Karl Popper famously defended the view, known as falsificationism, that what distinguishes science from non-science is falsifiability. On this view, a theory is scientific if and only if it’s falsifiable, at least in principle. What this means for a theory to be falsifiable is that one can think of a possible observation that would be inconsistent with the theory. For instance, since Newton’s law of universal gravitation implies that every particle exerts a force of attraction on every other particle, it would be falsified if we observed a particle that repels another particle. Since it’s at least conceivable that we could observe this, Newton’s law of universal gravitation is falsifiable and therefore scientific. Popper wants to contrast this with theories like psychoanalysis, which according to him can be reconciled with any conceivable observation, hence is not scientific.

I have often been struck, when talking to scientists, by the influence that Popper seems to have among them. I would go as far as saying that, in many scientific fields, falsificationism has become the official philosophy of science. It’s drummed into the heads of scientists when they’re in graduate school and, with a few exceptions, they never learn anything else about philosophy of science and spend the rest of their career thinking that Popper’s conception of science is still the gold standard. In fact, however, not only is falsificationism not the gold standard, but it never was. Indeed, I think it’s fair to say that, despite the incredible popularity it has achieved, falsificationism was always opposed by most philosophers of science and it’s certainly the case that nowadays virtually all of them reject it. Moreover, philosophers of science have excellent reasons to do so, because there are very strong arguments against falsificationism. In this post, I want to explain why I think falsificationism is false, because I keep running into people, especially scientists but not only, who are surprised to hear that and I figured it would be useful to be able to refer them to something accessible to non-philosophers that explains it.

Given this post’s intended audience, I will not discuss the more sophisticated versions of falsificationism that have been proposed, which often stem from remarks Popper himself made that showed he was aware of the difficulties his theory faced. (For instance, even though above I have formulated the criterion of falsifiability as a necessary and sufficient condition for a theory to be scientific, Popper sometimes claimed that it was only necessary but not sufficient.) Rather, I’m going to address the somewhat crude version of falsificationism that most people seem to have in mind when they talk about Popper’s conception of science, which I summarized at the beginning of this post. However, I want to say here that, as far as I can tell, I think it’s roughly what Popper himself believed. Again, he was a sophisticated thinker and knew there was problems with this view, but his goal was to find a purely logical criterion of demarcation between science and non-science and this is what the view I summarized above is supposed to do.

Once you modify it to address those issues, as Popper sometimes did in response to his critics or because he was anticipating their objections, it no longer delivers a purely logical criterion of demarcation and in my opinion the view loses much of its original appeal. But it’s okay if you disagree with this. I know there are philosophers of science who think that falsificationism can be salvaged and that a more sophisticated version of that view is correct, so they will predictably not be moved by this post, but I’m fine with that since it’s the crude version that most people, especially scientists, have in mind when they talk about falsificationism. I will be satisfied if all I have achieved with this post is to convince you that, at least in this crude version, falsificationism is false. Although I ultimately disagree, it’s okay with me if you think the lesson one should draw from this critique of naive falsificationism is that one should adopt a more sophisticated version of the view, instead of abandoning the project altogether.

Popper’s philosophy of science is a product of his view on the problem of induction, so let’s talk briefly about that. The problem of induction, made famous by David Hume in the 18th century (though it had been noticed before), is about how we can ever be justified in believing in the kind of empirical generalizations that figure prominently in science. For instance, take Newton’s law of universal gravitation, which says that every body exerts a force of attraction on every other body proportional to the product of their masses and inversely proportion to the square of the distance between them. Unlike the claim that every body either has a mass or does not have a mass, which is a truth of logic and can be known without resorting to observation, the law of universal gravitation cannot be deduced logically from self-evident principles and can only be established by observation. We look at the world and, every time we set out to check whether the law holds, it appears to do so, so we eventually come to believe that it’s true.

This type of inference, by which a generalization is established by repeated observation of instances of it, is known as induction. But it’s really not clear why one is justified in making that kind of inference. In the case of deduction, there is a sense in which the truth of the premises necessitates the truth of the conclusion, but that is not the case with induction. Even if the law of universal gravitation was observed to hold a thousand times in the past, it doesn’t mean that it will still hold the next time we check. It seems that, in order to make that inference, we need to assume some kind of uniformity in nature. But it also seems that we could only justify the belief in the uniformity of nature by induction, which is precisely the type of inference the principle of uniformity was supposed to justify, so we can’t do that on the pain of making a viciously circular argument. My favorite summary of the problem of induction was given by Bertrand Russell, the famous British philosopher and mathematician, in The Problems of Philosophy:

We know that all these rather crude expectations of uniformity are liable to be misleading. The man who has fed the chicken every day throughout its life at last wrings its neck instead, showing that more refined views as to the uniformity of nature would have been useful to the chicken.

Although we are more sophisticated than poultry, by assuming that nature is uniform in the relevant way, we may put ourselves in a similarly precarious epistemic position as the chicken in Russell’s story.

Most philosophers assume that the problem of induction must have a solution, and that empirical generalizations can be confirmed by observational evidence (where confirmation is a matter of degrees), although they disagree on what the correct account of confirmation is. On this view, while such generalizations can never be definitely proven, they can be more or less supported by the evidence and sometimes we can be reasonably certain they are true or at least approximately true. But Popper thought that the problem of induction was insoluble and that we must accept this fact. This means that a scientific theory can never be confirmed, but as Popper quickly noted, it can be falsified. In other words, while you can’t prove that a theory is true, you can at least show that it’s false.

Let’s go back to the law of universal gravitation. The problem of induction means that, no matter how often it has been observed to hold, it will never be proven. But all it would take to show that it’s false is just one counter-example. That’s because in order to show that a generalization is false, you just need to show that one of its instances is false. If I tell you that every cat is black, in order to show that it’s false, all you have to do is find a cat that is not black. On the other hand, even if all the cats you have seen so far have been black, there could always be another cat that is not black, so you can’t conclude that every cat is black no matter how many cats you have seen which seemed to confirm this generalization.

The problem of induction and this asymmetry between confirmation and falsification lie at the heart of Popper’s philosophy of science. For him, you can never show that a theory is true, but you can show that it’s false. So the way science works, according to Popper, is by conjuring up hypotheses such as the law of universal gravitation that can explain a whole array of phenomena and subjecting them to tests, whose aim is not to confirm them but to falsify them. On this view, science is essentially a deductive enterprise, not an inductive one. You come up with a theory, use deduction to derive observational consequences from it and perform experiments or make observations to check whether the results correspond to what your theory predicted. If they don’t, it means that your theory is false and you must look for another one, which is how scientific progress happens. A theory that can’t possibly be falsified, because it’s impossible to derive any observational predictions from it, is not scientific since it could never be shown to be false. Such a theory would be compatible with any observation whatsoever, which may be fine in some contexts (such as theology), but not in science, at least according to Popper.

What I have just described is, in a nutshell, falsificationism. Hopefully, at this point, you are feeling the appeal of that view. Perhaps you are even ready to officially embrace Popper’s philosophy of science. If you are, however, I recommend that you wait a little and keep reading, because I’m about to explain why most philosophers of science think falsificationism, despite its appeal, is actually mistaken. In order to understand why, we must take a closer look at what people actually do when they test a theory. Let’s go back once again to Newton’s law of universal gravitation and think about how one would go to test it. On its own, the law does not have any observable consequences, because it just says something about the forces that exist in the world and forces are not observable. You can’t see the gravitational force one physical object exerts on another. What you can see is only the effect it has on the motion of that object.

So you need something that provides the link between the forces that act on a physical object and its motion. This is precisely what Newton’s second law of motion does. This law of motion, expressed mathematically as F = ma, says that the total force on a physical object is equal to the product of its mass and its acceleration. Since the acceleration is the second derivative of position with respect to time, it connects something unobservable, i. e. the total force on the object, to something observable, i. e. the position of this object at different times. In order to use to test the law of universal gravitation with the help of Newton’s second law of motion, however, you also need to know something about the masses of the physical objects, as well as their position at different times. This requires using various instruments to measure position, mass and time, which in turn means that you have to make several assumptions about the way in which those instruments work, guaranteeing they are reliable.

Thus, in order to test even a theory as straightforward as Newton’s law of universal gravitation, you need to make a lot of auxiliary hypotheses. The law of universal gravitation by itself doesn’t make any observable prediction. This isn’t just true of the law of universal gravitation, it’s true of any theory whatsoever. Just take any theory you’d like and think about how you’d go about to test it and you’ll soon realize that, in order to so, you need to make a lot of hypotheses that are not part of the theory itself. In general, a theory is never testable on its own, but only with the help of various auxiliary hypotheses or background assumptions. This means that a theory is never falsifiable simpliciter, but only relative to a set of background assumptions. Therefore, if we say that a theory is only scientific if it’s falsifiable, then it follows that no theory, not even a theory as successful as Newton’s law of universal gravitation, is scientific. Of course, this is absurd, so falsificationism is false.

But this was a bit quick and perhaps you are not entirely convinced yet, so let’s continue to examine the implications of the fact that a theory is only falsifiable relative to a set of background assumptions. Suppose that you derive a prediction from the law of universal gravitation, plus a bunch of auxiliary hypotheses, but it doesn’t come true. From a purely logical point of view, the only conclusion you can draw from this is at least one of the hypotheses you used to derive the prediction is false, but logic doesn’t tell you which one. It could be the law of universal gravitation, but it could also be one or several of the various auxiliary hypotheses you had to make in order to derive the prediction, you just don’t know. Strictly speaking, since the failure of a prediction derived from a theory plus a bunch of background assumptions doesn’t logically imply the falsity of the theory, it can’t falsify it. The failure of the prediction only falsifies the theory if you assume that the background assumptions necessary to derive that prediction are true.

In practice, although they frequently pay lip service to falsificationism when they talk about methodology, scientists are perfectly aware of that and don’t behave at all like they should according to naive falsificationism. Indeed, when a prediction derived from a theory they take to be well-established fails, scientists don’t just throw away the theory and start looking for a replacement. Instead, they generally assume that one of the background assumptions necessary to derive the prediction, which are often left implicit, was false and try to figure out which one it was. Moreover, it’s not just that, as a matter of fact, scientists don’t behave as naive falsificationism implies they should, they are typically right not to do so.

After Newton formulated it, it took more than a hundred years before someone tested the law of universal gravitation experimentally, which incidentally shows that scientists are also not as obsessed with trying to falsify theories as Popper claimed they should. (Instead, they spend a lot of time devising methods to apply them to new problems, which is usually not straightforward and requires much ingenuity.) In fact, the purpose of the experiment wasn’t even to test the law of universal gravitation, but rather to estimate the mass of the earth. Suppose that, contrary to what actually happened, Cavendish’s experiment had yielded a result that was qualitatively incompatible with the law of universal gravitation. The reaction of physicists would certainly not have been to reject Newton’s theory of gravitation and start looking for another one.

Indeed, by the time Cavendish performed his experiment, Newton’s theory of gravitation had already been immensely successful. It had been used to explain a great variety of phenomena, from the motion of planets to that of projectiles on earth, but also the tides and many other things. It would have been completely irrational to throw such a successful theory away just because the experiment didn’t go as planned. The right thing to do would have been to assume some of the background assumptions was false, find which one and revise the experimental setup accordingly. For instance, perhaps the spheres used in the experiment were electrically charged, which according to Coulomb’s law, the electrostatic analogue to Newton’s law of universal gravitation, would have introduced another force that could have interfered with gravitation.

In general, when a prediction derived using a well-established theory fails, the rational thing to do is not to abandon that theory and start looking for another one. Doing so would considerably slow down scientific progress and could even stop it altogether. One thing Thomas Kuhn got right is that scientists only reject a theory they take to be well-established in the face of contrary experimental evidence if they have a viable alternative. (Kuhn was basically right about the sociology of theory change, but he drew crazy metaphysical/epistemological conclusions from that. Unfortunately, it’s the metaphysical/epistemological conclusions he drew that became popular, at least among non-philosophers. But this is a story for another day, so back to Popper and falsificationism.) It’s ironic that, although many scientists accept falsificationism uncritically, what they do every day is radically at odds with it.

That is not to say, of course, that it’s always a good thing that scientists operate in that way. One could argue that such a conservative bias toward currently accepted theory sometimes impedes the development of more satisfactory alternatives. For instance, many people think that, if economists initially dismissed empirical research suggesting that increasing the minimum wage might not reduce employment, it’s because they were committed to standard economic theory which predicts that increasing the price of labor will reduce employment since it assumes that demand curves in general are downward-slopping. Now, regardless of whether this particular instance of resistance to apparent falsification really is irrational, it certainly could be. So the point here is not that it’s never bad to stick to a theory even in the face of contrary evidence, but only that, before you throw away a theory that has served you good up until now, it’s perfectly rational to first examine whether the problem may not lay with one of the auxiliary hypotheses you used to derive the prediction that didn’t come true.

From a purely logical point of view, what scientists do when they blame the failure of a prediction on the background assumptions instead of theory they take to be well-established is no different from what charlatans peddling pseudoscience do when they conjure up bizarre hypotheses to explain away evidence that seem to contradict their theories, as when creationists posit that God planted fossils that seem very ancient to test our faith. The difference is that, when pseudoscientists do that, the hypotheses they conjure up are purely ad hoc and meant to protect theories that were never well-established in the first place and lack the kind of theoretical virtues that good theories have, but there is no purely formal criterion that allows you to decide that in a simple way. This is why the search for such a criterion of demarcation between science and non-science, whether in terms of falsifiability or not, is probably hopeless. If you want to argue that something is pseudoscience, there is no shortcut that will save you the pain of having to engage with it and grapple with the arguments of its proponents.

EDIT: I wrote a follow-up to this post in which I clarify a few things and reply to some critics. Due to the unexpected popularity of this post, I have already spent way too much time discussing falsificationism in the past few days, so I don’t plan on replying to comments either here or elsewhere. But the comments are open and, of course, you should feel free to criticize it. I apologize if you asked me a question or criticized my post and I didn’t reply to you, but like I said the response has been pretty overwhelming and I simply don’t have time.