is not valid a priori; an inborn expectation, no matter how strong and specific, may be mistaken. (The newborn child may b abandoned, and starve.)


Thus we are born with expectations; with ëknowledgeí which, although not valid a priori, is psychologically or genetically a priori, i.e. prior to all observational experience.Ý One of the most important of these expectations is the expectation of finding a regularity [assumption about how nature behaves; a ërule of natureí]. It is connected with an inborn propensity to look out for regularities, or with aÝ need Ýto find regularities, as we may see from the pleasure of the child who satisfies this need.


[Next it is Kantís turn. Kant believed human wisdom ìevolvesî as wonderfully as night crawlers have evolved into men. In other words, he thought that with enough time, men will always get smarter, not dumber. Such optimism raises a few questions.]


...Kantís reply to Hume came near to being right... But Kant proved too much. In trying to show how knowledge is possible, he proposed a theory which had the unavoidable consequence that our quest for knowledge must necessarily succeed, which is clearly mistaken. When Kant said, ëOur intellect does not draw its laws from nature but imposes its laws upon natureí, he was right. But in thinking that these laws are necessarily true, or that we necessarily succeed in imposing them upon nature, he was wrong.


But if, as he thought, we can explain the validity of Newtonís theory by the fact that our intellect imposes its laws upon nature, it follows, I think, that our intellect must succeed in this; which makes it hard to understand why a priori knowledge such as Newtonís should be so hard to come by.


Nature very often resists quite successfully, forcing us to discard our laws as refuted; but if we live we may try again....




Our propensity to look out for regularities [rules of nature], and to impose laws upon nature, leads to the psychological phenomenon of dogmatic thinking or, more generally, dogmatic behaviour: we expect regularities everywhere and attempt to find them even where there are none; events which do not yield to those attempts we are inclined to treat as a kind of ëbackground noicseí; and we stick to our expectations even when they are inadequate and we ought to accept defeat. This dogmaticism is to some extent necessary. It is demanded by a situation which can only be dealt with by forcing our conjectures upon the world. Moreover, this dogmatism allows us to approach a good theory in stages, by way of approximations: if we accept defeat too easily, we may prevent ourselves from finding that we were very nearly right.


It is clear that this dogmatic attitude, which makes us stick to our first impressions, is indicative of a strong belief; while a critical attitude, which is ready to modify its tenets, which admits doubt and demands tests, is indicative of a weaker belief.


(Ed: Wrong, actually. Humility about my own certainty about my own beliefs enables me to tread where few dare, because my openness only reduces the trauma of being disproved. As for others less humble, often the stubborness of clinging to a dogma has less to do with conviction than with apathy about knowing whether, indeed, a greater truth might be available. Truth is treated by many as some sort of sports event where who wins has little real importance, but the ìhome teamî is chosen for the most trivial and emotional of reasons. Their stubborness about leaving their favorite dogmas for the sake of mere evidence should not be confused for strong belief. It is not that their dogmas are that certain to them, but that the evidence is not that interesting to them. If another dogma comes along that seems more interesting, they will flit right to it, again with little concern for evidence. If it ìfeels rightî or better yet if it validates their prejudices against people and groups of people, that is enough ìevidenceî for many.Ý


Now according to Humeís theory, and to the popular theory, the strength of a belief should be a product of repetition; thus it should always grow with experience, and always be greater in less primitive persons. But dogmatic thinking, an uncontrolled wish to impose regularities, a manifest pleasure in rites and in repetition as such, are characteristic of primitives and children; and increasing experience and maturity sometimes create an attitude of caution and criticism rather than of dogmatism....




...the dogmatic attitude is clearly related to the tendency to verify our laws and schemata by seeking to apply them and to confirm them, even to the point of neglecting refutations [evidence proving us wrong], whereas the critical attitude is one of readiness to change themóto test them; to refute them; to falsify them, if possible. [These three words are synonyms for the same process.] This suggests that we may identify the critical attitude with the scientific attitude, and the dogmatic attitude with the one which we have described as pseudo-scientific....


[But] science must begin with myths, and with the criticism of myths; neither with the collection of observations, nor with the invention of experiments, but with the critical discussion of myths, and of magical techniques and practices. The scientific tradition is distinguished from the pre-scientific tradition in having two layers. Like the latter, it passes on its theories; but it also passes on a critical attitude towards them. The theories are passed on, not as dogmas, but rather with the challenge to discuss them and improve upon them....


The critical attitude, the tradition of free discussion of theories with the aim of discovering their weak spots so that they may be improved upon, is the attitude of reasonableness, of rationality. It makes far-reaching use of both verbal argument and observationóof observation in the interest of argument, however. The Greekís discovery of the critical method gave rise at first to the mistaken hope that it would lead to the solution of all the great old problems; that it would establish certainty; that it would help to prove our theories, to justify them. But this hope was a residue of the dogmatic way of thinking; in fact nothing can be justified or proved (outside of mathematics and logic). The demand for rational proofs in science indicates a failure to keep distinct the broad realm of rationality and the narrow realm of rational certainty; it is an untenable, un unreasonable demand.


Nevertheless, the role of logical argument, of deductive logical reasoning, remains all-important for the critical approach; not because it allows us to prove our theories, or to infer them from observation statements, but because only by purely deductive reasoning is it possible for us to discover what our theories imply, and thus to criticize them effectively. Criticism, I said, is an attempt to find the weak spots in a theory, and these, as a rule, can be found only in the more remote logical consequences which can be derived from it. It is here that purely logical reasoning plays an important part in science.


Hume was right in stressing that our theories cannot be validly inferred from what we can know to be trueóneither from observations nor from anything else. He concluded from this that our belief in them was irrational. If ëbeliefí means here our inability to doubt our natural laws, and the constancy of natural regularities, then Hume is again right: this kind of dogmatic belief has, one might say, a physiological rather than a rational basis. If, however, the term ëbeliefí is taken to cover our critical acceptance of scientific theoriesóa tentative acceptance combined with an eagerness to revise the theory if we succeed in designing a test which it cannot passóthen Hume was wrong. In such an acceptance of theories there is nothing irrational. There is not even anything irrational in relying for practical purposes upon well-tested theories, for no more rational course of action is open to us.


Assume that we have deliberately made it our task to live in this unknown world of ours; to adjust ourselves to it as well as we can; to take advantage of the opportunities we can find in it; and to explain it,Ý if possible (we need not assume that it is), and as far as possible, with the help of laws and explanatory theories. If we have made this our task, then there is no more rational procedure than the method of trial and erroróof conjecture and refutation: of boldly proposing theories; of trying our best to show that these are erroneous; and of accepting them tentatively if our critical efforts are unsuccessful.


From the point of view here developed all laws, all theories, remain essentially tentative, or conjectural, or hypothetical, even when we feel unable to doubt them any longer.


[Ed: This is a Biblical approach: to be like Bereans, ever willing to give ear to a new claimant to greater truth, yet ever willing to die for truths as revealed to date. For our trust is not in our faith, but our faith is in God who ever teaches us by the Holy Spirit and His Word. It is not inconsistent with treating truth as tentative, to act as if the truths in our limited minds are in fact well established. Indeed, scientists donít hesitate to apply what they have learned to practical projects. If they did, their knowledge would be useless and irrelevant. Practical applications do not compromise the tentative approach to truth; to the contrary, they further test the theories, acting like laboratory experiments which produce further data which helps refine the theory. In the same way, lives of dedication to God, and individual acts of obedience like laying hands on the sick to heal them, or tithing in the face of debt, or charging enemies too powerful for human strength to defeat, become spiritual laboratory experiments further refining our understanding of God and of how He likes to work with us and through us.


In fact, it is the more dogmatic, less open to challenge, less Berean-grade Christians who are most closed to the Spirit. Such an attitude is possible only to the extent one is content to accomplish nothing more for God than he has been. Any Christian who yearns to realize the wonderful promises of Jesus to answer prayer prayed in faith will not be content with theologies that ìexplain awayî the failure of Jesus to answer prayers as expected, but will try again, theorize again, risk reputation again, pray again, rededicate again, and keep learning until it happens the way Jesus promised. The yearning to accomplish more for God is a very conscious search for the dogmatic and spiritual errors remaining in oneís mind and heart which are impeding progress.


[This is the same for the Christian as for the scientist of the physical world. The scientistís neighbors may not share the scientistís passion for discovering every error in popular assumptions, but it is only because they do not share the scientistís passion for results greater than they already experience. If you donít care about bettering your situation, there is no need for you to question how you got into it.]


Before a theory has been refuted we can never know in what way it may have to be modified. That the sun will always rise and set within twenty-four hours is still proverbial as a law ëestablished by induction beyond reasonable doubtí. It is odd that this example is still in use, though it may have served well enough in the days of Aristotle and Pytheas of Massaliaóthe great traveller who for centures was called a liar because of his tales of Thule, the land of the frozen sea and the midnight sun.


The method of trial and error is not, of course, simply identical with the scientific or critical approachówith the method of conjecture and refutation. The method of trial and error is applied not only by Einstein but, in a more dogmatic fashion, in the amoeba also. The difference lies not so much in the trials as in a critical and constructive attitude towards errors; errors which the scientist consciously and cautiously tries to uncover in order to refute his theories with searching arguments, including appeals to the most severe experimental tests which his theories and his ingenuity permit him to design.


The critical attitude might be described as the result of a conscious attempt to make our theories, our conjectures, suffer in our stead in the struggle for the survival of the fittest. It gives us a chance to survive the elimination of an inadequate hypothesisówhen a more dogmatic attitude would eliminate it by eliminating us. (There is a touching story of an Indian community which disappeared because of its belief in the holiness of life, including that of tigars.) We thus obtain the fittest theory within our reach by the elimination of those which are less fit. (By ëfitnessí I do not mean merely ëusefulnessí but truth....) I do not think that this procedure is irrational or in need of any further rational justification.




...From what I have said it is obvious that there was a close link between the two problems which interested me at that time: demarcation [knowing where to draw the line between real science and imitation science], and induction or scientific method. It was easy to see that the method of science is criticism, i.e. attempted falsifications. Yet it took me a few years to notice that the two problemsóof demarcation and of inductionówere in a sense one.


Why, I asked, do so many scientists believe in induction? I found they did so because they believed natural science to be characterized by the inductive methodóby a method starting from and relying upon, long sequences of observations and experiments. They believed that the difference between genuine science and metaphysical or pseudo-scientific speculation depended solely upon whether or not the inductive method was employed. They believed (to put it in my own terminology) that only the inductive method could provide a satisfactory criterion of demarcation.


[The rules of] ëvalid inductioní is not even metaphysical: it simply does not exist. No rule can ever guarantee that a generalization inferred from true observations, however often repeated, is true. ... And the success of science is not based upon rules of induction, but depends upon luck, ingenuity, and the purely deductive rules of critical argument.


I may summarize some of my conclusions as follows:


(1) Induction, i.e. [valid] inference based on many observations, is a myth. It is neither a psychological fact, nor a fact of ordinary life, nor one of scientific procedure.


(2) The actual procedure of science is to operate with conjectures: to jump to conclusionsóoften after one single observation (as noticed for example by Hume and Born)


(3) Repeated observations and experiments function in science as tests of our conjectures or hypothesis, i.e. as attempted refutations.


(4) The mistaken belief in induction is fortified by the need for a criterion of demarcation which, it is traditionally but wrongly believed, only the inductive method can provide.


(5) The conception of such an inductive method, like the criterion of verifiability, implies a faulty demarcation.


(6) None of this is altered in the least if we say that induction makes theories only probable rather than certain.




...The logical problem of induction arises from (a) Humeís discovery (so well expressed by Born) that it is impossible to justify a law by observation or experiment, since it ëtranscends experienceí; (b) the fact that science proposes and uses laws ëeverywhere and all the timeí. (Like Hume, Born is struck by the ëscanty materialí, i.e. the few observed instances upon which the law may be based.) To this we have to add (c) the principle of empiricism which asserts that in science, only observation and experiment may decide upon the acceptance or rejection of scientific statements, including laws and theories.


These three principles, (a), (b), and (c), appear at first sight to clash; and this apparent clash constitutes the logical problem of induction.


Faced with this clash, Born gives up (c), the principle of empiricism (as Kant and many others, including Bertrand Russell, have done before him), in favour of what he calls a ëmetaphysical principleí; a metaphysical principle which he does not even attempt to formulate; which he vaguely describers as a ëcode or rule of craftí; and of which I have never seen any formulation which even looked promising and was not clearly untenable.


But in fact the princples (a) to (c) do not clash. We see this the moment we realize that the acceptance by science of a law or of a theory is tentative only; which is to say that all laws and theories are conjectures, or tentative hypotheses; (a position which I have sometimes called ëhypotheticism;); and that we may reject a law or theory on the basis of new evidence, without necessarily discarding the old evidence which originally led us to accept it. (Footnote: I do not doubt that Born and many others would agree that theories are accepted only tentatively. But the widespread belief in induction shows that the far-reaching implications of this view are rarely seen.)


The principle of empiricism (c) can be fully preserved, since the fate of a theory, its acceptance or rejection, is decided by observation and experimentóby the result of tests. So long as a theory stands up to the severest tests we can design, it is accepted; if it does not, it is rejected. But it is never inferred, in any sense, from the the empirical evidence. There is neither a psychological nor a logical induction.Only the falsity of the theory can be inferred from empirical evidence, and this inference is a purely deductive one.


Hume showed that it is not possible to [with any validity] infer a theory from observation statements; but this does not affect the possibility of refuting a theory by observation statements. The full appreciation of this possibility makes the relation between theories and observations perfectly clear.


This solves the problem of the alleged clash between the principles (a), (b), and (c), and with it Humeís problem of induction.




Thus the problem of induction is solved. But nothing seems less wanted than a simple solution to an age-old philosophical problem. Wittgenstein and his school hold that genuine philosophical problems do not exist; from which it clearly follows that they cannot be solved. Others among my contemporaries do believe that there are philosophical problems, and respect them; but they seem to respect them too much; they seem to believe that they are insoluble, if not taboo; and they are shocked and horrified by the claim that there is a simple, neat, and lucid, solution to any of them. If there is a solution it must be deep, they feel, or at least complicated.


One question which may be asked is this: how do we really jump from an observation statement to ...a good theory?í But to this the answer is: by jumping first to any theory and then testing it, to find whether it is good or not; i.e by repeatedly applying the critical method, eliminating many bad theories, and inventing many new ones. Not everybody is able to do this; but there is no other way.


...Another question sometimes asked is this: why is it reasonable to prefer non-falsified [not disproven] statements to falsified [disproven] ones? To this question some involved answers have bee produced, for example pragmatic answers. But from a pragmatic point of view the question does not arise, since false theories often serve well enough: most formulae used in engineering or navigation are known to be false, although they may be excellent approximations and easy to handle; and they are used with confidence by people who know them to be false.


The only correct answer is the straightforward one: because we search for truth (even though we can never be sure we have found it), and because the falsified theories are known or believed to be false, while the non-falsified theories may still be true. Besides, we do not prefer every non-falsified theoryóonly one which, in the light of criticism, appears to be better than its competitors: which solves our problems, which is well tested, and of which we think, or rather conjecture or hope (considering other provisionally accepted theories), that it will stand up to further tests.


It has also been said that the problem of induction is, ëWhy is it reasonable to believe that the future will be like the past?í, and that a satisfactory answer to this question should make it plain that such a belief is, in fact, reasonable. My reply is that it is reasonable to believe that the future will be very different from the past in many vitally important respects. Admittedly it is perfectly reasonable to actÝ on the assumption that it will, in many respects, be like the past, and that well-tested laws will continue to hold (since we can have no better assumption to act upon); but it is also reasonable to believe that such a course of action will lead us at times into severe trouble, since some of the laws upon which we now heavily rely may easily prove unreliable. (Remember the midnight sun!) One might even say that to judge from past experience, and from our general scientific knowledge, the future will not be like the past, in perhaps most of the ways which those have in mind who say that it will. Water will sometimes not quench thirst, and air will choke those who breathe it. An apparent way out is to say that the future will be like the past in the sense that the laws of nature will not change, but this is begging the question. We speak of a ëlaw of natureí only if we think that we have before us a regularity which does not change; and if we find that it changes then we shall not (end of page 56) continue to call it a ëlaw of natureí. Of course our search for natural laws indicates that we hope to find them, and that we believe that there are natural laws; but our belief in any particular natural law cannot have a safer basis than our unsuccessful critical attempts to refute it.


I think that those who put the problem of induction in terms of the reasonableness of our beliefs are perfectly right if they are dissatisfied with a Humean, or post-Humean, sceptical despair of reason. We must indeed reject the view that a belief in science is as irrational as a belief in primitive magical practicesóthat both are a matter of accepting a ëtotal ideologyí, a convention or a tradition based on faith. But we must be cautious if we formulate our problem, with Hume, as one of the reasonableness of our beliefs. We should split this problem into threeóour old problem of demarcation, or of how to distinguishÝ between science and primitive magic; the problem of the rationality of the scientific or critical procedure, and of the role of observation within it; and lastly the problem of the rationality of our acceptance of theories for scientific and for practical purposes. To all these three problems solutions have been offered here.


One should also be careful not to confuse the problem of the reasonableness of the scientific procedureÝ and the (tentative) acceptance of the results of this procedureói.e. the scientific theoriesówith the problem of the rationality or otherwise of the belief that this procedure will succeed. In practice, in practical scientific research, this belief is no doubt unavoidable and reasonable, there being no better alternative. But the belief is certainly unjustifiable in a theoretical sense, as I have argued (in section v). Moreover, if we could show, on general logical grounds, that the scientific quest is likely to succeed, one could not understand why anything like success has been so rare in the long history of human endeavours to know more about our world.


... we are interested in theories with a high degree of corroboration. ... it is a mistake to conclude from this that we are interested in highly probable theories. ... the probability of a statement (or set of statements) is always the greater the less the statement says: it is inverse to the content or the deductive power of the statement, and thus to its explanatory power. Accordingly every interesting and powerful statement must have a low probability; and vice versa: a statement with a high probability will be scientifically uninteresting, because it says little and has no explanatory power.Ý Although we seek theories with a high degree of corroboration, as scientists we do not seek highly probably theories but explanations; that is to say, powerful and improbable theories. ...








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