The Path of Science

(from “The Logic of Scientific Discovery”)

by Karl Popper

ONE MAY DISCERN something like a general direction in the evolution of physics—a direction from theories of a lower level of universality to theories of a higher level. This is usually called the “inductive” direction; and it might be thought that the fact that physics advances in this “inductive” direction could be used as an argument in favour of the inductive method.

Yet an advance in the inductive direction does not necessarily consist of a sequence of inductive inferences. Indeed we have shown that it may be explained in quite different terms—in terms of degree of testability and corroborability. For a theory which has been well corroborated can only be superseded by one of a higher level of universality; that is, by a theory which is better testable and which, in addition, contains the old, well corroborated theory—or at least a good approximation to it. It may be better, therefore, to describe that trend—the advance towards theories of an ever higher level of universality as “quasi-inductive.”

The quasi-inductive process should be envisaged as follows. Theories of some level of universality are proposed, and deductively tested; after that, theories of a higher level of universality are proposed, and in their turn tested with the help of those of the previous levels of universality, and so on. The methods of testing are invariably based on deductive inferences from the higher to the lower level;1 on the other hand, the levels of universality are reached, in the order of time, by proceeding from lower to higher levels.

The question may be raised: “Why not invent theories of the highest level of universality straight away? Why wait for this quasiinductive evolution? Is it not perhaps because there is after all an inductive element contained in it?” I do not think so. Again and again suggestions are put forward—conjectures, or theories—of all possible levels of universality. Those theories which are on too high a level of universality, as it were (that is, too far removed from the level reached by the testable science of the day), give rise, perhaps, to a “metaphysical system.” In this case, even if from this system statements should be deducible (or only semi-deducible, as for example in the case of Spinoza’s system), which belong to the prevailing scientific system, there will be no new testable statement among them; which means that no crucial experiment can be designed to test the system in question.2 If, on the other hand, a crucial experiment can be designed for it, then the system will contain, as a first approximation, some well corroborated theory, and at the same time also something new—and something that can be tested. Thus the system will not, of course, be “metaphysical.” In this case, the system in question may be looked upon as a new advance in the quasi-inductive evolution of science. This explains why a link with the science of the day is as a rule established only by those theories which are proposed in an attempt to meet the current problem situation; that is, the current difficulties, contradictions, and falsifications. In proposing a solution to these difficulties, these theories may point the way to a crucial experiment.

To obtain a picture or model of this quasi-inductive evolution of science, the various ideas and hypotheses might be visualized as particles suspended in a fluid. Testable science is the precipitation of these particles at the bottom of the vessel: they settle down in layers (of universality). The thickness of the deposit grows with the number of these layers, every new layer corresponding to a theory more universal than those beneath it. As the result of this process ideas previously floating in higher metaphysical regions may sometimes be reached by the growth of science, and thus make contact with it, and settle. Examples of such ideas are atomism; the idea of a single physical “principle” or ultimate element (from which the others derive); the theory of terrestrial motion (opposed by Bacon as fictitious); the age-old corpuscular theory of light; the fluid-theory of electricity (revived as the electron-gas hypothesis of metallic conduction). All these metaphysical concepts and ideas may have helped, even in their early forms, to bring order into man’s picture of the world, and in some cases they may even have led to successful predictions. Yet an idea of this kind acquires scientific status only when it is presented in falsifiable form; that is to say, only when it has become possible to decide empirically between it and some rival theory.

My investigation has traced the various consequences of the decisions and conventions—in particular of the criterion of demarcation—adopted at the beginning of this book. Looking back, we may now try to get a last comprehensive glimpse of the picture of science and of scientific discovery which has emerged. (What I have here in mind is not a picture of science as a biological phenomenon, as an instrument of adaptation, or as a roundabout method of production: I have in mind its epistemological aspects.)

Science is not a system of certain, or well-established, statements; nor is it a system which steadily advances towards a state of finality. Our science is not knowledge (epistēmē): it can never claim to have attained truth, or even a substitute for it, such as probability.

Yet science has more than mere biological survival value. It is not only a useful instrument. Although it can attain neither truth nor probability, the striving for knowledge and the search for truth are still the strongest motives of scientific discovery.

We do not know: we can only guess. And our guesses are guided by the unscientific, the metaphysical (though biologically explicable) faith in laws, in regularities which we can uncover—discover. Like Bacon, we might describe our own contemporary science—“the method of reasoning which men now ordinarily apply to nature”—as consisting of “anticipations, rash and premature,” and of “prejudices.”3

But these marvellously imaginative and bold conjectures or “anticipations” of ours are carefully and soberly controlled by systematic tests. Once put forward, none of our “anticipations” are dogmatically upheld. Our method of research is not to defend them, in order to prove how right we were. On the contrary, we try to overthrow them. Using all the weapons of our logical, mathematical, and technical armoury, we try to prove that our anticipations were false—in order to put forward, in their stead, new unjustified and unjustifiable anticipations, new “rash and premature prejudices,” as Bacon derisively called them.4

It is possible to interpret the ways of science more prosaically. One might say that progress can “...come about only in two ways: by gathering new perceptual experiences, and by better organizing those which are available already.”5 But this description of scientific progress, although not actually wrong, seems to miss the point. It is too reminiscent of Bacon's induction: too suggestive of his industrious gathering of the “countless grapes, ripe and in season,”6 from which he expected the wine of science to flow: of his myth of a scientific method that starts from observation and experiment and then proceeds to theories. (This legendary method, by the way, still inspires some of the newer sciences which try to practice it because of the prevalent belief that it is the method of experimental physics.)

The advance of science is not due to the fact that more and more perceptual experiences accumulate in the course of time. Nor is it due to the fact that we are making ever better use of our senses. Out of uninterpreted sense- experiences science cannot be distilled, no matter how industriously we gather and sort them. Bold ideas, unjustified anticipations, and speculative thought are our only means for interpreting nature: our only organon, our only instrument, for grasping her. And we must hazard them to win our prize. Those among us who are unwilling to expose their ideas to the hazard of refutation do not take part in the scientific game.

Even the careful and sober testing of our ideas by experience is in its turn inspired by ideas: experiment is planned action in which every step is guided by theory. We do not stumble upon our experiences, nor do we let them flow over us like a stream. Rather, we have to be active: we have to “make” our experiences. It is we who always formulate the questions to be put to nature; it is we who try again and again to put these questions so as to elicit a clear-cut “yes” or “no” (for nature does not give an answer unless pressed for it). And in the end, it is again we who give the answer; it is we ourselves who, after severe scrutiny, decide upon the answer to the question which we put to nature—after protracted and earnest attempts to elicit from her an unequivocal “no.” “Once and for all,” says Weyl,7 with whom I fully agree, “I wish to record my unbounded admiration for the work of the experimenter in his struggle to wrest interpretable facts from an unyielding Nature who knows so well how to meet our theories with a decisive No—or with an inaudible Yes.”

The old scientific ideal of epistēmē—of absolutely certain, demonstrable knowledge—has proved to be an idol. The demand for scientific objectivity makes it inevitable that every scientific statement must remain tentative forever. It may indeed be corroborated, but every corroboration is relative to other statements which, again, are tentative. Only in our subjective experiences of conviction, in our subjective faith, can we be “absolutely certain.”

With the idol of certainty (including that of degrees of imperfect certainty or probability) there falls one of the defences of obscurantism which bar the way of scientific advance. For the worship of this idol hampers not only the boldness of our questions, but also the rigour and the integrity of our tests. The wrong view of science betrays itself in the craving to be right; for it is not his possession of knowledge, of irrefutable truth, that makes the man of science, but his persistent and recklessly critical quest for truth.

Has our attitude, then, to be one of resignation? Have we to say that science can fullfil only its biological task; that it can, at best, merely prove its mettle in practical applications which may corroborate it? Are its intellectual problems insoluble? I do not think so. Science never pursues the illusory aim of making its answers final, or even probable. Its advance is, rather, towards an infinite yet attainable aim: that of ever discovering new, deeper, and more general problems, and of subjecting our ever tentative answers to ever renewed and ever more rigorous tests.


1. The “deductive inferences from the higher to the lower lever” are, of course, explanations...; thus the hypotheses on the higher level are explanatory with respect to those on the lower level.

2. It should be noted that I mean by a crucial experiment one that is designed to refute a theory (if possible) and more especially one which is designed to bring about a decision between two competing theories by refuting (at least) one of them—without, of course, proving the other...

3. Bacon, Novum Organum I, 26.

4. Bacon’s “anticipation” (“anticipatio”; Novum Organum I, 26) means almost the same as “hypothesis” (in my usage). Bacon held that, to prepare the mind for the intuition of the true essence or nature of a thing, it has to be meticulously cleansed of all anticipations, prejudices, and idols. For the source of an error is the impurity of our own minds; Nature itself does not lie. The main function of eliminative induction is (as with Aristotle) to assist the purification of the mind. (See also my Open Society, chapter 24; note 59 to chapter 10; note 33 to chapter 11, where Aristotle’s theory of induction is briefly described.) Purging the mind of prejudices is conceived as a kind of ritual, prescribed for the scientist who wishes to prepare his mind for the interpretation (the unbiassed reading) of the Book of Nature; just as the mystic purifies his soul to prepare it for the vision of God. (Cf. the Introduction to my Conjectures and Refutations [1963] 1965.)

5. P. Frank, Das Kausalgesetz und seine Grenzen, 1932. The view that the progress of science is due to the accumulation of perceptual experiences is still widely held (cf. my second Preface, 1958). My denial of this view is closely connected with the rejection of the doctrine that science or knowledge is bound to advance since our experiences are bound to accumulate. As against this, I believe that the advance of science depends upon the free competition of thought, and thus upon freedom, and that it must come to an end if freedom is destroyed (though it may well continue for some time in some fields, especially in technology). This view is more fully expounded in my Poverty of Historicism (section 32). I also argue there (in the Preface) that the growth of our knowledge is unpredictable by scientific means, and that, as a consequence, the future course of our history is also unpredictable.

6. Bacon, Novum Organum I, 123.

7. Weyl, Gruppentheorie und Quantenmechanik, 1931, p. 2. English translation by H. P. Robertson: The Theory of Groups and Quantum Mechanics, 1931, p. xx.