The general theory of relativity is sound science; ‘theories’ of psychoanalysis, as well as Marxist accounts of the unfolding of historical events, are pseudoscience. This was the conclusion reached a number of decades ago by Karl Popper, one of the most influential philosophers of science. Popper was interested in what he called the ‘demarcation problem’, or how to make sense of the difference between science and non-science, and in particular science and pseudoscience. He thought long and hard about it and proposed a simple criterion: falsifiability. For a notion to be considered scientific it would have to be shown that, at the least in principle, it could be demonstrated to be false, if it were, in fact false.

Popper was impressed by Einstein’s theory because it had recently been spectacularly confirmed during the 1919 total eclipse of the Sun, so he proposed it as a paradigmatic example of good science. Here is how in Conjectures and Refutations (1963) he differentiated among Einstein on one side, and Freud, Adler and Marx on the other:

Einstein’s theory of gravitation clearly satisfied the criterion of falsifiability. Even if our measuring instruments at the time did not allow us to pronounce on the results of the tests with complete assurance, there was clearly a possibility of refuting the theory. The Marxist theory of history, in spite of the serious efforts of some of its founders and followers, ultimately adopted [a] soothsaying practice. In some of its earlier formulations … their predictions were testable, and in fact falsified. Yet instead of accepting the refutations the followers of Marx re-interpreted both the theory and the evidence in order to make them agree. In this way they rescued the theory from refutation … They thus gave a ‘conventionalist twist’ to the theory; and by this stratagem they destroyed its much advertised claim to scientific status. The two psycho-analytic theories were in a different class. They were simply non-testable, irrefutable. There was no conceivable human behaviour which could contradict them … I personally do not doubt that much of what they say is of considerable importance, and may well play its part one day in a psychological science which is testable. But it does mean that those ‘clinical observations’ which analysts naively believe confirm their theory cannot do this any more than the daily confirmations which astrologers find in their practice.

As it turns out, Popper’s high regard for the crucial experiment of 1919 may have been a bit optimistic: when we look at the historical details we discover that the earlier formulation of Einstein’s theory actually contained a mathematical error that predicted twice as much bending of light by large gravitational masses like the Sun – the very thing that was tested during the eclipse. And if the theory had been tested in 1914 (as was originally planned), it would have been (apparently) falsified. Moreover, there were some significant errors in the 1919 observations, and one of the leading astronomers who conducted the test, Arthur Eddington, may actually have cherry picked his data to make them look like the cleanest possible confirmation of Einstein. Life, and science, are complicated.

This is all good and well, but why should something written near the beginning of last century by a philosopher – however prominent – be of interest today? Well, you might have heard of string theory. It’s something that the fundamental physics community has been playing around with for a few decades now, in their pursuit of what Nobel physicist Steven Weinberg grandly called ‘a theory of everything’. It isn’t really a theory of everything, and in fact, technically, string theory isn’t even a theory, not if by that name one means mature conceptual constructions, such as the theory of evolution, or that of continental drift. In fact, string theory is better described as a general framework – the most mathematically sophisticated one available at the moment – to resolve a fundamental problem in modern physics: general relativity and quantum mechanics are highly successful scientific theories, and yet, when they are applied to certain problems, like the physics of black holes, or that of the singularity that gave origin to the universe, they give us sharply contrasting predictions.

Physicists agree that this means that either theory, or both, are therefore wrong or incomplete. String theory is one attempt at reconciling the two by subsuming both into a broader theoretical framework. There is only one problem: while some in the fundamental physics community confidently argue that string theory is not only a very promising scientific theory, but pretty much ‘the only game in town,’ others scornfully respond that it isn’t even science, since it doesn’t make contact with the empirical evidence: vibrating superstrings, multiple, folded, dimensions of space-time and other features of the theory are impossible to test experimentally, and they are the mathematical equivalent of metaphysical speculation. And metaphysics isn’t a complimentary word in the lingo of scientists. Surprisingly, the ongoing, increasingly public and acerbic diatribe often centres on the ideas of one Karl Popper. What, exactly, is going on?

I had a front row seat at one round of such, shall we say, frank discussions last year, when I was invited to Munich to participate in a workshop on the status of fundamental physics, and particularly on what some refer to as ‘the string wars’. The organiser, Richard Dawid, of the University of Stockholm, is a philosopher of science with a strong background in theoretical physics. He is also a proponent of a highly speculative, if innovative, type of epistemology that supports the efforts of string theorists and aims at shielding them from the accusation of engaging in flights of mathematical fancy decoupled from any real science. My role there was to make sure that participants – an eclectic mix of scientists and philosophers, with a Nobel winner thrown in the mix – were clear on something I teach in my introductory course in philosophy of science: what exactly Popper said and why, since some of those physicists had hurled accusations at their critical colleagues, loudly advocating the ejection of the very idea of falsification from scientific practice.

In the months preceding the workshop, a number of high profile players in the field had been using all sorts of means – from manifesto-type articles in the prestigious Nature magazine to Twitter – to pursue a no-holds-barred public relations campaign to wrestle, or retain, control of the soul of contemporary fundamental physics. Let me give you a taste of the exchange, to set the mood: ‘The fear is that it would become difficult to separate such ‘science’ from New Age thinking, or science fiction,’ said George Ellis, chastising the pro-string party; to which Sabine Hossenfelder added: ‘Post-empirical science is an oxymoron.’ Peter Galison made crystal clear what the stakes are when he wrote: ‘This is a debate about the nature of physical knowledge.’ On the other side, however, cosmologist Sean Carroll tweeted: ‘My real problem with the falsifiability police is: we don’t get to demand ahead of time what kind of theory correctly describes the world,’ adding ‘[Falsifiability is] just a simple motto that non-philosophically-trained scientists have latched onto.’ Finally (but there is more, much more, out there), Leonard Susskind mockingly introduced the neologism ‘Popperazzi’ to label an extremely naive (in his view) way of thinking about how science works.

This surprisingly blunt – and very public – talk from prestigious academics is what happens when scientists help themselves to, or conversely categorically reject, philosophical notions that they plainly have not given sufficient thought to. In this case, it was Popper’s philosophy of science and its application to the demarcation problem. What makes this particularly ironic for someone like me, who started his academic career as a scientist (evolutionary biology) and eventually moved to philosophy after a constructive midlife crisis, is that a good number of scientists nowadays – and especially physicists – don’t seem to hold philosophy in particularly high regard. Just in the last few years Stephen Hawking has declared philosophy dead, Lawrence Krauss has quipped that philosophy reminds him of that old Woody Allen joke, ‘those that can’t do, teach, and those that can’t teach, teach gym,’ and science popularisers Neil deGrasse Tyson and Bill Nye have both wondered loudly why any young man would decide to ‘waste’ his time studying philosophy in college.

Loud debates on social media and in the popular science outlets define how much of the public perceives physics

This is a rather novel, and by no means universal, attitude among physicists. Compare the above contemptuousness with what Einstein himself wrote to his friend Robert Thorton in 1944 on the same subject: ‘I fully agree with you about the significance and educational value of methodology as well as history and philosophy of science. So many people today – and even professional scientists – seem to me like somebody who has seen thousands of trees but has never seen a forest. A knowledge of the historic and philosophical background gives that kind of independence from prejudices of his generation from which most scientists are suffering. This independence created by philosophical insight is – in my opinion – the mark of distinction between a mere artisan or specialist and a real seeker after truth.’ By Einstein’s standard then, there are a lot of artisans but comparatively few seekers of truth among contemporary physicists!

To put things in perspective, of course, Einstein’s opinion of philosophy may not have been representative even then, and certainly modern string theorists are a small group within the physics community, and string theorists on Twitter are an ever smaller, possibly more voluble subset within that group. The philosophical noise they make is likely not representative of what physicists in general think and say, but it matters all the same precisely because they are so prominent; those loud debates on social media and in the popular science outlets define how much of the public perceives physics, and even how many physicists perceive the big issues of their field.

That said, the publicly visible portion of the physics community nowadays seems split between people who are openly dismissive of philosophy and those who think they got the pertinent philosophy right but their ideological opponents haven’t. At stake isn’t just the usually tiny academic pie, but public appreciation of and respect for both the humanities and the sciences, not to mention millions of dollars in research grants (for the physicists, not the philosophers). Time, therefore, to take a more serious look at the meaning of Popper’s philosophy and why it is still very much relevant to science, when properly understood.

As we have seen, Popper’s message is deceptively simple, and – when repackaged in a tweet – has in fact deceived many a smart commentator in underestimating the sophistication of the underlying philosophy. If one were to turn that philosophy into a bumper sticker slogan it would read something like: ‘If it ain’t falsifiable, it ain’t science, stop wasting your time and money.’

But good philosophy doesn’t lend itself to bumper sticker summaries, so one cannot stop there and pretend that there is nothing more to say. Popper himself changed his mind throughout his career about a number of issues related to falsification and demarcation, as any thoughtful thinker would do when exposed to criticisms and counterexamples from his colleagues. For instance, he initially rejected any role for verification in establishing scientific theories, thinking that it was far too easy to ‘verify’ a notion if one were actively looking for confirmatory evidence. Sure enough, modern psychologists have a name for this tendency, common to laypeople as well as scientists: confirmation bias.

Nonetheless, later on Popper conceded that verification – especially of very daring and novel predictions – is part of a sound scientific approach. After all, the reason Einstein became a scientific celebrity overnight after the 1919 total eclipse is precisely because astronomers had verified the predictions of his theory all over the planet and found them in satisfactory agreement with the empirical data. For Popper this did not mean that the theory of general relativity was ‘true,’ but only that it survived to fight another day. Indeed, nowadays we don’t think the theory is true, because of the above mentioned conflicts, in certain domains, with quantum mechanics. But it has withstood a very good number of high stakes challenges over the intervening century, and its most recent confirmation came just a few months ago, with the first detection of gravitational waves.

Scientific hypotheses need to be tested repeatedly and under a variety of conditions before we can be reasonably confident of the results

Popper also changed his mind about the potential, at the least, for a viable Marxist theory of history (and about the status of the Darwinian theory of evolution, concerning which he was initially skeptical, thinking – erroneously – that the idea was based on a tautology). He conceded that even the best scientific theories are often somewhat shielded from falsification because of their connection to ancillary hypotheses and background assumptions. When one tests Einstein’s theory using telescopes and photographic plates directed at the Sun, one is really simultaneously putting to the test the focal theory, plus the theory of optics that goes into designing the telescopes, plus the assumptions behind the mathematical calculations needed to analyse the data, plus a lot of other things that scientists simply take for granted and assume to be true in the background, while their attention is trained on the main theory. But if something goes wrong and there is a mismatch between the theory of interest and the pertinent observations, this isn’t enough to immediately rule out the theory, since a failure in one of the ancillary assumptions might be to blame instead. That is why scientific hypotheses need to be tested repeatedly and under a variety of conditions before we can be reasonably confident of the results.

Popper’s initial work pretty much single-handedly put the demarcation problem on the map, prompting philosophers to work on the development of a philosophically sound account of both what science is and is not. That lasted until 1983, when Larry Laudan published a highly influential paper entitled ‘The demise of the demarcation problem,’ in which he argued that demarcation projects were actually a waste of time for philosophers, since – among other reasons – it is unlikely to the highest degree that anyone will ever be able to come up with small sets of necessary and jointly sufficient conditions to define ‘science,’ ‘pseudoscience’ and the like. And without such sets, Laudan argued, the quest for any principled distinction between those activities is hopelessly Quixotic.

‘Necessary and jointly sufficient’ is logical-philosophical jargon, but it is important to see what Laudan meant. He thought that Popper and others had been trying to provide precise definitions of science and pseudoscience, similar to the definitions used in elementary geometry: a triangle, for instance, is whatever geometrical figure has the internal sum of its angles equal to 180 degrees. Having that property is both necessary (because without it the figure in question is not a triangle) and sufficient (because that’s all we need to know in order to confirm that we are, indeed, dealing with a triangle). Laudan argued – correctly – that no such solution is ever going to be found to the demarcation problem, simply because concepts like ‘science’ and ‘pseudoscience’ are complex, multidimensional, and inherently fuzzy, not admitting of sharp boundaries. In a sense, physicists complaining about ‘the Popperazzi’ are making the same charge as Laudan: Popper’s criterion of falsification appears to be far too blunt an instrument not only to discriminate between science and pseudoscience (which ought to be relatively easy), but a fortiori to separate sound from unsound science within an advanced field like theoretical physics.

Yet Popper wasn’t quite as naive as Laudan, Carroll, Susskind, and others make him out to be. Nor is the demarcation problem quite as hopeless as all that. Which is why a number of authors – including myself and my longtime collaborator, Maarten Boudry – have more recently maintained that Laudan was too quick to dismiss the demarcation problem, and that perhaps Twitter isn’t the best place for nuanced discussions in the philosophy of science.

The idea is that there are pathways forward in the study of demarcation that become available if one abandons the requirement for necessary and jointly sufficient conditions, which was never strictly enforced even by Popper. What, then, is the alternative? To treat science, pseudoscience, etc. as Wittgensteinian ‘family resemblance’ concepts instead. Ludwig Wittgenstein was another highly influential 20th century philosopher, who hailed, like Popper himself, from Vienna, though the two could not have been more different in terms of socio-economic background, temperament, and philosophical interests. (If you want to know just how different, check out the delightful Wittgenstein’s Poker (2001) by journalists David Edmonds and John Eidinow.)

Wittgenstein never wrote about philosophy of science, let alone fundamental physics (or even Marxist theories of history). But he was very much interested in language, its logic, and its uses. He pointed out that there are many concepts that we seem to be able to use effectively, and that yet are not amenable to the sort of clear definition that Laudan was looking for. His favorite example was the deceptively simple concept of ‘game.’ If you try to arrive at a definition of games of the kind that works for triangles, your effort will be endlessly frustrated (try it out, it makes for a nice parlour, ahem, game). Wittgenstein wrote: ‘How should we explain to someone what a game is? I imagine that we should describe games to him, and we might add: ‘This and similar things are called games.’ And do we know any more about it ourselves? Is it only other people whom we cannot tell exactly what a game is? […] But this is not ignorance. We do not know the boundaries because none have been drawn […] We can draw a boundary for a special purpose. Does it take that to make the concept usable? Not at all!’

The point is that in a lot of cases we don’t discover pre-existing boundaries, as if games and scientific disciplines were Platonic ideal forms that existed in a timeless metaphysical dimension. We make up boundaries for specific purposes and then we test whether the boundaries are actually useful for whatever purposes we drew them. In the case of the distinction between science and pseudoscience, we think there are important differences, so we try to draw tentative borders in order to highlight them. Surely one would give up too much, as either a scientist or a philosopher, if one were to reject the strongly intuitive idea that there is something fundamentally different between, say, astrology and astronomy. The question is where, approximately, the difference lies.

Rather than laying into each other in the crude terms, scientists should work together not just to forge a better science, but to counter true pseudoscience

Similarly, many of the participants in the Munich workshop, and the ‘string wars’ more generally, did feel that there is an important distinction between fundamental physics as it is commonly conceived and what string theorists are proposing. Richard Dawid objects to the (admittedly easily derisible) term ‘post-empirical science,’ preferring instead ‘non-empirical theory assessment’, but whatever one calls it, he is aware that he and his fellow travellers are proposing a major departure from the way we have done science since the time of Galileo. True, the Italian physicist himself largely engaged in theoretical arguments and thought experiments (he likely never did drop balls from the leaning tower of Pisa), but his ideas were certainly falsifiable and have been, over and over, subjected to experimental tests (most spectacularly by David Scott on the Apollo 15 Moon landing).

The broader question then is: are we on the verge of developing a whole new science, or is this going to be regarded by future historians as a temporary stalling of scientific progress? Alternatively, is it possible that fundamental physics is reaching an end not because we’ve figured out everything we wanted to figure out, but because we have come to the limits of what our brains and technologies can possibly do? These are serious questions that ought to be of interest not just to scientists and philosophers, but to the public at large (the very same public that funds research in fundamental physics, among other things).

What is weird about the string wars and the concomitant use and misuse of philosophy of science is that both scientists and philosophers have bigger targets to jointly address for the sake of society, if only they could stop squabbling and focus on what their joint intellectual forces may accomplish. Rather than laying into each other in the crude terms sketched above, they should work together not just to forge a better science, but to counter true pseudoscience: homeopaths and psychics, just to mention a couple of obvious examples, keep making tons of money by fooling people, and damaging their physical and mental health. Those are worthy targets of critical analysis and discourse, and it is the moral responsibility of a public intellectual or academic – be they a scientist or a philosopher – to do their best to improve as much as possible the very same society that affords them the luxury of discussing esoteric points of epistemology or fundamental physics.