You Don’t Agree with Karl Popper

I’m teaching Philosophy of Science this semester. It’s a fun class. I just had occasion to discuss the philosophy of Sir Karl Popper, who was among the most influential philosophers of science of the last century (and therefore of all time). He had an enormous influence on our intellectual culture, especially on scientists, and he is the reason why you hear complaints about “unfalsifiability” from time to time (accusations that religion is “unfalsifiable”, etc.).

If you’re a philosopher of science, you probably don’t take Popper’s philosophy seriously. But if you’re an ordinary person, or a libertarian, or especially a scientist, there is a pretty good chance that you think you agree with Karl Popper.

So, as a public service, I am here to explain to you that no, you probably do not agree with Popper at all — unless you are completely out of your mind.

What He Said

You probably associate Popper with these ideas: It’s impossible to verify a theory, with any number of observations. Yet a single observation can refute a theory. Also, science is mainly about trying to refute theories. The way science proceeds is that you start with a hypothesis, deduce some observational predictions, and then see whether those predictions are correct. You start with the ones that you think are most likely to be wrong, because you’re trying to falsify the theory. Theories that can’t in principle be falsified are bad. Theories that could have been falsified but have survived lots of attempts to falsify them are good.

I wrote that vaguely enough that it’s kind of what Popper said. And you might basically agree with the above, without being insane. But the above paragraph is vague and ambiguous, and it leaves out the insane basics of Popper’s philosophy. If you know a little bit about him, there is a good chance that you completely missed the insane part.

The insane part starts with “deductivism”: the view that the only legitimate kind of reasoning is deduction. Induction is completely worthless; probabilistic reasoning is worthless.

If you know a little about Popper, you probably think he said that we can never be absolutely certain of a scientific theory. No, that’s not his point (nor was it Hume’s point). His point is that there is not the slightest reason to think that any scientific theory is true, or close to true, or likely to be true, or anything else at all in this neighborhood that a normal person might want to say.

Thus, there is no reason whatsoever to believe the Theory of Evolution. Other ways of saying this: we have no evidence for, no support for the Theory of Evolution. There’s no reason to think it’s any more likely that we evolved by natural selection than that God created us in 4004 B.C. The Theory of Evolution is just a completely arbitrary guess.

(This is not something special about Evolution, of course; he’d say that about every scientific theory.)

This is not a minor or peripheral part of his philosophy. This is the core of his philosophy. His starting point is deductivism, which very quickly implies radical skepticism. The deductivism is the reason for all the emphasis on falsification: he decided that since one can’t deduce the truth of a theory from observations, the goal of science must not be to identify truths. But, since we can deduce the falsity of a theory from observations, the goal of science must be to refute theories.

As I say, most people don’t realize that this is Popper’s view — even though he makes it totally clear and explicit. I think there are two reasons why people don’t realize this: (a) the view is so wildly absurd that when you read it, you can’t believe that it means what it says; (b) Popper’s emotional attitude about science is unmistakably positive, and he clearly doesn’t like the things that he calls “unscientific”. So one would assume that his philosophy must give us a basis for saying that scientific theories are more likely to be correct than unscientific ones. But then one would be wrong.

Now, in case you still can’t believe that Popper holds the irrational views I just ascribed to him, here are some quotations:

“We must regard all laws and theories as guesses.” (Objective Knowledge, 9)

“There are no such things as good positive reasons.” (The Philosophy of Karl Popper, 1043)

“Belief, of course, is never rational: it is rational to suspend belief.” (PKP, 69)

“I never assume that by force of ‘verified’ conclusions, theories can be established as ‘true’, or even as merely ‘probable’.” (Logic of Scientific Discovery, 10)

“[O]f two hypotheses, the one that is logically stronger, or more informative, or better testable, and thus the one which can be better corroborated, is always less probable — on any given evidence — than the other.” (LSD, 374)

“[I]n an infinite universe […] the probability of any (non-tautological) universal law will be zero.” (LSD, 375; emphasis in original)

About the last quotation, note that many scientific theories contain universal laws (e.g., the law of gravity). So, Popper is not just denying that we can be certain of these theories, and not just denying that they are likely to be true; he claims that they are absolutely certain to be false.

In the penultimate quotation, note the “on any given evidence” clause: When you get done testing your scientific theory, and it survives all tests, you can’t say that it’s likely to be correct; it’s less likely to be correct, even after you’ve gathered all the evidence, than some unfalsifiable, unscientific theory.

All of this is the sort of view that you would expect from the most extreme science-hater. The weird thing about Popper is that he inexplicably combines this stuff with a strong positive evaluation of science. We have no reason to believe in science, and pseudoscience is more likely to be correct, and in fact the paradigmatic scientific theories are definitely wrong . . . but hey, isn’t science great? Okay, now let’s get on with the wonderful science stuff!

I can’t explain this combination of attitudes. I don’t think Popper ever attempted to explain it himself.

By the way, the core idea — deductivism, and inductive skepticism — seems to be surprisingly popular among philosophers. It’s ridiculous. It’s like if a major position within geology were that there are no rocks.

How He’s Wrong

I’m only talking about objections that I like here, so I’ll ignore objections based on Thomas Kuhn.

The Duhem-Quine Thesis

This is something widely accepted in philosophy of science: a typical scientific theory doesn’t entail any observational predictions by itself. You at least need some auxiliary assumptions.

Ex.: Newton’s Theory of Gravity, together with Newton’s Laws of Motion, are sometimes said to entail predictions about the orbits of the planets (in particular, to predict Kepler’s laws). But that’s false. Newton’s second law only gives the acceleration of a body as a function of the total force acting on it. The Law of Gravity doesn’t tell you the total force on anything; nor is “total force” observable. So there are no observational predictions from this set of laws, even when combined with all our observations.

Of course, the laws and the observations make certain patterns of motion more plausible or likely than others. If the planets moved in squares around the night sky, it would be very implausible to explain that using Newton’s laws + the hypothesis of some unknown, unobservable forces. But that is completely irrelevant for Popper. Again, for Popper, the only thing that counts as scientific reasoning is deducing the falsity of a theory from observations. You’re not allowed to appeal to any probabilistic judgments to support a theory.

Probabilistic Theories

Another counter-example: Quantum Mechanics. It’s a scientific theory if anything is. But it is clearly unfalsifiable, because all of its observational predictions are probabilistic. It enables one to calculate the probabilities of different possible observed results, but a probabilistic claim (as long as the probability isn’t 0 or 1) can never be falsified — i.e., you can’t logically deduce the falsity of the probability claim from observations. And again, that’s the only thing you’re allowed to appeal to. So, on Popper’s view, quantum mechanics must be unscientific.

Evolution

But QM is weird. So let’s think about some perfectly ordinary, paradigmatic examples of scientific theories. Real scientific theories, by the way, are not normally of the form “All A’s are B” (as in philosophers’ examples).

Here’s one: the Theory of Evolution. Humans and other living things evolved by natural selection from simpler organisms, over a long period of time. Here’s an example of the evidence for this: some of the larger constrictor snakes have degenerate hind limbs underneath the skin. This can be explained, in the theory of evolution, by the hypothesis that they evolved from lizards. On the rival theory (Creationism), there’s no obvious explanation.

This isn’t a matter of deduction. The Theory of Evolution does not entail that the larger constrictors would have subcutaneous degenerate hind limbs. It merely gives a reasonable explanation of the phenomenon, which the rival theory doesn’t (but creationism doesn’t entail that there wouldn’t be such degenerate hind limbs; it merely fails to explain why there would).

The Dinosaur Extinction

Here’s another theory: the dinosaurs were driven extinct by a large asteroid impact. And here’s some evidence: there is an enormous crater at the edge of the Yucatan Peninsula in Mexico (the Chicxulub Crater), partly underwater. The crater has been dated to about the time of the Cretaceous–Paleogene extinction event. That’s evidence that an asteroid impact caused the mass extinction.

Again, that’s not deductive. The asteroid-impact theory of the extinction does not entail that we would find a crater. (It’s logically possible that the crater would have been filled in, or that the asteroid hit a mountain and didn’t leave a visible crater, or that the crater was somewhere we haven’t looked, etc.) It merely makes it much more likely that we would find a crater.

So, Popper’s philosophy entails that the Theory of Evolution and the asteroid-impact theory are unscientific, besides that we have no evidence at all for either of them.

The Obvious

Of course, the obvious problem is that it’s absurd to say that we don’t have any reason to think any scientific theory is true. We have excellent reasons, for example, to think that humans evolved by natural selection. There is not any serious doubt about that in biology, and that is not something that we should be arguing about. And in fact, I’m not going to argue about it, because I just don’t think that’s serious.

The Correct Theory

What is the correct view of scientific reasoning? Basically, the Bayesian view: it’s probabilistic reasoning.

Take the example of the degenerate hind limbs again: that is evidence for the theory of evolution because it’s more likely that we would see stuff like that if organisms evolved by natural selection, than it is if they were all created by God in 4004 B.C. (or, in general, if they did not evolve). In standard probability theory, that entails that Evolution is rendered more probable by our seeing things like the degenerate hind limbs.

Why Care About Falsifiability?

There really is something important about falsifiability. Intuitively, there is something bad about unfalsifiable theories, and we have Popper to thank for drawing attention to this.

Unfortunately, almost no one seems to have any idea why falsifiability matters, and Popper did not help with that situation, because his theory is incapable of accepting the correct explanation.

The correct explanation is a probabilistic/Bayesian account. In the Bayesian view, a hypothesis is supported when P(h|e) > P(h) (the probability of hypothesis h given evidence e is greater than the initial probability of h). It is a trivial theorem that, for any e, P(h|e) > P(h) iff P(h|~e) < P(h). In other words: e would be evidence for h if and only if the falsity of e would be evidence against h.

That means that if nothing counts as evidence against h, then nothing counts as evidence for h either. But if there’s no evidence for h, then one typically shouldn’t believe it. This is why you shouldn’t believe unfalsifiable theories. By contrast, falsifiable theories are supportable: when one tries and fails to falsify them, their probability goes up.

The point is more general than a point about Popperian falsifiability. Let’s say a theory is “falsifiable” iff one could deduce its falsity from some possible observations, and a theory is “testable” iff there are some observations that would lower the probability of the theory. Then the general point to make is that one can have evidence for a theory iff the theory is testable (it needn’t also be falsifiable), and good scientific theories should be testable.

Popper couldn’t say this, because he was obsessed with deduction, and this is a point about probabilistic reasoning, not deduction. Popper didn’t eschew all talk of probabilities; he just insisted on the most perverse probability judgments (e.g., that scientific theories are less likely to be correct than unscientific ones, even after they survive stringent tests). Which would make one wonder why anyone should prefer scientific theories. Obviously, the correct view is that scientific theories, after surviving tests, become more probable.

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Michael Huemer

Michael Huemer is a professor of philosophy at the University of Colorado. He is the author of more than seventy academic articles in epistemology, ethics, metaethics, metaphysics, and political philosophy, as well as six amazing books that you should immediately buy.

29 thoughts on “You Don’t Agree with Karl Popper”

  1. This is a bit distressing. I’d heard a lot of good about Popper before, mostly talking vaguely about falsifiability. The vague versions of his philosophy that seem to have filtered into popular understanding (say Nassim Taleb’s Antifragile) always seemed to me very robust. I had no idea about his obsession with deduction. Was Popper just crazy?
    Also, I look forward to your next post, where you find a way to rag on Bayesian methods too.

  2. The story he tells about how the idea of being able to falsify a theory is quite amazing. The story was he asked psychoanalyst what is the proof of his theory. The answer given was “I have a thousand case files showing this. Popper answered him” and if you will have another one it will just show what you already think.” That is when it occurred to him the crux of the issue is not how much proof you have. But whether there is something that can show it is false. That is when it occured to him that psychology is pseudo science because no matter what theory anyone has, they never give any kind of case that could show they are wrong.

    1. Inductivist theories of scientific reasoning can accommodate this idea without adopting falsificaionism. For example, in Bayesianism, you can prove that some evidence can only support a hypothesis insofar as its negation would undermine that hypothesis. In the limit, the hypothesis gets the maximum probabilistic boost from discovering some evidence (ceteris paribus) if the hypothesis entails the evidence, but if the hypothesis entails the evidence, then searching for the evidence raised the possibility of falsification.

      Making bold predictions is good, but not for the reasons Popper said. It’s because making bold predictions and having those predictions confirmed makes for strong evidence.

  3. Thanks for this public service post, Michael. Sadly, I was one of those brought up to revere “Popperian falsifiability” and knew nothing of his extreme, inane views. Perhaps most importantly, you are right to shift the discussion from falsifiability to testability, another distinction to which I had failed to attend. Thank you for your clear, insightful, and thorough post.

  4. There is much that is poignant and important about Popper’s ideas. Popper must be accorded recognition in science and (of) philosophy. However, we do not jettison others because Popper appears so dominant. Verification is still relevant; justification is required for coherent and valid argumentation. Many of Popper’s claims have been and are being disproven, eg the rejection of Hegelian, Marxist and Freudian theories and hypotheses. Popper should be understood in context – in the contexts of science, discovery and justification, and philosophy, and in his own theories where picking out phrases and interpreting his theory in this light is inadequate. The contexts of his statements must be included.

  5. Great post, Mike! Reminds me of a paper by Carol Cleland from 2001.

    “rejecting a hypothesis in the face of a failed prediction is sometimes the wrong thing to do; it is not an accident that logic gives us the option of rejecting an auxiliary assumption instead. In short, logic does not dictate that scientists behave like good falsificationists, and scientists do not in fact behave like good falsificationists. …. a close look at the work of experimental scientists suggests that they are primarily concerned with protecting their hypotheses against false negatives and false positives, as opposed to ruthlessly attempting to falsify them.”

    The paper: https://www.icloud.com/iclouddrive/0_m7CjeTpqFItBx4rnRCncB0Q#cleland-carol-2001-historical-science-experimental-science-and-the-scientific-method

  6. “but a probabilistic claim (as long as the probability isn’t 0 or 1) can never be falsified”

    Actually, not even this is true. Supposed you have to pick a random real number between 0 and 1 (assuming any continuous probability distribution). The probability that you will pick 0.5 is 0, but it clearly is possible and might happen once you do the experiment, since 0.5 is a real number between 0 and 1. Also, the probability of NOT picking 0.5 is 1, but it’s possible it doesn’t happen since you can pick 0.5 as a random real number between 0 and 1.

    So even events with probability 0 may happen, and events with probability 1 may not happen.

  7. We can apply a useful test to this article: to judge a critics’ reply to an argument, first check how well they set out the original argument itself.

    I’m unconvinced here.

    Is there a longer version of this article that better establishes the rather remarkable claims about Popper’s original writings?

    And does the author accept that other writers critical of Popper have nevertheless somehow come to a different reading of his arguments?

    1. David, I am not writing an article about Popper, because I have a lot of other things to do. If you want to verify what I ascribed to Popper, look at the sources I quoted. Those quotes are all really in there, and they are not taken out of context.

      I of course did not explain the reasons Popper gives for his ideas, nor his responses to objections, because this is a blog post, not a book.

      The main point about Popper is really not open to dispute. There is no question that Popper was an inductive skeptic. It’s stated clearly, repeatedly, and explicitly, and it’s at the core of his entire philosophy. This is not something that a reasonable person who reads the text can disagree with. And that is the core of my objection to him — that is the thing I am saying is “insane”.

      Some people didn’t like the way I quoted his remark about the zero probability of laws, without explaining his reasons and what he meant by that. But, again, (a) this is a blog post, not a book, and (b) the main problem is his insane inductive skepticism.

  8. I want to opine – as a professional philosopher of science who has written often on Popper (including for encyclopaedias and the like) – that this blog entry neither accurately reflects Popper’s views nor presents reasonable criticisms thereof. The post contains several significant errors: on the impossibility/’absolutely certain’ falsity of zero probability statements (on a _logical_ interpretation of probability), the nature of probabilistic reasoning, and so forth. (Why would Popper define corroboration in terms of probabilities, create new axioms of probability, and propose new interpretations of probability, while holding that probabilistic reasoning is ‘useless’?) I worry Clapsaddle is correct that the author has done his students a disservice.

    I only post this for the benefit of those who stumble on this blog. I am not committed to defending Popper’s views; indeed, I have criticised them often in print (and nowadays hardly ever write on them). (Incidentally, there are no new criticisms in the post. Such criticisms, some of which are over fifty years old, can be found in the Schilpp volume on Popper – e.g., in the contributions by Putnam and Salmon – in the work of Lakatos, Newton-Smith, Stove, and so on. They have been responded to in print by Popper and other philosophers, such as Bartley, Miller, and Musgrave, on numerous occasions. It requires a great deal of time and effort to read this material carefully and appreciate it. It is intellectually difficult. I believe one should approach the matter with humility, and not represent the extensive scholarly debate on these matters in such a black-and-white fashion.)

    1. Darrell, would you deny that Popper was a deductivist? And an inductive skeptic? And that he gave falsifiability as the criterion for demarcating scientific theories? Because those are the main things I ascribed to Popper.

      I also provided quotations from Popper, saying the things I ascribed to him. Readers who doubt me are invited to look up those quotes.

      About the lack of new criticisms in the post: I did not claim that my criticisms of Popper were new. I note a couple of odd things, though:

      – If Popper really didn’t say the things I ascribed to him, it would be odd that these other authors would have made the same objections to Popper that I did.
      – It would also be odd if they made those objections even though (as your first sentence says?) none of them are reasonable objections – unless of course Putnam, Salmon, Lakatos, Newton-Smith, and Stove are bad and unreasonable philosophers.

      1. Michael, Popper wrote a lot on verisimilitude, especially in his later years (see the SEP article on Truthlikeness). He also wrote a lot about how theories become corroborated. Those two facts – simple enough to check, I’m sure – go directly contra this sentence of yours:

        > Thus, there is no reason whatsoever to believe the Theory of Evolution [according to Popper]

        This is *very* wrong, just on the face of it. It is also very wrong because Popper wrote extensively on Evolution, seee.g. https://ncse.ngo/what-did-karl-popper-really-say-about-evolution

        Now, with that out of the way, there is a plethora of reasons not to be a Popperian about science, and a lot of quite bad reasons scientists (especially social scientists) cling to Popper. I suspect half of it is that Popper is the entirety of philosophy of science they learn about; but also that Popper can be read normatively in a way that scientists find rather flattering.

        So please, take the criticism of Dannell, who I’m sure knows much more about Popper than me.

        1. Dominik, the word “corroboration”, in ordinary English, refers to evidential support. But that is not Popper’s use; he is using it as a special, technical term, and he clearly explains that it does not mean that you have evidence in favor of the theory or that the theory is likely to be correct.

          Popper’s endorsement of inductive skepticism is really extremely clear. He says it totally explicitly, repeatedly. We should not be arguing about that.

          As to his writing about evolution: I never said that he didn’t write about evolution. I also didn’t say that he didn’t write about verisimilitude.

          1. Again, I’m pretty baffled by your reply. It is very clear from reading Popper or good secondary literature (shout-out to the SEP) that Popper’s project is as follows:

            1) Inductivism doesn’t work, and scientists don’t actually use it

            2) Science is too important to leave it at that. We need a criterion of demarcation

            3) Enter falsification: Science *actually* proceeds by hypothesis testing and deductivism

            4) This implies that we can only falsify theories by finding evidence against hypotheses/predictions

            5) if a theory survives sufficient rounds of falsification testing, we may change its status to well-corroborated

            Please note that “corroboration” enters the discussion because, it appears, Popper did not want to use the term “confirmation”, cause that came with a lot of baggage from the logical positivists. See also the chapter on page 273 in the LSD.

            Now, unfortunately you are quote-mining to support your wrong interpretation of Popper. Let me briefly point out what is meant by Popper and why it doesn’t support what you think it does:

            “We must regard all laws and theories as guesses.” (Objective Knowledge, 9)

            The point here is that Popper thinks scientists shouldn’t get attached to their theories. One gaping hole in early Popper is, indeed, that he has no clear idea of how scientists come up with theories. He makes up for that by saying it is not important; what is important is falsification. Once a theory survives that, we can call it corroborated or – if you must – confirmed, in a weak sense, since theories can always still be falsified, or a better theory can come about.

            “There are no such things as good positive reasons.” (The Philosophy of Karl Popper, 1043)

            Again, the point here isn’t that Popper is against science. Positive reasons being inductive reasons, Popper thinks they can never be logically valid – hence the whole spiel about falsification.

            “Belief, of course, is never rational: it is rational to suspend belief.” (PKP, 69)

            Again, this does not imply anything on whether some theories are better (or more likely to be true) than others. Rather, I would see this as “not getting attached” again.

            “I never assume that by force of ‘verified’ conclusions, theories can be established as ‘true’, or even as merely ‘probable’.” (Logic of Scientific Discovery, 10)

            Again, if you read this quote in context, it means something very different. On this page of the LSD, Popper writes that his theory is superior because it makes no use of inductive, or positivist, terms. His project is precisely to show how science can establish knowledge without terms such as “true” or “probable”. This notably does not mean that Popper thinks there is nothing at all that supports successful theories. Actually, it’s the opposite: Repeatedly surviving falsification testing makes the theory of evolution, and other scientific theories, much more than a completely arbitrary guess.

            “[O]f two hypotheses, the one that is logically stronger, or more informative, or better testable, and thus the one which can be better corroborated, is always less probable — on any given evidence — than the other.” (LSD, 374)

            Note where this is, and what context there is to it. The idea here is that the logically stronger theory allows for less statements. “Probable” should be read as “on the outset, this theory allows for a smaller circle of statements” – there are less ways the world are that are compatible with the logically stronger theory. As a simplistic example, the theory of relativity is logically stronger than Newtonian physics, because it is *more precise*.

            Again, that has nothing to do with not believing in it.

            “[I]n an infinite universe […] the probability of any (non-tautological) universal law will be zero.” (LSD, 375; emphasis in original)

            Note, again, that this is a highly technical phrase in an appendix. Two paragraphs further down, it is clarified that probability means somethign else than what you have in mind.

            Again, let me reiterate that I am not a Popperian. I have no stake in this except that I am fascinated with why (social) scientists cling to Popper’s philosophy of science. Unfortunately, you severely misinterpret Popper.

  9. Popper argued no such thing as there being no reason to prefer one theory over another. He has claimed that there can be no *positive* reason to accept a scientific theory (you even quotes him on this), which is to say that an additional data point, another observation, cannot add to the certainty of a theory or increase its likelihood of being true. (A trillionth observation of a white swan isn’t reason to accept the theory that “all swans are white”; there could be a single black one.) There can only ever be a *negative* reason to (temporarily) accept a scientific theory: its rivals may all have failed to solve the relevant problem whereas *it* hasn’t (yet).

    Popper rejected induction, but that doesn’t mean he lauded deduction. His *actual* argument was that the starting point—the thing from which all progress springs, in all of life and not just in science—is problems.

    The starting point couldn’t be observation, because we can only ever observe an unimaginably tiny fraction of everything, and there is no principle or law that enables us to extrapolate from the particular to the universal. Theory comes first, because data are interpreted in light of some explanation. An observation is necessarily for or against something, some assumption, expectation or view.

    Popper suggested that the actual process by which we gain knowledge is that we identify our problem, guess what might be its solution, and evaluate (which is to say criticize) our guesses. We can safely reject any attempt at a solution that doesn’t survive criticism. But since we are fallible, and cannot generalize from the particular to the universal or know that a single future observation won’t be inconsistent with our theory, Popper argued that there is no reason to rush to accept its truth only because it has yet survived criticism. Such a theory can at best be promising, and subjected to more stringent tests down the line. It must certainly be said to *contain* some truth that its rivals did not, or to lack some falsity that its rivals did not—simply to approximate truth better than its rivals. But we don’t know that the theory itself isn’t true, and we shouldn’t pretend otherwise.

    You express confusion over Popper’s claim that the probability that a theory is true decreases with its content. But this is an uncontroversial claim. The so-called “Linda problem” illustrates the same thing. As a matter of pure probability, any two propositions in conjunction are less probable than either of them by itself. An unscientific theory is one that contains few to no testable claims; its statements about reality will be able to be reinterpreted in light of any evidence for or against it. A scientific theory is one that makes bold, testable claims, and the more it makes, the more testable it is. Such a theory must rationally be said to be preferred, since it can survive criticism or collapse under it, leaving hope of discovering something better. But as a matter of pure probability, the scientific theory is less probable. Popper did not say this to say that the aim of science should be *improbable theories*, but that probability has nothing whatsoever to do with progress in science. The aim of science is to explain reality by problem-solving through error-correction.

    Before Einstein, it was taken for granted that Newton had discovered the truth about physics, and there were countless observations to back that up. But he hadn’t. You may now argue that Einstein has, but just as people in Newton’s time couldn’t anticipate the fundamentally different explanation that relativity provided, we cannot today anticipate whatever discoveries might happen that would help overthrow relativity. Eventually, we have to grow tired of the game of continually calling our latest theory the *actual* truth when the best one before that has been shown to be false.

    This is why Popper claimed that all our best theories are almost certainly false, and therefore that we mustn’t *believe in* them, but only look for their errors and correct them. This is not to say that there is no reason to prefer one over another, but to acknowledge our fallibility and hope for indefinite progress in science. Popper imagined science to be an unended chain of conjectures and refutations, moving towards ever better approximations to the truth.

    Thanks a lot anyway for publishing these thoughts and therefore subjecting them to criticism. I hope I’ve helped make clear, together with Bill Clark and Jonathan Clapsaddle, that there’s a lot of mistakes in here.

    1. “Eventually, we have to grow tired of the game of continually calling our latest theory the *actual* truth when the best one before that has been shown to be false.”

      Sounds like inductive reasoning to me…

    2. Thank you for your thoughts, Noa. But I don’t see how you’ve identified any mistakes.

      Some of what you said was simply (re)description of Popper’s views that was compatible with everything I said. Most of your text is ambiguous and doesn’t take a stand on the main problem (you write a lot of sentences that could be interpreted as: “Popper thinks we can’t be *certain* of theories, but at least we have good reasons to think they’re correct.” — which of course is not his point at all).

      I note, in particular, that my main objection was not that Popper supported deduction, nor was it that he denies that we can be absolutely certain of our theories. My main problem — the thing I thought was “insane” — was about his rejection of induction, and his saying that we can’t even conclude that scientific theories are probable. And there just is no question that those were Popper’s views. Those were the central core of his philosophy. He makes these completely explicit, repeatedly.

      Your interpretation of the “no positive reasons” quote seems to be “Oh, so he must be saying that we have a different kind of reason for believing a theory, a ‘negative reason’.” That is not his point at all. His point is that you can never have a reason for believing a theory; you can only have a reason to reject a theory.

      Again, this is at the core of his whole philosophy, and really is not something that we should be arguing about. This is like arguing about whether Trump is anti-immigration or not.

      1. Thanks for the response, and for some clarification. I hope I can make some myself. I’m gonna focus only on your characterization of Popper’s views.

        Yes, (part of) the core of Popper’s philosophy was that 1) induction is not a thing, 2) good scientific theories aren’t more probable than any other and 3) there is no reason to believe in them. I agree we shouldn’t argue about that. But you claim yourself that you’re unable to resolve the conflict between these three points and Popper’s strong advocacy for the scientific method (you use expressions such as “The weird thing about…,” “he inexplicably…” and “I can’t explain…”). You draw the conclusion from the conflict that Popper is insane and irrational. I guess a simpler explanation: you’ve misunderstood all three.

        As for 1: Induction supposes that we derive our theories from data, and that there is some principle that enables us to draw universal conclusions from particular observations, especially with increasing number, depth and precision. But this idea itself was not derived from any data. It is a guess that that is how knowledge grows. Another issue is that *no* knowledge is derived from data, because data are necessarily interpreted in light of some preconceived theory. The actual purpose of observation is to test such a theory: if a supposition (like “all swans are white”) does not survive contact with an observation (like that of a black swan) then it’s false. A bigger issue still is that we cannot predict the content of future knowledge, so any measure of the probability of a current theory to be true is a gross underestimation of our most important discoveries: those that haven’t happened yet, which may well rule out nearly everything we believe today.

        2: The probability of A is higher than the probability of the conjunction of A and B: Pr(A ∧ B) ≤ Pr(A), and also Pr(A ∧ B) ≤ Pr(B). A scientific theory that makes fewer bold claims (say, Newtonian physics) than another (say, the general theory of relativity) is more probable than the other for the same reason.

        Your mistake here is that you conflate Popper’s ideas about probability with his ideas about truth. You say for instance: “His point is that there is not the slightest reason to think that any scientific theory is true, or close to true, or likely to be true.” You’re right about the “likely to be true” part, on the analysis of the calculus of probability above, but dead wrong on the “close to true” part, which is a different argument entirely.

        Popper says of ideal scientific theories that they are improbable not to say that we should *aim* for low probability, or for that matter that it’s unimportant whether a theory has some truth to it or not. It’s to say that probability is entirely *outside* the aim of science; that it is a justificationist distraction; that the *truth* is approximated using a fundamentally different method than the calculus of probability. The method, he suggests, is one of error-correction. When we make conjectures about how the world works, and they don’t survive observational tests, we either abandon them to the benefit of other, better conjectures (better only in the sense that they survive those tests), or improve them by abandoning flaws in their explanation.

        Popper has written extensively about the approximation to truth, and the fact that some theories (Einstein’s) are *truer* than others (Newton’s), in the sense of containing more truth or less falsity. Note that Einstein’s theory is not Newton’s theory plus more data; it is the replacement of the conjecture of a force of gravity with the bold conjecture of spacetime, which happens to have survived all criticism thought of so far. So it’s Newton’s theory minus some egregious errors.

        The reason Popper didn’t go as far as to say that Einstein’s theory is itself *true* is that we can’t know which future improvements will be made to it. If a better theory crops up, by all means with more numerous and precise data (but more crucially with a better testable, bolder, harder-to-vary explanation), then rationally we will have to conclude that general relativity is false. (This is even to be expected, since there is a radical conflict between it and quantum mechanics, our two deepest theories about reality at the moment.)

        This does not mean that the huge benefit of the general theory of relativity disappears, or that we should mindlessly rely on earlier, killed off theories or randomly guessed, untested theories to explain reality.

        3: Popper rejects belief for the reasons I’ve described here. If we expect that all our best theories are to be superseded, then it would be irrational to invest any “faith” in them. All we should do is aim to create ever better explanations of reality and hope to correct errors where they inevitably occur.

        In a rather banal sense of the word “believe,” scientists “believe” in their theories when they put them forth and use them to make predictions. But Popper’s point was not that it’s irrational to *use* theories or to prefer one over another; it was that it’s irrational to think we have stumbled upon wholesale truth in our latest one of endless attempts to understand the world better and to commit to that attempt for whatever psychological reasons implied by the word “belief.” We may well be wrong.

        One major part of the core of Popper’s philosophy that you’ve neglected to mention is fallibilism, that we and all our knowledge are fallible (liable to contain error). It is partly because there is no upper bound on the mistakes we can make that it’s irrational to suppose that we can ever establish the truth of any theory. There is also a practical reason: if we do suppose so, then there is incentive to stop looking for something better, and that helps stall the growth of scientific knowledge.

        Summary of the main points:
        • Scientific theories that are testable are better than those that aren’t, because if a bold conjecture survives all our best criticism, it is a useful approximation to the truth, and it can be subjected to further criticism and carry the search for knowledge forwards. If a bold conjecture is immune to all criticism, then it is useless and whatever truth it contains impossible to come by.
        • Since a theory is less probable the more it says about reality (in the sense of the calculus of probability, e.g. as illustrated by the conjunction fallacy), probability cannot help determine the truth of scientific theories. The aim must be something else.
        • There is never any guarantee that our best theory for the moment will survive future criticism, so it is pointless to invest belief in it or expect that the accumulation of more confirming instances will increase its chances of being true. We should instead try to think of ever better theories.

        (I appreciate that you only interact with people who respond without personal attacks! I’m only engaging with this post because I thought it was fun, and I’m trying not to come across as criticizing *you* but only what I consider to be some misconceptions. Pardon me for the wordiness.)

  10. The article seems to confuse belief with truth. The Bayesian approach is simply a description of how we react subjectively to the ongoing success of a theory—we tend to believe it more as time goes on. However, the truth of the theory is independent of our belief. Newton’s theory was not true, even though it was widely believed and quite successful in responding to apparent falsifications for over 200 years (1687-1919 if I remember correctly).

    1. (1) At least in epistemology, Bayesianism is a normative theory, not a descriptive theory. For example, no-one actually has a deductively closed algebra over their entire language, but Bayesians argue that this is a normative ideal for belief, i.e. if you have a degree of belief for a set of statements {H1, H2 … Hn}, then you should have degrees of belief for every statement that can be validly derived from elements of that set.

      (2) As Lakatos argued, Newtonian physics isn’t actually falsifiable in the Popperian sense. I know that Popper tries to respond to this criticism in The Philosophy of Karl Popper, Volume 2, but he begs the question against Lakatos (“… if [Popper’s tea-cup] moved in a non-Newtonian direction…”). You can only falsify Newtonian physics by inferring – by induction! – various scientific hypotheses providing bridges between the Newtonian formulas and observable reality, e.g. in the case of Mercury, that there is no force acting on it to make it move in a supposedly “non-Newtonian” path.

  11. Test

    It appears comments with urls to youtube appear only after blog owner approves of them

    It appears comments without urls automatically get accepted immediately?

    Looks like comment system maybe working better if you just allow people to freely comment and if someone says something you dont like then just delete the comment or ignore it

  12. Michael in under 5 mins my 2 short video clips of the late Richard Rorty and Mr Peterson succinctly summarize what (i think) youre trying to say with this essay above. I would like to share with you and fellow readers for improved clarity, how can I? Your comment system appears to be blocking links to youtube

  13. Sorry for the repeated posts. Maybe I just wastnt patient enough.
    Hope you enjoyed the Rorty and Peterson vids.

    Kind Regards

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