Chapter III

The Historical Source Located

[ Preface | Part One | Part Two | Notes | Bibliography | Cover ]


Popper, Kuhn, Lakatos and Feyerabend have succeeded in making irrationalist philosophy of science acceptable to many readers who would reject it out of hand if it were presented to them without equivocation and consistently. It was thus that the question arose to which the first Part of this book was addressed: namely, how did they achieve this? My answer was, that they did so principally by means of two literary devices discussed in Part One. The question to which the present Part of this book is addressed is: how was irrationalist philosophy of science made acceptable to these authors themselves?

Some part of the answer to this question no doubt lies in those very misuses of language which have already been discussed. For there is no reason to suppose that our authors' characteristic treatment of logical expressions and success-words has imposed on the writers any less than on their readers. But obviously, there must be some much more basic answer than this to the historical question which I have just raised. How did irrationalism about science come to recommend itself at all and in the first place, to some leading philosophers from about 1920 onwards, as it did not, and could not have done, to their counterparts a hundred or two hundred years earlier? It must be in principle possible to explain this phenomenon, just as it is possible in principle to explain any other large-scale movement in the history of thought.

It is not to be assumed, of course, that the origins of recent irrationalist philosophy of science are purely intellectual: that this philosophy came into being solely as a result of our authors accepting some thesis or other, and duly accepting its logical consequences. The common-sense assumption is in fact the other way. Any large-scale movement of thought is likely to be brought about, at least in part, by non-intellectual causes; and the present case is presumably no exception.

Nevertheless it will be taken for granted here that the origins of the movement of thought with which we are concerned are at any rate principally intellectual: that is, that the irrationalist conclusions of our authors' philosophy are embraced by them principally because they are logical consequences of some premises which these authors accept. Not to take this for granted would amount to intolerable condescension towards the authors in question, similar in kind to that by which Marxist writers `explain' Darwin as though he were some some simple mechanical toy.

The question is, then, what are the intellectual origins of recent irrationalism in the philosophy of science? Since we are looking for intellectual origins, the answer must consist in some thesis or other. And since were are looking for origins, the thesis must be one which functions in our authors' philosophy as a premise, and not as a consequence of other theses. Further still: what we seek to identify is that one among their premises which is the key premise of their irrationalism, in the sense that without it their philosophy of science would not have (that is, the other premises of it do not have) any irrationalist consequences at all.

Our question, then, is purely historical. The answer to it, however, is not of historical interest only. It would indeed be extremely interesting, as a matter of the history of thought, to know what is the key premise, in the sense just explained, of recent irrationalist philosophy of science. But the philosophical interest which indirectly attaches to our enquiry is greater still. What philosophers will want most to know, concerning the key premise of our authors' philosophy, is whether or not it is true. But in order for that to be known, it is an obviously indispensable preliminary, that it be known what this proposition is.

In this book only the preliminary and historical task, of identifying this proposition, is attempted; not the philosophical task of determining its truth-value. But if we can do even this much, then there will be some immediate and substantial benefit to philosophers. Controversies constantly take place between our authors (or their followers) and other philosophers who, while they share some of our authors' premises, disagree with their irrationalist conclusions. If our authors' key premise were once identified, then it would be known, to both sides in such controversies, where their disagreements begin. How valuable such information is, in enabling pointless discussion to be avoided, and yet how hard to come by in philosophy, no philosopher need be told.


Since most of the quotations in Part One illustrated ways in which our authors' irrationalism is disguised, we should here satisfy ourselves that the phenomenon which we wish to explain really does exist: that is, that our authors' philosophy of science really is irrationalist. The best way to do this with reasonable brevity is to put before the reader (who is assumed to be familiar with their writings) a few concrete and pungent reminders of those writings: to cite some things our authors say about science, which, while they are indisputably representative of their philosophy, are at the same time extremely and overtly irrationalist. This is what is done in the present section.

First, then: if there has been a great increase in knowledge in recent centuries, then a fortiriori there sometimes are such things as positive good reasons to believe a scientific theory; but Popper says expressly, repeatedly, and emphatically, that there are not and cannot be such things. This thesis is so startlingly irrationalist that other philosophers, as Popper himself tells us, sometimes "cannot quite bring [themselves] to believe that this is my opinion". But it is: "There are no such things as good positive reasons" [1] to believe any scientific theory. "Positive reasons are neither necessary not possible" [2].

These opinions will be admitted to be irrationalist enough: and they are too deliberately and emphatically expressed to be unrepresentative.

A scientific theory, Popper never tires of reminding his readers, is never certain in relation to, or in other words deducible from, those propositions that constitute (in most people's eyes) the reasons to believe it. Of course I do not cite this as an irrationalist thesis. It is only a fallibilist one: it asserts no more than the logical possibility of the conjunction of the evidence for any given scientific theory, with the negation of that theory. This thesis is so far from being one which is peculiar to the authors with whom we are concerned, that it is nowadays a commonplace with almost all philosophers of science. But Popper goes much further than this. It is a favorite thesis with him that a scientific theory is, not only never certain, but never even probable, in relation to the evidence for it [3]. More than that: a scientific theory, he constantly says, cannot even be more probable, in relation to the empirical evidence for it, than it is a priori, or in the absence of all empirical evidence [4].

These two theses will be acknowledged to be irrationalist enough; and they are ones upon which Popper repeatedly insists. He goes much further still, however. The truth of any scientific theory or law-statement, he constantly says, is exactly as improbable, both a priori and in relation to any possible evidence, as the truth of a self-contradictory proposition [5]; or, to put the matter in plain English (as Popper does not), it is impossible.

Again: scientific knowledge is usually thought to have at least some connection with rational belief, but Popper writes: "Belief, of course, is never rational: it is rational to suspend belief" [6]. One hardly knows what to wonder at more here, the thesis itself, or the arrogance of the author's "of course". His thesis, as will be evident, goes far beyond the philosophy of science. But it certainly does go as far as that, and will be admitted to express, in that domain, an irrationalism sufficiently uncompromising.

Again: Popper endorses the notorious sceptical thesis of Hume concerning inductive arguments, or arguments from the observed to the unobserved. This is the thesis that no proposition about the observed is a reason to believe any contingent proposition about the unobserved; or in other words, that the premise of an inductive argument is never a reason to believe its conclusion. Popper constantly and emphatically, and with detailed references to Hume, expresses his assent to this thesis. He writes, for example: "I agree with Hume's opinion that induction is invalid and in no sense justified" [7]. And again: "Are we rationally justified in reasoning from repeated instances of which we have experience to instances of which we have had no experience? Hume's unrelenting answer is: No, we are not justified [...] My own view is that Hume's answer to this problem is right [...]" [8]. There are many other statements by Popper to exactly the same effect [9].

Scepticism about induction is an irrationalist thesis itself, but its irrationalist character is enormously amplified if it is combined, as it is in Hume and in Popper, with the thesis of empiricism: that is, with the thesis that no propositions other than propositions about the observed can be a reason to believe a contingent proposition about the unobserved. For then it follows at once (since inductive scepticism says that there can be no reason from experience), that there can be no reason at all, to believe any contingent propositions about the unobserved: which class of propositions includes, of course, all scientific theories. Hume, being an empiricist, did draw from his inductive scepticism this even more irrationalist conclusion: `scepticism about the unobserved', as we may call it. And Popper, for the same reason, does the same.

Hume's inductive scepticism, while it is an irrationalist thesis among others in Popper's philosophy of science, is also more than that: it is one on which all the others logically depend. Whenever Popper undertakes, as he often does, to explain the grounds of his philosophy of science, and especially of whatever is most irrationalist in it, the reader is sure to meet with yet another of Popper's expositions, with detailed reference to Hume's writings and with unqualified endorsement of Hume's scepticism about induction [10]. If we take any other representative expression of Popper's irrationalism (for example, those mentioned above in the second to the sixth paragraph of this section), and ask ourselves "Why does Popper believe this?", then part at least of the answer is always the same, and always obvious. It is because he shares Hume's scepticism about induction.

It would be easy to extend indefinitely a list of irrationalist theses which are representative of our authors; but there is no need to do so here. The examples given above suffice for the present purpose, which was only to satisfy ourselves that the philosophy of science here in question really is irrationalist. It is a sufficient condition for a philosophy of science to be irrationalist (as we said at the beginning of this book) if consistency with it requires reluctance to admit that there has been a great increase of knowledge in recent centuries. Popper's philosophy of science, it will be evident even from the few samples of it given above, fulfills this condition amply.

The examples of irrationalist theses given above were not only few in number, but were all drawn from Popper, none of them from any of our other three authors. But this too is perfectly proper, and in fact appropriate. Popper's philosophy of science is at any rate not more irrationalist than that of Feyerabend, Kuhn, or Lakatos, and at the same time, as a matter of well-known history, Popper's philosophy owes nothing to theirs, while Kuhn's philosophy owes much, and the philosophy of Lakatos and Feyerabend owes nearly everything, to Popper.


Our object, then, is to identify the key premise (in the sense explained earlier) of the reasoning by which our authors have been led to such irrationalist conclusions about science as have been cited in the preceding section.

There is no reason to expect this identification to be very easily made. It is always harder to identify a person's premises than to identify his conclusions. The reason is obvious. A reasoner's premises or starting-points are those propositions which he feels most entitled to take for granted. They are, therefore, the parts of his reasoning which are least likely to be explicit enough to enable other people to identify them easily. Indeed, it is sometimes difficult or even impossible for the reasoner himself to identify all his premises. For a proposition can be a premise of a person's reasoning without his ever having put it into words, and even without his being conscious of believing it at all.

It is nowhere of more importance than in philosophy to make clear what our reasoning is, and hence what our premises are; and most philosophers accordingly, at least aim to achieve these things. But, whether from differences in temperament or in training, their actual achievements in this respect are very unequal, and many philosophers simply are not clear enough reasoners to enable their premises to be identified with any confidence. Again, it will be difficult to identify a philosopher's premises, however clear a reasoner he may be, in proportion as his philosophy is derivative from some one else's. If, for example, what one philosopher does is principally just to illustrate a position which he takes to have been placed beyond dispute by another philosopher, then it will hardly be possible to discover, from his writings, what the ultimate grounds are on which that position rests.

For these reasons, it would be idle to try to identify the key premise of recent irrationalist philosophy of science, from the writings of Lakatos, Feyerabend, or Kuhn. Lakatos is the only one of these three who is a clear enough reasoner to hold out any hope of such identification. But it is in fact impossible in all three, because of the extremely derivative character of their philosophy. In their writings, irrationalism about scientific theories functions, not as a conclusion at all, but as a premise, and as an inexplicit and scarcely-conscious one at that. Of what such irrationalism is a consequence, it is the least of their concerns to make clear. They are hardly to be looked to even for the enunciation of general irrationalist theses about science, such as Popper scatters so freely over his pages; still less, therefore, are they to be looked to for the arguments for them. In recent irrationalist philosophy of science, these authors are fortunate heirs, and like most persons of that kind, they are more concerned to enjoy their inheritance than to enquire into the grounds of it. Feyerabend and Kuhn made some slight additions to their irrationalist inheritance; Lakatos made some trifling abridgements of it, as though he were slightly uneasy about it; but what all of them chiefly did was simply to illustrate it, from chosen episodes in the history of science.

Popper on the other hand, writing as he was a generation before these authors, and for a less enlightened age, was obliged, as they never were, to work for his irrationalist theses: to argue for them. He it was in fact, and no one else, who made `straight in the desert a highway' for these writers, so that irrationalism could thereafter be treated as a settled thing and a starting-point. It is to Popper, therefore, and to him alone, that we must look, in our attempt to identify the key premise of recent irrationalism. But since he is also a clearer reasoner than any of our other authors, we can do so with some prospect of success.

In such theses as those of Popper which were mentioned in the preceding section, there is nothing new. What were there cited as representative expressions of new irrationalism, could equally be cited as representative expressions of old scepticism. That it is always rational to suspend belief, is a thesis of Pyrrho as well as of Popper: that from what has been experienced, nothing can be rationally inferred about what has not, is a thesis of Hume as well as of Popper; and so on. It is new, of course, to have such sceptical or irrationalist theses as these filling huge books called "The Growth of Scientific Knowledge" [11], "The Logic of Scientific Discovery", etc., etc. But then (as was said at the beginning of the book), when it is obvious that knowledge has increased, authors who wish to imply the opposite and yet retain plausibility must write in ways apt to mislead their readers. But in the substance, as distinct from the literary form, of Popper's philosophy, nothing is new. In particular, Popper himself makes clear (as I have said), that the scepticism of Hume about inductive arguments is not only one of his own irrationalist theses, but part of the immediate grounds of all the others.

In this dependence of Hume, Popper is only an extreme case of a general condition. For the influence of Hume on 20th-century philosophy of science in general is in fact so great that it is scarcely possible to exaggerate it. He looms like a colossus over both of the main tendencies in philosophy of science in the present century: the logical positivist one, and the irrationalist one. His empiricism, his insistence on the fallibility of induction, and on the thesis which follows from those two, of the permanent possibility of the falsity of any scientific theory, are fundamental planks in the platform of both of these schools of thought. Where the two schools separate is that the irrationalists further accept, while the logical positivists reject, Hume's further, sceptical, thesis about induction: that the premise of an inductive argument is no reason to believe its conclusion. This is why the logical positivists, in the 1940's and '50's set about constructing what they called `confirmation-theory', `non-deductive logic', `the theory of logical probability', or `inductive logic': a branch of logic which, while being consistent with empiricism and inductive fallibilism, would allow scientific theories to be objects of rational belief without being certain. The irrationalists, on the other hand, being Humean sceptics and not merely fallibilists about induction, deny the possibility of any such theory; and Popper, accordingly, makes the chief landmark of `inductive logic', Carnap's Logical Foundations of Probability, a principal target of his criticism [12].

In the sharpest possible contrast to all this, the influence of Hume on philosophy of science in the 19th century was but slight. For this extraordinary reversal in the importance attached to Hume's philosophy of science, the historical reason is obvious enough, at least in broad terms. The crucial event was that one which for almost two hundred years had been felt to be impossible, but which nevertheless took place near the start of this century: the fall of the Newtonian empire in physics. This catastrophe, and the period of extreme turbulence in physics it inaugurated, changed the entire climate of philosophy of science. Almost all philosophers of the 18th and 19th centuries, it was now clear, has enormously exaggerated the certainty and the extent of scientific knowledge. What was needed, evidently, was a far less optimistic philosophy of science, a rigorously fallibilist philosophy, which would ensure that such fearful hubris as had been incurred in connection with Newtonian physics should never be incurred again. Well, the very thing needed was lying at hand, though long neglected; and Hume, 150 years after his death, finally and fully came into his own.

Thus the revival of Hume's philosophy of science in this century was a movement of retreat from that confidence in science which was so high, and constantly rising, in the two preceding centuries, and which had proved to be misplaced precisely where it was highest. This retreat was general, all empiricist philosophers taking part in it. Popper and his followers are simply those with whom the retreat turned into a rout. They fell back all the way to Hume: not just to his fallibilism but to his scepticism about induction; and hence (since they were empiricists) to his scepticism in general about the unobserved.

Their only object was, and has remained, to ensure that no scientific theory should ever again become the object of over-confident belief; since only in that way can it be guaranteed that such a fall as overtook Newtonian pride will never be repeated. Now, it was the belief that a scientific theory can be certain, which had made that fall possible. So it must be re-affirmed, with Hume, that a scientific theory is never deducible from the observational evidence for it. On this negative logical Popper and his followers accordingly insist, and insist ad nauseum, even though no empiricist any longer dreams of denying it. They insist on it to the exclusion of every other logical relation which might exist between a scientific theory and the evidence for it, and they deny, with Hume, that propositions about the observed can ever be a positive reason to believe a scientific theory. They must do so: otherwise, it might one day happen that a scientific theory should again be mistaken for a certainty. And that, for these philosophers, is what must at any cost be prevented.

This same consuming anxiety, it is worthwhile to point out, finds expression even in the very germ of Popper's philosophy: that is, in his opinions as to what constitutes a scientific theory, and what makes one such theory better than another. The very mark of a scientific theory, he thinks, is that it should be able to be disproved by experience [13]; and one scientific theory is better than another (other things being equal), he thinks, if it is more disprovable than the other [14]. No opinions could express more poignantly than these the depth of Popper's dread lest Newtonian hubris should ever have a sequel. For this is to say that the very mark of a scientific theory is that it be possible for us to repel any claims it might have on our belief, and that a theory is the better, the more easily the burden of belief which it threatens to impose on us can be put off. And nothing, evidently, could have suggested so strangely inverted a conception of science, except the most intense recollection of the traumatic consequences of having once fully believed a false theory.

Such is the genesis of Popper's philosophy of science. It is a story of one kind of reaction to the disappointment of extreme expectations: that kind of reaction, namely, of which the best epitome is given in Aesop's fable of the fox and the grapes. The parallel would be complete if the fox, having become convinced that neither he nor anyone else could ever succeed in tasting grapes, should nevertheless write many long books on the progress of viticulture.

We have made a beginning, then, in our attempt to identify the key premise of recent irrationalist philosophy of science. That premise is to be looked for, among our authors, in Popper and nowhere else. The irrationalism of Popper about scientific theories has turned out to be no other than the scepticism of Hume concerning contingent propositions about the unobserved. We know what are the immediate grounds, both in Hume and in Popper, of that irrationalism or scepticism: the conjunction of the theses of empiricism and inductive scepticism.

It is obvious, furthermore, which of these two immediate grounds is the key to the irrationalism of this consequences to their conjunction. It is the thesis of inductive scepticism. From the empiricist ground on its own no irrationalist consequence follows.

But all this is only a beginning, since what we have so far identified are only the immediate grounds of Popper's irrationalism concerning scientific theories. What we want to know, however, are the ultimate grounds of it. At least, we want to know that ultimate ground without which his philosophy of science would have no irrationalist implications.

The thesis of inductive scepticism cannot possibly be itself that ultimate ground or premise of Popper's irrationalism. It operates as a tacit premise, indeed, in the philosophy of Feyerabend, Kuhn, and Lakatos; but then, that is just the principal respect in which these philosophers are careless beneficiaries of Popper's labors. At no earlier period than theirs in the entire history of philosophy could a respected philosopher (not to say a sane man) have started from the assumption that the observed can furnish no reason to believe anything about the unobserved. Certainly Popper, writing in an earlier and less enlightened age, had to have some argument for so startlingly irrationalist a thesis.

Our search for the key premise of our authors' irrationalism leads us, then, to the question: what are the premises of Popper's argument for scepticism about induction? How was inductive scepticism itself established?

Just as in general our other authors are derivative thinkers in relation to Popper, so Popper in turn, here at any rate, is a derivative thinker in relation to Hume. Indeed, on this all-important matter of the grounds of inductive scepticism, he is entirely so. Popper's argument for scepticism about induction is simply Hume's argument for it. He has neither fault to find with Hume's reasoning for this conclusion, nor anything to add to it. "I regard Hume's formulation and treatment of the logical problem of induction [...] a flawless gem" [15]. What Hume gave us, Popper says, is "a gem of priceless value [...]: a simple, straightforward, logical refutation of any claim that induction could be a valid argument, or a justifiable way of reasoning" [16].

This being so, we know at any rate this much about the key premise of Popper's argument for inductive scepticism: that it is the key premise, whatever that is, of Hume's argument for the same conclusion. For these arguments are one and the same.

The reader of Popper is naturally led to expect, by such passages as have just been quoted, that he is about to be told what this perfect and simple argument of Hume's was. But the reader is disappointed in this expectation. Hume's conclusion is there stated and endorsed by Popper, but his argument for it is only praised, not stated. There are, however, other places in his writings where he does attempt to say what Hume's argument was [17]. These accounts differ widely in how much of the detail of Hume's argument they disclose. Some of them are mere hints of the argument, too brief or obscure to make any of its internal structure visible at all [18]. In other cases Popper's account does succeed in making some of the structure of Hume's argument clear [19]. For our purposes, however, what is required is an account of Hume's argument which enables us to identify its premises, and all of them. From this point of view all Popper's accounts of Hume's argument are extremely deficient. It would be only with the greatest difficulty, if at all, that anyone could learn from Popper what even one of Hume's premises was.

It should not surprise us that Popper has reproduced only very incompletely the argument which he praises so lavishly. On the contrary, this was to be expected. It is simply another instance of the obvious rule which was stated earlier: that the more derivative a thinker is in relation to another, that is, the more he regards that other as having placed a certain conclusion beyond dispute, the less likely he is to make clear what the original grounds were on which that conclusion rested.

The deficiencies of Popper's account of Hume's argument do not, however, impose any obstacle to our enquiry. They are simply an additional reason why the historical focus of that enquiry must now go back beyond Popper. We must simply identify that premise of Hume's argument for inductive scepticism, without which it would not have its irrationalist conclusion. The fact that Popper's accounts of that argument are very imperfect, does not matter at all. Had he given ever so good an account of it, still, since the argument in question, by Popper's own testimony, is Hume's, it is Hume's argument to which we ought to turn our attention.

The shift of the focus of our enquiry back to Hume, while it is in any case necessary, is also attended by marked advantages. For one thing, Hume is a clearer reasoner than any of our four modern authors. Secondly, and even more important, the circumstances in which Hume argued for inductive scepticism were much more conducive to explicitness of argument on this point than those in which Popper did. Popper did so in a period of catastrophic collapse of confidence in science (as well as of confidence in much else) [20], a period in which irrationalist theses, such as inductive scepticism, were greedily embraced by many of his readers almost faster than Popper could write them down. Hume, living in a less enlightened age, had no such assistance. On the contrary, he had to argue for scepticism about induction, not only from a standing start (as it were), but entirely against the prevailing current of opinion. The current of Newtonian confidence, in particular, was already then so strong as to be irresistible except by the hardiest of sceptics. Popper, therefore, even if his native talent for clear reasoning had been as great as Hume's, was bound to be, on this subject, the less explicit reasoner of the two.

That Hume's philosophy of science is the source of a great deal of subsequent irrationalism, has been, of course, widely recognized: for example, by Bertrand Russell [21]. Indeed, it is emphasised by Popper himself [22]. Popper does not admit, of course, that his own philosophy of science is irrationalist, but it is as obvious to him as it is to everyone else that Hume's is [23], and he has been admirably explicit (as we have seen) in acknowledging the debt he owes to Hume.

In this respect our other authors compare very unfavorably with Popper. Their debt to Hume's philosophy (which means in the end, as we have seen, their debt to his sceptical thesis about induction) is not less than Popper's; it is only less direct. Yet one would look in vain in their writings for any direct, indeed almost for any indirect, acknowledgment of this indebtedness. Indeed, one has only to recall the thesis to which they are indebted (namely, the premise of an inductive argument is no reason to believe its conclusion), to see at once how utterly out of place it would have been for these authors even to mention it. Popper had made Humean scepticism about induction so much de rigueur, that even to affirm it had become extremely unfashionable; almost as much so, indeed, as to deny it. For the author of The Structure of Scientific Revolutions, or of The Methodology of Scientific Research Programmes, to introduce this simple old thesis into their works, would have been felt as an intolerable piece of rusticity. The proprietor of a pornographic book shop may be dimly conscious of a debt to the author of Areopagitica, but Milton is the last person he wants to see in his shop.

In later works, however, there are two small and indirect indications that these authors do after all recognize, in this homely thesis of Hume, the progenitor of their own irrationalism.

Lakatos' philosophy of science was no sooner published than it was outflanked on the left (so to speak) by the still more irrationalist philosophy of his friend Feyerabend. Thus by a manoeuvre not the less amusing for being familiar, Lakatos found himself placed, late in his life, in the unaccustomed role of defender of science, against neo-Popperian irrationalism. In this extremity (we are told by Feyerabend, who is here referring mainly to unpublished discussions between them), Lakatos was reduced to objecting that even irrationalist philosophers do not "walk out of the window of a 50-story building instead of using the lift" [24]. Feyerabend admits he was baffled by this objection "for quite a while"; as anyone might have been, by an objection so extremely recherche. Finally, however, he found a reply which the irrationalist can make to it, and he gravely explains what it is. This reply is fully as original as the objection, and is in fact, though apparently all-unknowingly, pure Hume. It does not matter, Feyerabend tells us, what he or anyone else "does or does not do", or feel, about walking out of high windows; what matters is that neither he nor anyone else "can give reasons for his fear" of doing so [25].

Kuhn provides a less picturesque but equally clear belated acknowledgment of the central part played in his philosophy of science by scepticism about induction. In an article first published in 1977, he tells us that, if he finds himself unable to avoid certain views of science which some people regard as irrationalist, "that is only another way of saying that I make no claim to have solved the problem of induction" [26].

The ordinary philosopher comes across these two passages with mingled relief, astonishment, and indignation. Relief, because what he had privately believed all along, he now finds indirectly admitted, and admitted by the emperors themselves: that they have no clothes at all, except such as are woven out of Hume's scepticism about induction. Astonishment and indignation, because previously and apart from these two passages, nothing in these authors had prepared him for such an admission, and everything had in fact pointed the other way. There is not one word in Kuhn's The Structure of Scientific Revolutions from which a reader could infer that Kuhn believes that a problem of induction exists; much less infer that he believes it to have something to do with his philosophy of science. As for the debate about the rationality of believing one can safely walk out of high windows: what is this `pastoral-comical' scene, in which Lakatos plays Beattie to Feyerabend's Hume, but an admission that what is principally at stake between irrationalists and their critics is the sceptical thesis of Hume about the possibility of learning from experience? A thesis which was old when Sextus Empiricus wrote, and which requires for its discussion examples no more esoteric than Hume's own about walking out of windows [27], or the one always associated with Pyrrho, of walking over cliffs [28]! "But until now", the indignant reader exclaims, "these authors had led me to believe that, before I could enter the lists against their philosophy of science, I would have to have read at least as much as they have written about Galileo and the telescope, about Lavoisier and oxygen, about the Bohr-Kramers-Slater theory, about the Lummer-Pringsheim experiments, etc., etc. What! Was all of this really quite inessential all along? Was it bestowed on me, then principally, ad terrorum?" Alas, poor reader, it was.

However belated or infrequent their own acknowledgment of it, then, the philosophy of these authors depends, no less critically than the philosophy of Popper does, on the scepticism of Hume about induction. This historical fact has some extremely curious corollaries. For example, that had it not been for the author of the most famous of all attacks on the credibility of miracles, the author of Against Method would not have believed a vulgar charlatan who claimed to become a raven from time to time [29]. But it is in any case a fact, and we must now turn to the argument of Hume on which this all-important thesis of irrationalist philosophy rests. For the key premise, whatever it is, of Hume's argument for inductive scepticism, is also the key to our whole enquiry.

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