Railway lines of thought

One of the themes which I developed some years ago concerned the way ideas take hold and exert an influence on our thoughts and our research projects which is very hard to identify, to subject to critical appraisal and to change. I used the term “railway lines of thought” to capture the image of a vehicle that has to stay “on track” more or less regardless of the  wishes of the passengers.

The late Liam Hudson dropped onto the same theme in his wonderful book The Cult of the Fact and he gave me permission to reproduce several chapters on my web site. This material is now an appendix to the collection “Jacques Barzun and Others“. I urge you to read it!

Here is the Preamble to convey the flavor.

This is a book about professional psychologists and the visions they pursue. It expresses a growing dissatisfaction with the self-consciously scientific psychology in which I myself was trained – an activity that, increasingly over the last ten years, has taken on the air of a masquerade. It has been written in the hope that, somewhere behind the paraphernalia of false science and apparent objectivity, there lies the possibility of a more genuinely dispassionate study of human nature and human action.

Such a book is bound to some extent to be autobiographical; and it is bound also to concern itself not simply with the ‘facts’, but with the unspoken assumptions that we all use when deciding which facts are interesting, and which trivial, a bore…One must question not so much what university teachers think they teach, nor what students think they are learning, but the more subterranean traffic in ideals and prejudices that all powerful teaching institutions create, and that governs thereafter the intellectual lives their products lead.

In attempting this, I have set myself to transgress certain barriers that at present hem in academic discussion, and render much of it inconsequential. Each of these barriers takes the form of a distinction, persuasive but false. The first is that between Science and Art: my belief, unfashionable though this may still be, is that all arguments bearing on human life deserve to be heard within the same arena of debate. The second is between the Serious and the Frivolous: we are moving, if the tastes of the student body are any guide, from an era in which wit, like Art, has been seen as an irrelevant frill, into one – at once gloomier and more Teutonic – in which wit is outlawed as an affront to moral rectitude. The systematic, technical and cheerless are automatically preferred to the literate and humane. Although this new Calvinism satisfies simple psychic needs, I have written in defiance of it – also on the chance that the tide of piety is one that can still be turned.

Lying behind these false distinctions, and serving to unite them, is a further and more general distinction, itself false: that between Style and Content. In the entrenched sciences, it is possible to transmit the truth in prose that is as crabbed as it is evasive. But where foundations are shakier, style not merely limits what we find it natural to express; it is, in important respects, the very essence of that expression. For it is through our style, our mode of address, that we transmit all those messages that lie beyond the literal meaning of our utterance. And it is precisely on such ‘meta-messages’ that the focus of this book lies.

My account begins, conventionally, with the circumstances of its own conception. Also, less conventionally, with a foray into literary criticism, and into the history of a particular myth. This may seem at first sight irrelevant, a diversion. But if I have judged matters aright, this brief literary exploration heralds my main theme – Myths, Ancient and Modern – and also serves to identify the metaphorical nature of its own motive force: the spring that moves the mechanism along. My assumption is that human thought, before it is squeezed into its Sunday best, for purposes of publication, is a nebulous and intuitive affair: in place of logic there brews a stew of hunch and partial insight, half submerged. And although we accept that our minds’ products must eventually be judged by the puritan rules of evidence and insight – the strait gate through which they must pass – we seem in practice to draw what inspiration we possess from a hidden stockpile of images, metaphors and echoes, ancient in origin, but fertile and still growing. This work is no exception. Its energy is drawn from a clutch of human sentiments that, over and again down the centuries, have found expression in potent, metaphoric form. What these sentiments are, and what their relation is to a putative science of human life, should with luck become clearer as the narrative progresses.

To begin with, though, the story is simple enough – in fact, it has about it the beguiling air of a fable. In it, the intrepid young psychologist is packed off by his mentors across the deserts of ignorance and superstition. In mid-journey, with rations running low and a dead-line approaching, this outrider of the rational order is set upon – or so it seems – by the agents of unreason. Bloodlessly, as on the silver screen, his assailants tumble to the ground. But the dead will not lie still. They dust themselves down, and demand to be heard. Our hero finds that parley he must, and around the camp-fire all wax philosophical.

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Popper vs von Mises on the philosophy of science


A long thread on the Critical Rationalist facebook page began by drawing on  von Mises’s criticism of Popper in The Ultimate Foundation of Economic Science “he [von Mises] addressed the claim of Karl Popper that scientific propositions must be falsifiable. Although Popper was not a positivist, he intended his falsification criterion to separate scientific from non-scientific statements.”

That is not a helpful statement without providing an account of the problem situation which the positivists and Popper addressed. For the positivists, the use of the inductive method was a distinctive feature of science, but Popper considered that induction was logically incoherent.  Instead, he was looking for a convention make a clear distinction between (a) theories that claimed to be scientific (due to their alleged basis on evidence) which are nevertheless not refutable and (b) theories that do lay themselves open to falsification (in principle).

As described in The Guide to The Logic of Scientific Discovery,  he made a significant departure from the usual approaches to decide these matters, either by logical analysis or by observation of the way scientists work (the naturalistic approach). He articulated the “rules of the game” or “conventions” approach. This is closely related to his rejection of certainty as an aim of science . He introduced the theme of conjectural knowledge as a permanent feature of scientific theories and not a transient situation or a “bug” in a new theory, to be superseded by further investigation and “confirmation”.

His criterion of demarcation is a proposal for an agreement or convention. He noted that his convention will be rejected by people who think that science can generate a system of “absolutely certain, irrevocably true statements”.

The test for his proposals is to examine their logical consequences, and to explore their fertility in solving problems in the theory of knowledge and scientific investigation. Essentially, it is a test of practice and practical results.

One of the practical implications of  Popper’s criterion is that it can be used early in an argument to discover where the various parties stand on the use of evidence in the debate. It also prompts scientist to be constantly mindful of the importance of testing, with all that implies for the design of experiments and the attitude adopted towards adverse findings.

Popper’s program was radically different from the positivists, a fact obscured by people who can only see Popper’s falsifiability criterion as a rival of the positivists criterion of MEANING  (they royally confused the issue by taking up testability as a criterion of meaning, as though Popper was working on the same problem).

Part of the problem here is the great significance ascribed to Science in the wake of Newton, when Science gained the reputation for finding ultimate truths. Previously the terms science or scientific merely implied  systematic investigation with a view to  obtaining useful principles, and so there was the science of angling and every other thing.

Part of the power of Popper’s program was to get away from the hopeless quest of the positivists/empiricists for a criterion of meaning (or cognitive significance) and the attempt to save inductive logic. The falsifiability criterion had logical coherence which the verification  criterion lacked, and although falsification could not be decisive in practice, it did have the practical effect of pointing up the need for more critical attention to conventions to guide scientific practice (hence the program charted by Ian Jarvie).

One more important point: the focus of critical discussion for Popper was/were the laws of science, expressed as universal generalizations. That is what makes the logic of testing so strong (compared with verification).  I don’t understand how  a pure logical analysis  can demonstrate  that both the verification criterion and the falsifiability criterion are worthless.   What is the point of Popper’s demarcation principle, given the larger contours of his program? Where is the universal statement that is tested by the basic statement “there is a chair in this room”? Is it a universal statement of any interest in the real world of scientific investigation?

This is the original argument.

In point of fact, the criterion is worthless, since every statement comes out verifiable under it. Suppose that “p” is a non-controversially verifiable statement, e.g., “there is a chair in this room.” Let us take “q” to be a statement logical positivists reject as meaningless. A good example is one that Rudolf Carnap held up to ridicule when he called for an end to metaphysics. He cited the following from Martin Heidegger’s Being and Time (1927): “The not nothings itself.” I shall not attempt to explain this: one can see why Carnap presented it as a paradigm instance of a meaningless statement.

Does the verification principle eliminate it? Surprisingly, it does not. From p, we deduce p or q. (This step is non-controversial.) Assuming that a logical consequence of a verifiable proposition is itself verifiable, (p or q) is verifiable. Further, if p is verifiable, then the negation of p is verifiable; this principle seems difficult to question. Now, consider this argument:

p or q not -p ______ q
This argument is valid, and each of its premises is verifiable. Then, q is a logical consequence of verifiable propositions, and it, too, is verifiable. Clearly, if the verification criterion cannot eliminate “the not nothings itself,” it is not worth very much.

A falsification criterion fairs no better. If p is falsifiable, then (p and q) is falsifiable. Once more, not-p should be falsifiable if p is, though Karl Popper has implausibly denied this. By an argument parallel with that for verification, we conclude that q is falsifiable.

One might think that this is a mere trick, readily avoidable through slight modification of the principle. There have been many attempts to formulate a criterion that comes up with the “right” results, but so far all have failed to withstand criticism.

What is the “right result”  or the criterion for a “right result”?

Looked at in the context of testing (universal) scientific theories, what is wrong with the principle of falsifiability in logic and in practice for working scientists?  With scientifically relevant statements in place of the ps and qs in the argument above would the result still look like a knockdown victory over Popper’s arguments?
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Non-justificationism: A distinction that makes a difference

When the linguistic philosophers inspired by  Ryle, Wittgenstein and Austin ruled the roost in some universities, notably Oxford, critics sometimes referred to “distinctions that don’t make a difference”. What about the distinction between justificationism and non-justificationism? A critic of Popperism in the Critical Café (email list)  was prepared to read widely and he complained that the Popperians were the only people in the world who used this term non-justificationism. He wondered why we should be bothered with it, given that it was not a topic of interest or concern in the wider philosophical world. That was a decade or so ago and the ensuing discussion did so little to change the minds of the critics that some of us have not been back to the Café on a regular basis since that time. I suppose we decided that life is too short to spend a lot of it engaging with people who appear to be completely closed to the ideas of critical rationalism. I will add to this later and I just want to put this ball in play before I go out. The point is to take up a challenge that Bruce and I offered to each other, to be more hospitable to critical visitors to the site and take some time to explain more carefully the things that we have been taking for granted for a long time. Things like non-justicationism, inductivism  objective knowledge.


to be continued

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Updating the Popper Guides and Misreading Popper

I am preparing to publish Misreading Popper and some other of my ebooks as print-on-demand paperbacks. This facility is available through a publishing associate of Amazon called CreateSpace.

There are still people out there who want books to hold in their hands and I want  to give them that opportunity at a price  that is not much more than the ebook

The reason for this notice is to ask people to let me know about (a) mistakes of all and any kind that you have noticed and (b) any kind of improvements that you think can be made. Please use my email address for this,  not the comments here. The email is rchampATbigpondDOTnetDOTau

The order of publication will probably be

Misreading Popper

Reason and Imagination

The five guides in one volume.

So please focus on those books if you can find the time to dip into some of them again (or even for the first time!).

The full set.


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Hans-Joachim Niemann on Karl Popper, biology and evolution

A copy of Karl Popper and the Two New Secrets of Life has arrived, courtesy of the author. This books is a fine addition to Popperian exegesis,  just when you thought the  cottage industry on the intellectual development of KRP was running out of material.  Check it out on Amazon, I agree with Luc Castelein that it is a five star performance! Unlike some of the over-priced books on the market this one is affordable and real value for money.

I am short of time and I would like to do a slow read, chapter by chapter to explain the new information and the insights of this wonderful book, but I will have to spread this over several days or even weeks.

There is a handy summary.

The story of how humans and all living things came into existence is told in two widely believed versions: the Book of Genesis and Darwin’s Origin of Species. It was the philosopher Karl Popper who presented us with a third story, no less important. His New Interpretation of Darwinism denies the creative power of blind chance and natural selection and establishes knowledge and activity of all living beings as the real driving forces of evolution. Thus, spiritual elements are back in the theory of evolution, and in Popper’s view “the entire evolution is an adventure of the mind.”

In this book, Hans-Joachim Niemann establishes Karl Popper as an eminent philosopher of biology. In the first chapter, biographical details are unearthed concerning how Popper’s biological interests were inspired by a biological meeting in the old windmill at Hunstanton in 1936. The second chapter focusses on the year 1986 when Popper, in several lectures, summarized the results of his life-long biological thinking. The most important of these, the Medawar Lecture given at the Royal Society London, was lost for a long time and is now printed in the Appendix. A new world view begins to emerge that is completely different from Creationism or Darwinism.

Twenty years after Popper’s death, the last chapter looks back on his biological thoughts in the light of new results of molecular biology. His then attacks on long-lasting dogmas of evolutionary theory turned out to be largely justified. The new biology seems even well suited to support Popper’s endeavour to overcome the gloomy aspects of Darwinism that have made organisms passive parts of a machinery of deadly competition. Neither blind chance nor natural selection are the creative forces of all life but knowledge and activity. How they came into existence is still a secret and a worthwhile research programme.

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The debacle of philosophy in the 20th century

Can philosophy progress, or is there an eternal  dialogue on fundamentals that just keeps the issues in play without resolving any of them and moving on, like the successful natural sciences?  Unlike the progress of science,  there are signs of decline in philosophy judging from some of the schools of thought that achieved great prestige in the 20th century.

This situation could be seen as a comedy or a farce, but it should be seen as a tragedy because ideas have consequences and bad ideas are likely to have bad consequences. Some examples are Heidegger’s phenomenology, Sartre’s existentialism, Wittgenstein in both his early and later stages and the logical positivists/empiricists

On the bright side

Of course there are alternatives to the bad philosophers and a some  good philosophy may be done by people who have a notional allegiance to some of the defective movements such as “linguistic philosophy” inspired by Wittgenstein in his second phase.

Phillip Kitcher is reviving the science and practice-oriented philosophy of the pragmatists (Peirce and Dewey) and there are the critical rationalists.

I have taken out most of the post because I want to use some of it for a magazine. It will be substantially different but there is some overlap and I don’t want to prejudice their policy of publishing original work.

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Topics and themes

Planning a paper version of  Reason and Imagination  I have made some minor improvements and Bruce made a major suggestion, to have a new Introduction to spell out the six themes that I have used to introduce the guides and Misreading Popper.

This reminds me of some wise words from C Wright Mills on the treatment of themes and topics in writing a book. He talked about  themes and topics (a distinction which he attributed to a great editor, Lambert Davis).  A topic is a subject which might be treated in a chapter of the book. The order of chapters brings up the issue of themes.

“A theme is an idea, usually of some signal trend, some master conception, or a key distinction, like rationality and reason, for example. In working out the construction of a book, when you come to realise the two or three, or as the case may be, the six or seven themes, then you will know that you are on top of the job. ”

These themes will keep turning up in connection with the different topics, they may appear to be repetitious, they may at first be confused and poorly formulated.

“What you must do is sort them out and state them in a general way as clearly and briefly as you can…cross classify them with the full range of the topics…At some point all the themes should appear together, in relation to one another…maybe at the beginning of the book, certainly near the end…It is easier to write about this than to do it, for it is usully not so mechanical a matter as it may appear…Sometimes you may find that a book does not really have any themes. It is just a string of topics, surrounded of course by methodological introductions to methodolgy, and theoretical introductions to theory. These are indeed quite indispensable to the writing of books by men without ideas. And so is lack of intelligibility”.

So in addition to the introduction to the six themes (1) conjectural knowledge, (2) objective knowledge, (3) no obsession with terms,  (4) the social turn (‘rules of the game’), (5) revival of metaphysics and (6)  significance of evolution I will need to look at each paper to see if it will help to identify how one or more of the themes occur in that chapter.

Or maybe the Introduction can flag the way the themes are related to particular topics in each chapter.


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Papineau on Popper

A student who is studying Popper in London  sent this paper by David Papineau for comment.

His immediate task is to write a paper on induction so I will not dwell on other matters except to suggest that Popper deserves some credit for writing one of the great books of political philosophy in the 20th century and also his “three world” theory is exciting and fertile.

It will help to get clear about the several things which travel under the label of induction.

1. Inductive discovery of  regularities (sometimes unhelpfully called laws), like the sun rises every morning in the east.

2. The (subjective) belief that the sun will rise every morning in the east.

3. Inductive proof of the law or the belief, by observing the sun rising in the east.

4. The shift to probabilities after Hume’s critique has been taken on board and people accept that you can’t actually have inductive proof. So inductive logic becomes the matter of putting probability values on theories (not to be confused with the probability of events or the probability values assigned in statistical analysis  to indicate the probability that the result could have come about by accident rather than a genuine causal relationship).

5. The assertion that induction means the belief that there are regularities in the world, so the future will be like the past, as long as the laws or regularities persist. This has got nothing to do with discovery, or testing, or probabilities and it tends to be the last resort of inductivists when the  other forms of induction are criticized.  It is better described as a metaphysical theory about the world.

After various critiques of the CERTAINTY of scientific knowledge and induction were accepted, the core of the program of inductivism became the quest for inductive probabilities.  In recent decades it seems that Carnap’s quest for objective probabilities had been given up and the core these days is Bayesian subjectivism


In paragraph 5 he wrote “Popper’s philosophy of science centres on his rejection of inductive reasoning. This is the kind of reasoning by which we judge that some hitherto observed pattern will continue to hold good in the future“.

That is induction of the fifth type identified above – the belief in regularities in the world.  Did Popper reject the idea that there are regularities in the word? For him the theoretical or generalizing sciences were all about the quest for (true) laws of nature, that is regularities (or propensities) for systems to behave in particular ways.

That is a metaphysical idea about the way the world works in general, it has nothing to say about the methodology of science or epistemology (how we learn and test our ideas).

Moving on to para 7 we find explicit criticism of Popper’s strategy of conjectures and refutations because it “can only deliver negative knowledge. It shows certain scientific theories are false, but it never shows that any theory is true”.



Papineau mocks Popper by producing two examples of theories – that cigarettes cause lung cancer and matter is made of atoms. What sort of theories are these?  Not all smokers contract lung cancer, so clearly the proposition that cigarettes cause lung cancer is not a universal law. Similarly, atoms are very complex phenomena and they might well be explained in terms of force fields, not the  little billiard balls  of classical atomic theory.

The causes of lung cancer and the structure of matter are much more complex than the picture painted by Papineau, there are many conjectural elements and more work remains to be done. The theory of conjectural knowledge is not demolished by reference to cigarettes causing cancer and the atomic theory of matter.

Moving on to para 11 and 12 Papineau attacks Popper’s demarcation of scientific propositions on the basis of testability. He did not take account of the historical problem situation that created the issue of demarcation: that was the aim of the logical positivist to eliminate metaphysics using the demarcation principle of  VERIFICATION and also the hallmark of science was THE METHOD OF INDUCTION, BASED ON EMPIRICAL DATA, EVIDENCE OR SENSE EXPERIENCE.

Popper argued that the principle of verification could not work (eventually that turned out to be undeniable despite major efforts by the positivists and logical empiricists to use it) and neither would inductive logic to either PROVE  theories to be true or to make them PROBABLE  (with a numerical p  value).

Note that we are  talking about general, explanatory theories that explain how things work, not just statements about things that exist like black swans or atoms.

Popper suggested that to should use evidence to test our theories (because we cannot verify them) and this proposal has two advantages over the positivists.

1. It makes us check to see whether evidence can be brought to bear in the argument that we are having. It may be that the theory at stake is in principle not testable, that does not mean that it is meaningless or trivial, it just means that criticism has to proceed using criteria other than evidence.

2. It liberated the positivists from the quest to eliminate metaphysics  (by making it meaningless) by finding an empirical criterion of meaning.  But they did not want to be liberated and so wasted some decades until nowadays metaphysics has made a comeback to the point where Popper’s theory of metaphysical research programs may be taken seriously.

Papineau went on to write (para 12) that Popper could not say that physics is less speculative than astrology because he cannot claim that atomic theory  is firmly established by a large amount of evidence (as the inductivists claim). And so Popper “is stuck with the non-problem of explaining why some speculations are better than others”.

That is not difficult: we favour theories that are  better tested, that explain more, that predict more accurately, that lead to fertile research programs.



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Popper Symposium featuring Notturno on Hayek

There will be a Popper Symposium in Pennsylvania, September 16-18, 2014.

The twentieth anniversary of Karl Popper’s death (September 17, 1994) provides a fitting occasion to reflect on Popper’s contributions to many fields of inquiry …

A session of the symposium will be devoted to the recent work of Dr. Mark Notturno regarding differences between Popper and Hayek …

The symposium will take place on the campus of Lebanon Valley College in Annville, Pennsylvania …

Here is the link:

This should be interesting. I’ve alway thought there is a lot of possible synergy to be generated between the ideas of Hayek and Popper via their methodological individualism, their theories of knowledge, their institutional approach toward society, and their interest in extended orders.

A good example of this might be Larry Boland’s paper that explores the limits of equilibrium models starting with comments by Hayek and supplementing them with Popper’s ideas about the growth of knowledge:
Equilibrium process vs. equilibrium attainment (pdf).

However, it seems that the Notturno aspect of this symposium will mainly be focused on arguing Hayek had “propensities toward scientism and economism.” There will also be criticism presented of Hayek’s ideas as far as they concern democracy and the rule of law.

If you are interested in attending the symposium, there is still time to submit a paper. Please follow the link and check out the details.

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More Thoughts on Critical Preference

Karl Popper held that the effort of reaching a preference of one theory against others is the key to escaping the trap of the logical error of induction. This position is not a late appendage but is clearly stated for the English-speaking world in “The Logic of Scientific Discovery” (1959), the translated version of “Logik der Forschung” (1934). In Paul Arthur Schilpp’s “Philosophy of Karl Popper” (1974) Volume 2 Popper replies to his critics and gives credit for the fine-tuning of his comments to his associate David Miller. On Page 52 of his own book “Critical Rationalism a Restatement and Defence” (1994), Miller states: “There are no such things as good reasons; that is, sufficient or even partly sufficient favourable (or positive) reasons for accepting a hypothesis rather than rejecting it, or for rejecting it rather than accepting it, or for implementing a policy, or for not doing so”. Miller seems to be addressing reasons that are not critical preferences.

Thomas Stearn Eliot’s much quoted paragraph from “Four Quartets”, “Little Gidding” (1942), echoes my experience when reading Popper:
“We shall not cease from exploration
And the end of all our exploring
Will be to arrive where we started
And know the place for the first time.”

The quest for certainty in 20th and now 21st century philosophy is unfortunately given an epistemic significance that fills volume after volume but leads to no escape from Hume’s problem of induction nor Kant’s problem of demarcation. Relegating Popper to footnotes or comfortable low rungs in textbook chapter organization does not help this quest and, even worse, the strawman naive-falsificationist legend of Popper actually hinders it.

The extract, below, from “The Logic of Scientific Discovery” highlights Popper’s understanding of the problem of the empirical basis in that acceptance of evidence (basic statements) is the result of human decisions, agreements. Science is a human activity, indeed a communal activity. What decides the fate of a theory is decisions not on aesthetic considerations such as how simple (Occam’s Razor) the theory is worded but decisions on what basic statements are to be accepted for attempted rebuttal of theories. The value of simplicity is to improve testability. One must also differentiate between existential trends e.g. statistical samples of events and so-called probability of hypotheses. Popper rejects the latter.
In the 1963 essay “Models Instruments and Truth”, included in the anthology”The Myth of the Framework” (1994) Popper is critical of the ridiculous phrase “truth is relative”. This phrase confuses the choice we make relative to competing theories’ perceived closeness to truth with TRUTH, unsullied by our opinions and efforts. No matter how well we test our theories they may still not be fair reflections of reality.

In the “The Logic of Scientific Discovery” (1959 orig., ninth impression July 1977, Hutchinson & Co, London), Page 108 – Section 30, “Theory and Experiment” Popper states:

“It may now be possible for us to answer the question: How and why do we accept one theory in preference to others?

The preference is certainly not due to anything like an experiential justification of the statements composing the theory; it is not due to a logical reduction of the theory to experience. We choose the theory which best holds its own in competition with other theories; the one which, by natural selection, proves itself the fittest to survive. This will be the one which not only has hitherto stood up to the severest tests, but the one which is also testable in the most rigorous way. A theory is a tool which we test by applying it, and which we judge as to its fitness by the results of its applications.

From a logical point of view, the testing of a theory depends upon basic statements whose acceptance or rejection, in its turn, depends upon our decisions. Thus it is decisions which settle the fate of theories. To this extent my answer to the question, ‘how do we select a theory?’ resembles that given by the conventionalist; and like him I say that this choice is in part determined by considerations of utility. But in spite of this, there is a vast difference between my views and his. For I hold that what characterizes the empirical method is just this: that the convention or decision does not immediately determine our acceptance of universal statements but that, on the contrary, it enters into our acceptance of the singular statements – that is, the basic statements.

For the conventionalist, the acceptance of universal statements is governed by his principle of simplicity: he selects that system which is the simplest. I, by contrast, propose that the first thing to be taken into account should be the severity of tests. (There is a close connection between what I call ‘simplicity’ and the severity of tests; yet my idea of simplicity differs widely from that of the conventionalist; see section 46.) And I hold that what ultimately decides the fate of a theory is the result of a test, i.e. an agreement about basic statements. With the conventionalist I hold that the choice of any particular theory is an act, a practical matter. But for me the choice is decisively influenced by the application of the theory and the acceptance of the basic statements in connection with this application; whereas for the conventionalist, aesthetic motives are decisive.
Thus I differ from the conventionalist in holding that the statements decided by agreement are not universal but singular. And I differ from the positivist in holding that basic statements are not justifiable by our immediate experiences, but are, from the logical point of view, accepted by an act, by a free decision. (From the psychological point of view this may perhaps be a purposeful and well-adapted reaction.)”

For those interested, some further references in his works can be found as follows:

“After the Open Society” (2008), page 10 “Optimist,Pessimist and Pragmatist Views of Scientific Knowledge” (1963)

“The Myth of the Framework” (1994), Models, Instruments and Truth (orig 1963)

“Conjectures and Refutations” (1963), Page 235 – Chapter 10 Truth, Rationality, and the Growth of Knowledge, XIII
Page 248 – XXII

“Objective Knowledge” (1972)
Page 20 – Chapter 1. Conjectural Knowledge, Section 8 Corroboration: The Merits of Improbability

“Realism and the Aim of Science” (1983)
Page 65 – Chapter 1 Induction, Section 4 A Family of Four Problems III
page 71 Chapter 1 Induction, Section 4 A Family of Four Problems VI

“Unended Quest” (1974), standalone printing. Unended Quest is the autobiography included in the two volume Schilpp “The Philosophy of Karl Popper”

P. A. Schilpp, “The Philosophy of Karl Popper” (1974)
Page 1024 + Replies to My Critics -Section 14 The Psychological and Pragmatic Problems of Induction

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