does a scientific hypothesis have to be falsifiable

Karl Popper: Theory of Falsification

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Saul Mcleod, PhD., is a qualified psychology teacher with over 18 years of experience in further and higher education. He has been published in peer-reviewed journals, including the Journal of Clinical Psychology.

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Karl Popper’s theory of falsification contends that scientific inquiry should aim not to verify hypotheses but to rigorously test and identify conditions under which they are false. For a theory to be valid according to falsification, it must produce hypotheses that have the potential to be proven incorrect by observable evidence or experimental results. Unlike verification, falsification focuses on categorically disproving theoretical predictions rather than confirming them.

• Karl Popper believed that scientific knowledge is provisional – the best we can do at the moment.
• Popper is known for his attempt to refute the classical positivist account of the scientific method by replacing induction with the falsification principle.
• The Falsification Principle, proposed by Karl Popper, is a way of demarcating science from non-science. It suggests that for a theory to be considered scientific, it must be able to be tested and conceivably proven false.
• For example, the hypothesis that “all swans are white” can be falsified by observing a black swan.
• For Popper, science should attempt to disprove a theory rather than attempt to continually support theoretical hypotheses.

Theory of Falsification

Karl Popper is prescriptive and describes what science should do (not how it actually behaves). Popper is a rationalist and contended that the central question in the philosophy of science was distinguishing science from non-science.

Karl Popper, in ‘The Logic of Scientific Discovery’ emerged as a major critic of inductivism, which he saw as an essentially old-fashioned strategy.

Popper replaced the classical observationalist-inductivist account of the scientific method with falsification (i.e., deductive logic) as the criterion for distinguishing scientific theory from non-science.

All inductive evidence is limited: we do not observe the universe at all times and in all places. We are not justified, therefore, in making a general rule from this observation of particulars.

According to Popper, scientific theory should make predictions that can be tested, and the theory should be rejected if these predictions are shown not to be correct.

He argued that science would best progress using deductive reasoning as its primary emphasis, known as critical rationalism.

Popper gives the following example:

Europeans, for thousands of years had observed millions of white swans. Using inductive evidence, we could come up with the theory that all swans are white.

However, exploration of Australasia introduced Europeans to black swans.  Poppers’ point is this: no matter how many observations are made which confirm a theory, there is always the possibility that a future observation could refute it.  Induction cannot yield certainty.

Karl Popper was also critical of the naive empiricist view that we objectively observe the world. Popper argued that all observation is from a point of view, and indeed that all observation is colored by our understanding. The world appears to us in the context of theories we already hold: it is ‘theory-laden.’

Popper proposed an alternative scientific method based on falsification.  However, many confirming instances exist for a theory; it only takes one counter-observation to falsify it. Science progresses when a theory is shown to be wrong and a new theory is introduced that better explains the phenomena.

For Popper, the scientist should attempt to disprove his/her theory rather than attempt to prove it continually. Popper does think that science can help us progressively approach the truth, but we can never be certain that we have the final explanation.

Critical Evaluation

Popper’s first major contribution to philosophy was his novel solution to the problem of the demarcation of science. According to the time-honored view, science, properly so-called, is distinguished by its inductive method – by its characteristic use of observation and experiment, as opposed to purely logical analysis, to establish its results.

The great difficulty was that no run of favorable observational data, however long and unbroken, is logically sufficient to establish the truth of an unrestricted generalization.

Popper’s astute formulations of logical procedure helped to reign in the excessive use of inductive speculation upon inductive speculation, and also helped to strengthen the conceptual foundation for today’s peer review procedures.

However, the history of science gives little indication of having followed anything like a methodological falsificationist approach.

Indeed, and as many studies have shown, scientists of the past (and still today) tended to be reluctant to give up theories that we would have to call falsified in the methodological sense, and very often, it turned out that they were correct to do so (seen from our later perspective).

The history of science shows that sometimes it is best to ’stick to one’s guns’. For example, “In the early years of its life, Newton’s gravitational theory was falsified by observations of the moon’s orbit”

Also, one observation does not falsify a theory. The experiment may have been badly designed; data could be incorrect.

Quine states that a theory is not a single statement; it is a complex network (a collection of statements). You might falsify one statement (e.g., all swans are white) in the network, but this should not mean you should reject the whole complex theory.

Critics of Karl Popper, chiefly Thomas Kuhn , Paul Feyerabend, and Imre Lakatos, rejected the idea that there exists a single method that applies to all science and could account for its progress.

Popperp, K. R. (1959). The logic of scientific discovery . University Press.

Further Information

• Thomas Kuhn – Paradigm Shift Is Psychology a Science?
• Steps of the Scientific Method
• Positivism in Sociology: Definition, Theory & Examples
• The Scientific Revolutions of Thomas Kuhn: Paradigm Shifts Explained

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What does it mean for science to be falsifiable?

Posted on July 31, 2021 by Evan Arnet

Science is falsifiable. Or at least, this is what I (like many Americans) learned in many of my high school and college science classes. Clearly, the idea has appeal among scientists and non-scientists alike:

But what exactly does “falsifiable” mean? And why is it valued by some scientists, but dismissed or even considered actively harmful by others?

Imagine you are an infectious disease expert investigating COVID-19. You want to determine whether, absent vaccination, COVID-19 always causes at least some lung damage. To prove this claim is true, you would have to check every case and see if every time a patient has COVID, there is also lung damage. And for every case you check, there are more new cases to check.

However, to prove this claim is false, you merely need to document a single case in which someone who previously had COVID has no lung damage. This is an extension of the logical point that to prove a general claim, you need to confirm every instance, but to disprove a general claim, you only need a single counterexample. 

The legendary philosopher of science Karl Popper argued that good science is falsifiable, in that it makes precise claims which can be tested and then discarded (falsified) if they don’t hold up under testing. For example, if you find a case of COVID-19 without lung damage, then you falsify the hypothesis that it always causes lung damage. According to Popper, science progresses by making conjectures, subjecting them to rigorous tests, and then discarding those that fail.

He contrasted this with ostensibly unscientific systems, like astrology. Let’s say your horoscope says “something of consequence will happen in your life tomorrow.” Popper argued that a claim like this is so vague, so devoid of clear content, that it can’t be meaningfully falsified and, therefore, isn’t scientific. 

Contemporary scholars who study scientific methodology are often frustrated by the implication that science is logically falsifiable. The problem is that scientists can always make excuses to avoid falsifying a claim. The discovery of Neptune is a famous case. Astronomers had noticed irregularities in the orbit of Uranus. One possibility would be that these irregularities violated the theory currently used to explain planetary motion, called Newtonian mechanics, and that this theory should be rejected. At face value, these observations seemed to falsify Newtonian mechanics. But, no one actually argued for this. Instead, they searched for explanations for the irregularities — including the possibility of another planet. Two astronomers, Urban Leverrier in France and John Couch Adams in England, independently used mathematics to predict the location of this previously unknown planet. Astronomical observations by Johann Gottfried Galle confirmed the existence of a planet and, thus, Neptune was discovered.

Put simply, to test a hypothesis, you have to make a bunch of other assumptions, or auxiliary hypotheses. You have to assume that your instruments are working, that you did the math correctly, that you didn’t miss any relevant causes (like Neptune), etc. When something goes awry, you can then choose whether the real error lies in your main hypothesis or in an auxiliary hypothesis. 

For an illustration of this problem, imagine you are baking lasagna. You Google lasagna recipes, find a recipe that looks good, and get cooking. You take your lasagna out of the oven, take a bite, and…it tastes terrible. Does this mean you can falsify the hypothesis that the lasagna recipe is good? Not necessarily. Maybe you didn’t follow the recipe correctly, or the olive oil was rancid, or any number of problems other than the recipe itself.

Similar to the COVID example above, we can imagine a scientist arguing that because of poor resolution in a CT scan, lung damage was not detected when it did in fact occur. In other words, the presumed false hypothesis is not that COVID always causes lung damage. Instead, what is allegedly false is the assumption, or auxiliary hypothesis, that the CT scan was detailed enough to detect the lung damage.

This general argument against falsification is sometimes attributed to the philosopher W. V. O. Quine in a famous 1951 article, but it was actually a widely-expressed concern, including by Karl Popper himself. However, Popper thought that features necessary for the testing of scientific claims would be accepted as background conditions by the scientific community and, therefore, falsification could proceed. For example, after it is accepted that the oven temperature is correct and the ingredients are in good condition and measured properly, then one can test whether the lasagna recipe is any good.

Regardless, when a scientist touts the falsifiability of science, it is rare that they are a strict devotee of Popper. (He held some unorthodox views, e.g., we can never actually gain confidence in a theory, we can only eliminate alternatives.) Usually they mean that, unlike some other systems, science makes deliberately clear predictions and actively attempts to disprove claims.

One of the amazing things about science is not so much its tight logical structure — the scientific process can actually be quite messy — but rather, that science aims to test claims and consider countermanding evidence. The sociologist of science Robert Merton referred to this as “organized skepticism.” (Incidentally, despite his reputation for prioritizing logical falsification, Karl Popper was attentive to this social aspect of science.)

Falsification as a matter of scientific practice, rather than logic, is especially significant because humans like to be right. We are inclined to seek out evidence which supports rather than challenges our existing opinions, a well-known phenomenon that is often referred to as confirmation bias . Science fights against this cognitive tendency by encouraging individual scientists to think critically about their own work and for the broader community to be skeptical of each other. 

Falsification does not stand alone as the mark of the scientific, and a lot of scientific research aims to confirm claims or to evaluate claims on metrics other than strict truth or falsity. Nonetheless, the willingness and intent to vigorously confront claims with evidence remains a key aspect of the scientific community. This requires attention to the formulation of claims to ensure they are testable. But, even more important is the careful coordination across the scientific community that allows scientific skepticism to lead to productive research.

Edited by Jennifer Sieben and Joe Vuletich

This was a fantastic explanation of a concept that I’ve always had difficulty understanding.

Great article, you really explain it well! I was looking for the line, “science tries to disprove itself by falsification,” and this article was on the list.

At the health sciences center where I worked for 8 years, the idea was widespread that anybody could come up with an explanation or hypothesis for some physiology or biochemical facts, so much so that you couldn’t be bothered if all it did was explain the data. A lecture with a mathematical model involving modeling biochemistry with 100 different equation in a seminar led to the reaction (from me) , how would you know if one or more equation was wrong? Feynman, the skeptical physicist from the Bronx would make a characteristic short reply to a non-falsifiable claim “how would you know?”. The writers above in this thread point out that a community that uses publication of scientific results in the newly public publications of the new scientific societies of the 16nth century that made replication of studies possible and publication is a key factor. I have heard chemists reply disdainfully of the guy whose published synthesis can never be repeated. You may have heard about the humor magazine “journal of irreproducible results”. Doubting your own assumptions maybe 1 per day, is a potentially painful exercise that is at the heart of being a scientist. A person who tends to rote memorization, or good boy behavior may not be a scientists if they do not think in terms of falsification but simply truthiness. It is disturbing that some people propose that string theory does not need to generate testable results and can get by on beauty alone.

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Being Scientific: Falsifiability, Verifiability, Empirical Tests, and Reproducibility

If you ask a scientist what makes a good experiment, you’ll get very specific answers about reproducibility and controls and methods of teasing out causal relationships between variables and observables. If human observations are involved, you may get detailed descriptions of blind and double-blind experimental designs. In contrast, if you ask the very same scientists what makes a theory or explanation scientific, you’ll often get a vague statement about falsifiability . Scientists are usually very good at designing experiments to test theories. We invent theoretical entities and explanations all the time, but very rarely are they stated in ways that are falsifiable. It is also quite rare for anything in science to be stated in the form of a deductive argument. Experiments often aren’t done to falsify theories, but to provide the weight of repeated and varied observations in support of those same theories. Sometimes we’ll even use the words verify or confirm when talking about the results of an experiment. What’s going on? Is falsifiability the standard? Or something else?

The difference between falsifiability and verifiability in science deserves a bit of elaboration. It is not always obvious (even to scientists) what principles they are using to evaluate scientific theories, 1 so we’ll start a discussion of this difference by thinking about Popper’s asymmetry. 2 Consider a scientific theory ( T ) that predicts an observation ( O ). There are two ways we could approach adding the weight of experiment to a particular theory. We could attempt to falsify or verify the observation. Only one of these approaches (falsification) is deductively valid:

Popper concluded that it is impossible to know that a theory is true based on observations ( O ); science can tell us only that the theory is false (or that it has yet to be refuted). He concluded that meaningful scientific statements are falsifiable.

Scientific theories may not be this simple. We often base our theories on a set of auxiliary assumptions which we take as postulates for our theories. For example, a theory for liquid dynamics might depend on the whole of classical mechanics being taken as a postulate, or a theory of viral genetics might depend on the Hardy-Weinberg equilibrium. In these cases, classical mechanics (or the Hardy-Wienberg equilibrium) are the auxiliary assumptions for our specific theories.

These auxiliary assumptions can help show that science is often not a deductively valid exercise. The Quine-Duhem thesis 3 recovers the symmetry between falsification and verification when we take into account the role of the auxiliary assumptions ( AA ) of the theory ( T ):

That is, if the predicted observation ( O ) turns out to be false, we can deduce only that something is wrong with the conjunction, ( T and AA ); we cannot determine from the premises that it is T rather than AA that is false. In order to recover the asymmetry, we would need our assumptions ( AA ) to be independently verifiable:

Falsifying a theory requires that auxiliary assumption ( AA ) be demonstrably true. Auxiliary assumptions are often highly theoretical — remember, auxiliary assumptions might be statements like the entirety of classical mechanics is correct or the Hardy-Weinberg equilibrium is valid ! It is important to note, that if we can’t verify AA , we will not be able to falsify T by using the valid argument above. Contrary to Popper, there really is no asymmetry between falsification and verification. If we cannot verify theoretical statements, then we cannot falsify them either.

Since verifying a theoretical statement is nearly impossible, and falsification often requires verification of assumptions, where does that leave scientific theories? What is required of a statement to make it scientific?

Carl Hempel came up with one of the more useful statements about the properties of scientific theories: 4 “The statements constituting a scientific explanation must be capable of empirical test.” And this statement about what exactly it means to be scientific brings us right back to things that scientists are very good at: experimentation and experimental design. If I propose a scientific explanation for a phenomenon, it should be possible to subject that theory to an empirical test or experiment. We should also have a reasonable expectation of universality of empirical tests. That is multiple independent (skeptical) scientists should be able to subject these theories to similar tests in different locations, on different equipment, and at different times and get similar answers. Reproducibility of scientific experiments is therefore going to be required for universality.

So to answer some of the questions we might have about reproducibility:

• Reproducible by whom ? By independent (skeptical) scientists, working elsewhere, and on different equipment, not just by the original researcher.
• Reproducible to what degree ? This would depend on how closely that independent scientist can reproduce the controllable variables, but we should have a reasonable expectation of similar results under similar conditions.
• Wouldn’t the expense of a particular apparatus make reproducibility very difficult? Good scientific experiments must be reproducible in both a conceptual and an operational sense. 5 If a scientist publishes the results of an experiment, there should be enough of the methodology published with the results that a similarly-equipped, independent, and skeptical scientist could reproduce the results of the experiment in their own lab.

Computational science and reproducibility

If theory and experiment are the two traditional legs of science, simulation is fast becoming the “third leg”. Modern science has come to rely on computer simulations, computational models, and computational analysis of very large data sets. These methods for doing science are all reproducible in principle . For very simple systems, and small data sets this is nearly the same as reproducible in practice . As systems become more complex and the data sets become large, calculations that are reproducible in principle are no longer reproducible in practice without public access to the code (or data). If a scientist makes a claim that a skeptic can only reproduce by spending three decades writing and debugging a complex computer program that exactly replicates the workings of a commercial code, the original claim is really only reproducible in principle. If we really want to allow skeptics to test our claims, we must allow them to see the workings of the computer code that was used. It is therefore imperative for skeptical scientific inquiry that software for simulating complex systems be available in source-code form and that real access to raw data be made available to skeptics.

Our position on open source and open data in science was arrived at when an increasing number of papers began crossing our desks for review that could not be subjected to reproducibility tests in any meaningful way. Paper A might have used a commercial package that comes with a license that forbids people at university X from viewing the code ! 6

Paper 2 might use a code which requires parameter sets that are “trade secrets” and have never been published in the scientific literature . Our view is that it is not healthy for scientific papers to be supported by computations that cannot be reproduced except by a few employees at a commercial software developer. Should this kind of work even be considered Science? It may be research , and it may be important , but unless enough details of the experimental methodology are made available so that it can be subjected to true reproducibility tests by skeptics, it isn’t Science.

• This discussion closely follows a treatment of Popper’s asymmetry in: Sober, Elliot Philosophy of Biology (Boulder: Westview Press, 2000), pp. 50-51.
• Popper, Karl R. “The Logic of Scientific Discovery” 5th ed. (London: Hutchinson, 1959), pp. 40-41, 46.
• Gillies, Donald. “The Duhem Thesis and the Quine Thesis”, in Martin Curd and J.A. Cover ed. Philosophy of Science: The Central Issues, (New York: Norton, 1998), pp. 302-319.
• C. Hempel. Philosophy of Natural Science 49 (1966).
• Lett, James, Science, Reason and Anthropology, The Principles of Rational Inquiry (Oxford: Rowman & Littlefield, 1997), p. 47
• See, for example www.bannedbygaussian.org

5 Responses to Being Scientific: Falsifiability, Verifiability, Empirical Tests, and Reproducibility

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“If we cannot verify theoretical statements, then we cannot falsify them either.

Since verifying a theoretical statement is nearly impossible, and falsification often requires verification of assumptions…”

An invalid argument is invalid regardless of the truth of the premises. I would suggest that an hypothesis based on unverifiable assumptions could be ‘falsified’ the same way an argument with unverifiable premises could be shown to be invalid. Would you not agree?

“Falsifying a theory requires that auxiliary assumption (AA) be demonstrably true.”

No, it only requires them to be true.

In the falisificationist method, you can change the AA so long as that increases the theories testability. (the theory includes AA and the universal statement, btw) . In your second box you misrepresent the first derivation. in the conclusion it would be ¬(t and AA). after that you can either modify the AA (as long as it increase the theories falsifiability) or abandon the theory. Therefore you do not need the third box, it explains something that does not need explaining, or that could be explained more concisely and without error by reconstructing the process better. This process is always tentative and open to re-evaluation (that is the risky and critical nature of conjectures and refutations). Falsificationism does not pretend conclusiveness, it abandoned that to the scrap heap along with the hopelessly defective interpretation of science called inductivism.

“Contrary to Popper, there really is no asymmetry between falsification and verification. If we cannot verify theoretical statements, then we cannot falsify them either.” There is an asymmetry. You cannot refute the asymmetry by showing that falsification is not conclusive. Because the asymmetry is a logical relationship between statements. What you would have shown, if your argument was valid or accurate, would be that falsification is not possible in practice. Not that the asymmetry is false.

Popper wanted to replace induction and verification with deduction and falsification.

He held that a theory that was once accepted but which, thanks to a novel experiment or observation, turns out to be false, confronts us with a new problem, to which new solutions are needed. In his view, this process is the hallmark of scientific progress.

Surprisingly, Popper failed to note that, despite his efforts to present it as deductive, this process is at bottom inductive, since it assumes that a theory falsified today will remain falsified tomorrow.

Accepting that swans are either white or black because a black one has been spotted rests on the assumption that there are other black swans around and that the newly discovered black one will not become white at a later stage. It is obvious but also inductive thinking in the sense that they project the past into the future, that is, extrapolate particulars into a universal.

In other words, induction, the process that Popper was determined to avoid, lies at the heart of his philosophy of science as he defined it.

Despite positivism’s limitations, science is positive or it is not science : positive science’s theories are maybe incapable of demonstration (as Hume wrote of causation), but there are not others available.

If it is impossible to demonstrate that fire burns, putting one’s hand in it is just too painful.

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The idea that a scientific theory can be ‘falsified’ is a myth

It's time we abandoned the notion.

Mano Singham

Transit of Mercury across the Sun; Newton's theory of gravity was considered to be "falsified" when it failed to account for the precession of the planet's orbit. Credit: Allexxandar/Getty

10 September 2020

Allexxandar/Getty

Transit of Mercury across the Sun; Newton's theory of gravity was considered to be "falsified" when it failed to account for the precession of the planet's orbit.

J.B.S. Haldane, one of the founders of modern evolutionary biology theory, was reportedly asked what it would take for him to lose faith in the theory of evolution and is said to have replied, “Fossil rabbits in the Precambrian.”

Since the so-called “Cambrian explosion” of 500 million years ago marks the earliest appearance in the fossil record of complex animals, finding mammal fossils that predate them would falsify the theory.

But would it really?

The Haldane story, though apocryphal, is one of many in the scientific folklore that suggest that falsification is the defining characteristic of science.

As expressed by astrophysicist Mario Livio  in his book  Brilliant Blunders :

"[E]ver since the seminal work of philosopher of science Karl Popper, for a scientific theory to be worthy of its name, it has to be falsifiable by experiments or observations. This requirement has become the foundation of the ‘scientific method.’”

But the field known as science studies (comprising the history, philosophy and sociology of science) has shown that falsification cannot work, even in principle.

This is because an experimental result is not a simple fact obtained directly from nature. Identifying and dating Haldane's bone involves using many other theories from diverse fields, including physics, chemistry, and geology.

Similarly, a theoretical prediction is never the product of a single theory, but also requires using many other theories.

When a “theoretical” prediction disagrees with “experimental” data, what this tells us is that that there is a disagreement between two sets of theories, so we cannot say that any particular theory is falsified.

The rule-breakers

Fortunately, falsification — or any other philosophy of science — is not necessary for the actual practice of science.

The physicist Paul Dirac was right when he  said , "Philosophy will never lead to important discoveries. It is just a way of talking about discoveries which have already been made.”

Actual scientific history reveals that scientists break all the rules all the time, including falsification.

As philosopher of science Thomas Kuhn noted, Newton's laws were retained despite the fact that they were  contradicted for decades  by the motions of the perihelion of Mercury and the perigee of the moon.

It is the single-minded focus on finding what works that gives science its strength, not any philosophy.

Albert Einstein said that scientists are not, and should not be, driven by any single perspective, but should be willing to go wherever experiment dictates and  adopt whatever works .

Unfortunately, some scientists have disparaged the entire field of science studies, claiming that it was  undermining public confidence  in science by denying that scientific theories were objectively true.

This is a mistake, since science studies play vital roles in two areas.

A better argument

The first is that it gives scientists a much richer understanding of their discipline.

As Einstein  said :

"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."

The actual story of how science evolves results in inspiring more confidence in science, not less.

The second is that this knowledge equips people to better argue against antiscience forces that use the same strategy over and over again, whether it is about the dangers of tobacco, climate change, vaccinations or evolution.

Their goal is to exploit the slivers of doubt and discrepant results that always exist in science in order to challenge the consensus views of scientific experts. They fund and report their own results that go counter to the scientific consensus in this or that narrow area and then argue that they have falsified the consensus.

How to combat the critics

In their book  Merchants of Doubt , historians Naomi Oreskes and Erik M. Conway  say that for these groups  “[t]he goal was to fight science with science — or at least with the gaps and uncertainties in existing science, and with scientific research that could be used to deflect attention from the main event.”

Science studies provide supporters of science with better arguments to combat these critics, by showing that the strength of scientific conclusions arises because credible experts use comprehensive bodies of evidence to arrive at consensus judgments about whether a theory should be retained or rejected in favour of a new one.

These consensus judgments are what have enabled the astounding levels of success that have revolutionized our lives for the better. It is the preponderance of evidence  that is relevant in making such judgments, not one or even a few results.

So, when anti-vaxxers or anti-evolutionists or climate change deniers point to this or that result to argue that they have falsified the scientific consensus, they are making a meaningless statement.

What they need to do is produce a preponderance of evidence in support of their case, and they have not done so.

Falsification as a 'myth-story'

Falsification is appealing because it tells a simple and optimistic story of scientific progress, that by steadily eliminating false theories, we can eventually arrive at true ones. As Sherlock Holmes put it, “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.”

Such simple but incorrect narratives abound in science folklore and textbooks.

Richard Feynman in his book  QED , right after “explaining” how the theory of quantum electrodynamics came about, said:

"What I have just outlined is what I call a “physicist’s history of physics,” which is never correct. What I am telling you is a sort of conventionalized myth-story that the physicists tell to their students, and those students tell to their students, and is not necessarily related to the actual historical development which I do not really know!"

But if you propagate a “myth-story” enough times and it gets passed on from generation to generation, it can congeal into a fact, and falsification is one such myth-story.

It is time we abandoned it.

This piece was originally posted on Scientific American . Read the original article .

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Falsifiability

Karl popper's basic scientific principle, karl popper's basic scientific principle.

Falsifiability, according to the philosopher Karl Popper, defines the inherent testability of any scientific hypothesis.

This article is a part of the guide:

• Inductive Reasoning
• Deductive Reasoning
• Hypothetico-Deductive Method
• Scientific Reasoning
• Testability

Browse Full Outline

• 1 Scientific Reasoning
• 2.1 Falsifiability
• 2.2 Verification Error
• 2.3 Testability
• 2.4 Post Hoc Reasoning
• 3 Deductive Reasoning
• 4.1 Raven Paradox
• 5 Causal Reasoning
• 6 Abductive Reasoning
• 7 Defeasible Reasoning

Science and philosophy have always worked together to try to uncover truths about the universe we live in. Indeed, ancient philosophy can be understood as the originator of many of the separate fields of study we have today, including psychology, medicine, law, astronomy, art and even theology.

Scientists design experiments and try to obtain results verifying or disproving a hypothesis, but philosophers are interested in understanding what factors determine the validity of scientific endeavors in the first place.

Whilst most scientists work within established paradigms, philosophers question the paradigms themselves and try to explore our underlying assumptions and definitions behind the logic of how we seek knowledge. Thus there is a feedback relationship between science and philosophy - and sometimes plenty of tension!

One of the tenets behind the scientific method is that any scientific hypothesis and resultant experimental design must be inherently falsifiable. Although falsifiability is not universally accepted, it is still the foundation of the majority of scientific experiments. Most scientists accept and work with this tenet, but it has its roots in philosophy and the deeper questions of truth and our access to it.

What is Falsifiability?

Falsifiability is the assertion that for any hypothesis to have credence, it must be inherently disprovable before it can become accepted as a scientific hypothesis or theory.

For example, someone might claim "the earth is younger than many scientists state, and in fact was created to appear as though it was older through deceptive fossils etc.” This is a claim that is unfalsifiable because it is a theory that can never be shown to be false. If you were to present such a person with fossils, geological data or arguments about the nature of compounds in the ozone, they could refute the argument by saying that your evidence was fabricated to appeared that way, and isn’t valid.

Importantly, falsifiability doesn’t mean that there are currently arguments against a theory, only that it is possible to imagine some kind of argument which would invalidate it. Falsifiability says nothing about an argument's inherent validity or correctness. It is only the minimum trait required of a claim that allows it to be engaged with in a scientific manner – a dividing line between what is considered science and what isn’t. Another important point is that falsifiability is not any claim that has yet to be proven true. After all, a conjecture that hasn’t been proven yet is just a hypothesis.

The idea is that no theory is completely correct , but if it can be shown both to be falsifiable  and supported with evidence that shows it's true, it can be accepted as truth.

For example, Newton's Theory of Gravity was accepted as truth for centuries, because objects do not randomly float away from the earth. It appeared to fit the data obtained by experimentation and research , but was always subject to testing.

However, Einstein's theory makes falsifiable predictions that are different from predictions made by Newton's theory, for example concerning the precession of the orbit of Mercury, and gravitational lensing of light. In non-extreme situations Einstein's and Newton's theories make the same predictions, so they are both correct. But Einstein's theory holds true in a superset of the conditions in which Newton's theory holds, so according to the principle of Occam's Razor , Einstein's theory is preferred. On the other hand, Newtonian calculations are simpler, so Newton's theory is useful for almost any engineering project, including some space projects. But for GPS we need Einstein's theory. Scientists would not have arrived at either of these theories, or a compromise between both of them, without the use of testable, falsifiable experiments. 

Popper saw falsifiability as a black and white definition; that if a theory is falsifiable, it is scientific , and if not, then it is unscientific. Whilst some "pure" sciences do adhere to this strict criterion, many fall somewhere between the two extremes, with  pseudo-sciences  falling at the extreme end of being unfalsifiable. 

Pseudoscience

According to Popper, many branches of applied science, especially social science, are not truly scientific because they have no potential for falsification.

Anthropology and sociology, for example, often use case studies to observe people in their natural environment without actually testing any specific hypotheses or theories.

While such studies and ideas are not falsifiable, most would agree that they are scientific because they significantly advance human knowledge.

Popper had and still has his fair share of critics, and the question of how to demarcate legitimate scientific enquiry can get very convoluted. Some statements are logically falsifiable but not practically falsifiable – consider the famous example of “it will rain at this location in a million years' time.” You could absolutely conceive of a way to test this claim, but carrying it out is a different story.

Thus, falsifiability is not a simple black and white matter. The Raven Paradox shows the inherent danger of relying on falsifiability, because very few scientific experiments can measure all of the data, and necessarily rely upon generalization . Technologies change along with our aims and comprehension of the phenomena we study, and so the falsifiability criterion for good science is subject to shifting.

For many sciences, the idea of falsifiability is a useful tool for generating theories that are testable and realistic. Testability is a crucial starting point around which to design solid experiments that have a chance of telling us something useful about the phenomena in question. If a falsifiable theory is tested and the results are significant , then it can become accepted as a scientific truth.

The advantage of Popper's idea is that such truths can be falsified when more knowledge and resources are available. Even long accepted theories such as Gravity, Relativity and Evolution are increasingly challenged and adapted.

The major disadvantage of falsifiability is that it is very strict in its definitions and does not take into account the contributions of sciences that are observational and descriptive .

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Martyn Shuttleworth , Lyndsay T Wilson (Sep 21, 2008). Falsifiability. Retrieved May 18, 2024 from Explorable.com: https://explorable.com/falsifiability

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Falsifications and scientific progress: Popper as sceptical optimist

• Published: 30 January 2014
• Volume 1 , pages 179–184, ( 2014 )

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• Carlo Veronesi 1  

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A scientific theory must be falsifiable, and scientific knowledge is always tentative, or conjectural. These are the main ideas of Popper’s Logic of Scientific Discovery . Since 1960 his writings contain some essential developments of these views and make some steps towards epistemological optimism. Although we cannot justify any claim that a scientific theory is true, the aim of science is the search of truth and we have no reason to be sceptical about the notion of getting nearer to the truth. Our knowledge can grow, and science can progress. Nevertheless, Popper’s theory of approximation to the truth is problematic and is still the subject of studies and discussions.

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1 References to two fundamental problems of knowledge

Popper’s philosophy of science takes as a starting point two fundamental problems of knowledge theory: the problem of induction, which Popper calls ‘Hume’s problem’, and the problem of demarcation, which is called ‘Kant’s problem’.

The problem of demarcation consists in the search for a criterion that makes it possible to distinguish empirical science from metaphysical speculation, philosophical systems and other forms of human knowledge. One answer to this problem is widely agreed upon: science is based on facts and is distinguished by its inductive method, which derives universal laws by generalising the results of observations and experiments. Thus, to demarcate science recourse is made to a ‘principle of induction’, which can be expressed as follows: if an observable property is valid for a certain number of members of a class, then it is valid for all members of the class. Popper, like Hume, declares himself contrary to induction and, like Kant, maintains that science begins with hypotheses and not with the gathering of experimental data, but all the same he thinks that it is possible to provide a criterion of demarcation. Hume maintained that induction, as a method of formulating laws or habits, was an irrational procedure, and according to Popper as well, it is not legitimate to go from particular cases to a universal law, that is, one that is valid for a potentially infinite set of cases; it is only permissible to go, in the presence of contrasting observations, to the falsification of the law. Using an example that is by now well known, Popper argues that, no matter how many white swans are observed, no one can be certain that the law ‘All swans are white’ will always be valid in all regions of space and time: a single observation of a black swan can render it false. It is precisely the possibility of falsification that characterises the empirical sciences and which, according to Popper, draws the line of demarcation between the theories of science and the doctrines of metaphysics or pseudoscience.

Elsewhere [ 15 ] we remarked that a young Popper arrived to this idea under the impression of the great upheaval in physics wrought by Einstein’s theory of relativity. Newton’s theory of gravitation, based on action at a distance of masses, had carried the day for more than two centuries but was replaced, at the beginning of the twentieth century, by relativistic physics. In 1919 the English astronomer Arthur Eddington organised two scientific expeditions to measure, during an eclipse of the sun in the southern hemisphere, an effect of general relativity which, in a normal day of sunshine, would have been impossible to observe. It was during this expedition that confirmation arrived that the trajectory of the luminous rays of the stars, even though these are without mass in the classic sense of the term, are curved when they pass near the sun. This result, predicted by Einstein and unforeseeable according to Newton’s theory, led to the global triumph of the new relativistic physics, even appearing on the front page of the New York Times . In that same year––1919––young Popper was able to attend a lecture given by Einstein in Vienna, remaining impressed by the fact that the physics of Newton, which appeared indisputable, could be replaced by a better theory, especially because Einstein himself had explained that in its turn the theory of relativity could also be confuted. Popper became convinced that we can never be sure in science that the truth has been reached, nor could even the most thoroughly tested theories escape the risk of falsification.

The falsifiability of scientific theories, which for Popper was suggested by logic and by the history of science, could also appear to be a rule of common sense. No theory should be taken into account if it evades all checks and if it cannot be contradicted by any observable fact. The predictions of soothsayers and astrologers, often so vague and imprecise as to be suitable for any kind of situation, are neither reliable nor scientific. Thus, science cannot reasonably include theories that render the search for counter-examples impractical, even though this means excluding theories that enjoy greater consideration than astrology.

Even the psychoanalysis of Freud, according to Popper, is closer to metaphysics than to science, because no kind of human behaviour can be either predicted or excluded on the basis of this theory. Instead, Popper’s position on Marxism is rather more detailed: in his opinion Marx’s doctrine was born as a falsifiable theory, with historically verifiable predictions, but then the followers of Marxism fitted it out with a battery of auxiliary hypotheses to prevent its being clearly contradicted by the facts. In this way they often managed to reinterpret theories and facts so as to make them agree, but in saving the theory, they sacrificed its scientificity.

2 The optimistic turn of Popper’s thinking

In his important 1935 Logik der Forschung [translated into English as The Logic of Scientific Discovery (1959)], Popper, in order to give credence to the thesis that verifications of theories (no matter how numerous) are in any case insufficient, took care to clear the field from another idea of an inductivist or verificationist nature, that is, that evidence in favour, even if unable to lead to the truth, can in any case increase the probability of theories (obviously in the absence of counter-examples). According to him, the probability of a universal law turns out to be equal to zero because the number of favourable cases, necessarily finite, must be seen in reference to the infinity totality of possible cases. Hence the results in favour can in no way increase the probability of a theory; they can only increase the degree of ‘corroboration’. Popper uses the term ‘corroboration’ to provide an indication of how well a theory has stood up to the attempts to confute it, as well as its provisional acceptability. To this end, the number of examples in favour does not count. Banal evidence, such as that which might derive from the repetition of the same experiment, does not increase the corroboration. A theory can be said to be corroborated only if it passes rigorous tests of risky predictions, that is, those at high risk of falsification. More precisely, Popper connects the degree of corroboration of a theory to the success in the prediction of events that are unexpected, surprising and considered improbably in light of previous knowledge. The prediction that the distance between two fixed stars, measured during the day, would be different from that measured at night, would have been unthinkable without Einstein’s theory of gravitation (which predicts that light must be attracted to the sun in exactly the same way that heavy bodies are). Thus, confirmation of this prediction by the British expeditions during the 1919 eclipse provided extraordinary corroboration of Einstein’s theory. However, the corroboration of a theory is a temporal account of its past successes, which provides no guarantee of its ability to pass future tests. Newton’s physics, over the course of two centuries, had registered a series of confirmations as well as of corroborating successes, culminating in the discovery of Neptune. This planet, whose existence has been postulated to explain the anomaly of the orbit of Uranus, was discovered in 1846 by the German astronomer Johann Gottfried Galle in exactly the region of space in which the earlier calculations (based on Newton’s celestial mechanics) by John Couch Adams and Urbain Le Verrier had situated it. The sensational success of this prediction provided a huge amount of support for the Newtonian theory, but did not prevents its later refutation.

For the reasons we have just given, in the final pages of The Logic of Scientific Discovery (which is still a youthful work, published when the author was just over 30), Popper observes that corroboration is not a value of truth, and that in his logic of science it was possible to avoid the use of concepts of ‘true’ and ‘false’ [ 7 : pp. 273–274]. In this order of ideas, he spoke of progress only as the elimination of erroneous theories in favour of others that were more comprehensive, or as the discovery of new problems that were deeper and more general [ 7 : p. 281], almost as if, as Imre Lakatos noted, scientific progress consisted in ‘an increased awareness of ignorance rather than a growth of knowledge’ [ 2 : p. 155]. However, after the publication of Logic , Popper came into contact with Alfred Tarski’s theory of truth, which led him to change the tone of his own philosophy and integrate the logic of discovery, in which it only seemed possible to reveal the error, with the theory of verisimilitude and the approximation to truth.

The Polish logician Tarski––explains Popper––had rehabilitated a theory of truth as a ‘correspondence to the facts’, which is another common sense idea of the truth. Following Tarski it is possible to write that ‘the sentence “snow is white” is true if and only if snow is white’ [ 13 : p. 64].

The discussion seems rather trivial, but what Tarski made evident––and this is the decisive element of his discovery––was that to speak about the correspondence of a sentence with the facts we need an ‘object language’ and a ‘metalanguage’. The object language is used to speak about facts, things and properties of the world, such as snow and its colour. Metalanguage is used to speak about both statements in object language, such as the statement in quotation marks ‘snow is white’, and about the facts of the world to which the statement refer. Footnote 1 In his comment Popper goes on to say that once the need for this metalanguage has been understood, it is not difficult to see how a statement can correspond to the facts, and it is also possible to explain the traps of everyday language, such as the classic ‘antinomy of the liar’, according to which the statement ‘I am lying’ is self-contradictory (the contradiction deriving from the fact that in everyday language no distinction is made between the levels of language and metalanguage).

Encouraged by Tarski’s results, Popper began to think that it might be possible to speak of objective truth, that is, of truth as correspondence to facts, without fear of falling into paradoxes, and that hence there was no longer any reason to abstain from speaking of the truth of science. One scientific theory could correspond to the facts better than another, that is, it could be closer to the truth. It would be rather unreasonable to think that Einstein’s physics, which had been successful in risky predictions, with precise measurements of phenomena not predicted by previous theories, did not contain something of the truth, or that it was no closer to the truth than all the rival theories that had preceded it [ 11 : pp. 1192–1193]. Popper became convinced that theories could come close to reality, and that it was also possible to recognise progress made towards the truth. If a theory had passed the tests that had been failed by a previous theory, then we have reason to believe that it is more verisimilar: we can therefore think that a highly corroborated theory is closer to the truth than one with a lower degree of corroboration.

Popper developed these concepts in the writings of his later years, and it is rather peculiar that his philosophy is known above all for its falsificationist methodology and much less known for these more articulated positions, in spite of these having been illustrated in lectures, talks and articles over the course of several decades. In one essay that joins two lectures given in the years 1960 and 1961, Popper himself wrote that, after having become aware of Tarski’s ideas on truth and becoming convinced that the idea of truth was not so ‘dangerously vague and metaphysical’ [ 8 : p. 314], he was able to contribute ‘essential further developments’ [ 8 : p. 291] to the ideas expressed in his Logic of Scientific Discovery . Footnote 2 According to this new outlook, science is something more than an incessant discovery of failures, and is not limited to revealing error and replacing erroneous theories. Scientific progress is not made only by means of conjectures and refutations; it is progress by means of conjectures, refutations and corroborations. Thus, corroborations, which were initially the point of departure for ulterior attempts at refutation [ 7 : Appendix IX, p. 419], became signs of progress and steps forward towards the truth, because a corroborated scientific theory, even if it can still be refuted, can be in the running as an approximately true theory and in any case contain a part of the truth. In this way Popper can also explain the possible paradox of false theories, such as Newtonian physics, which in any case function for centuries and continue to be used even after they have been falsified; this without taking refuge in pragmatism or instrumentalism, concepts according to which scientific theories are only convenient instruments for working without any pretext of aiding our understanding of the world. To the contrary, Popper states that the aim of science is precisely to search for the truth and that, in spite of difficulties and limited successes, it even manages to approach it.

3 Truth and approximation of truth

A further difficulty of the concept of truth derives from the conviction that a satisfying theory of truth must comprehend a criterion for believing in it in a way that is established and rational. According to Popper, this idea confuses what is true with what we know to be true, and does not take account of the fact that a theory can be true even if no one believes it. Popper maintains that truth must be separated from subjective experience of believing in it, and that the concepts of truth and certainty must not be confused. The aim of science cannot be to search for certainty, because all knowledge is fallible and thus uncertain, but the search for truth nevertheless remains. The theory of objective truth supported by Popper makes it possible to say that we search for the truth even if, as the ancient Greek philosopher Xenophanes (c.570–c.475 BCE) pointed out, we might not ever reach it, or recognise it when we do reach it. Using a famous metaphor, Popper compares the status of truth to a mountaintop wrapped in clouds. A climber might not only have trouble reaching it, but recognising it when he does: up in the clouds he might not be able to distinguish the main peak from the smaller peaks surrounding it. However, he can understand when he has not reached it, as when, for instance, he discerns one even higher, and he can consequently decide to continue on in that direction. As a rule we do not have a criterion of truth, that is, a procedure for recognising it, but we do have criteria for moving towards it.

However, the search for truth might also reveal itself to be a secondary ideal if it is limited to the trivial aspects of reality, because in science we seek something more than simple truth. Much in the same way as in mathematics, where we are not content with saying that two plus two equals four, in science as well we desire truths that are interesting and difficult to attain. We thus prefer a bold conjecture, even if it should turn out to be false, to a series of assertions that are true but uninteresting. From failure we can learn much about the truth; we can, eliminating our errors, come closer to it. Popper adds––and this is the new element––that to approach truth it is generally not sufficient to correct the errors of a previous theory. Certainly what is needed is a new theory that solves the difficulties of the earlier one, but this theory must also make it possible to predict facts never before observed, and to pass some of the tests regarding these new predictions. ‘An unbroken sequence of refuted theories would soon leave us bewildered and helpless’ [ 8 : p 330]: we need success and empirical corroboration in order to understand if we are on the right path, and also to appreciate the meaning of successful refutations.

All of these considerations take off from an intuitive base: the idea that scientific progress is made by means of a sequence of false (or presumably so) theories, ever closer to the truth, and that these can arise both by means of the correction of the aspects that are gradually falsified as well as by means of the support of new consequences or verified predictions. To explain precisely what he means, Popper considers two theories, A and B , both of which are false ( A can be considered an earlier theory and B a later one that replaced it) and states that B is closer to the truth than A if in the passage from A to B the set of false consequences is reduced without impairing the set of true consequences, or the set of true consequences is reinforced without incrementing at the same time the set of false consequences. This definition appears well posed logically: while a true theory has only true consequences, affirmations both true and false can follow from false premises. Moreover, common sense seems to agree with the idea that one false theory can contain fewer errors than another given the same amount of true information, or a greater amount of true information given equal false information. Unfortunately, a few years later some critics [ 5 , 14 ] showed that none of the conditions established by Popper for approaching truth can be verified, because the true consequences and false consequences of a theory increase and decrease together.

Given the importance of this negative result, which opened a new line of epistemological research, we want to expound on it in some detail. To this end, given the two false theories A and B , let A T indicate the truth-content (=the set of true logical consequences) of theory A , and A F its falsity-content (=the set of consequences of A that do not belong to A T ). Analogously, B T and B F are respectively the truth-content and falsity-content of theory B . Using these symbols, Popper’s comparative definition of verisimilitude or truthlikeness can be rewritten as follows: theory B is closer to the truth than theory A if and only if ( A T  ⊂  B T and B F  ⊆  A F ) or ( B F  ⊂  A F and A T  ⊆  B T ). Now let us show, following Tichý and Miller, that these two conditions cannot be satisfied if the theories are false and thus no false theory B can be closer to the truth than a false theory A on the basis of Popper’s criterion.

Let us first suppose that A T  ⊂  B T and that b is a true consequence of B but not of A (in the passage from A to B the truth-content is incremented, by example with the proposition b ). Since B is false, B F is not empty: I can thus consider a false consequence f of B and form the conjunction b&f . This conjunction is false (it would be true if and only if both b and f were true) and is a consequence of B (because b&f is a consequence of the propositions b and f , and moreover both b and f are consequences of B ): it therefore belongs to the falsity-content of B . The conjunction b&f cannot also belong to the falsity-content of A , because in that case both b and f would have to be consequences of A , contrary to our assumption that b is not. Therefore, if A T  ⊂  B T , there exists a proposition, b&f , which belongs to B F and not to A F , and there cannot be B F  ⊆  A F as required by Popper’s case 1. If the truth-content of the new theory B exceeds the truth-content of A , contemporarily the falsity-content of B also exceeds that of A .

Let us now suppose that B F  ⊂  A F and that g is a false consequence of A but not of B (in the passage from A to B the falsity-content is deprived of proposition g ). Let us consider a false proposition f of B to form the implication f  →  g . This implication is true (it would be false only for true f and false g ) and it is a consequence of A (because the implication f  →  g is a consequence of proposition g , and proposition g is a consequence of A ): it thus belongs to the truth-content of A . The statement f  →  g cannot also belong to the truth-content of B because in that case both f and g would have to be consequences of B , contrary to our assumption that g is not. Therefore, in the case where B F  ⊂  A F , there exists a statement, f  →  g , which belongs to A T and not to B T and there cannot be A T  ⊆  B T as required by case 2 of Popper’s definition. In the passage from A to B the falsity-content cannot diminish without at the same time also diminishing the truth-content.

This negative result, according to which two false scientific theories cannot be compared, might appear to be a mere logical artifice. Looking at the history of science, it seems reasonable to think that theories that are gradually falsified can still be considered increasingly better approximations to an unknown truth. The astronomical system of Copernicus has come to be considered better with respect to that of Ptolemy, and the theories of Newton and Einstein are considered even better. We might also cite trivial examples of false statements that we judge to be closer to the truth than others: the statement ‘There are ten planets in the solar system’ seems to be less false and thus closer to the truth than the statement ‘There are ten thousand planets in the solar system’.

In any case, even if the results of Tichý and Miller seem rather counter-intuitive, Popper acknowledged his logical error and attempted to correct the initial definitions of verisimilitude, and so did some of his students and other scholars in the years that followed. One idea might be that of placing a few restrictions on the classes of logical consequences that might work for or against the verisimilitude of theories, for example, comparing only the truth contents or privileging atomic or elementary propositions. This search for an approach to truth can be conducted, as Popper suggested, ‘in a kind of metrical or at least topological space’ [ 8 : p. 314] but, in spite of a great plethora of approaches, the search has not produced results that are unanimously shared. Thus the intuitive idea of a progressive approach to truth is not easily captured by formal definitions and the problem of verisimilitude remains an open one. Footnote 3

4 Concluding observations and a glance at ulterior problems

In an autobiographical note about his youthful interests, Popper wrote that in the autumn of 1919, when he tackled his first problem of the philosophy of science, he was not worried about the truth of theories: ‘My problem was different: I wished to distinguish between science and pseudo-science, knowing very well that science often errs and that pseudo-science may happen to stumble on the truth’ [ 9 : p. 44]. Be that as it may, already at the origin of all of Popper’s discourse, and his efforts to distinguish science from other forms of knowledge, it is possible to recognise an undeclared assumption: the basic idea that the requirement of falsifiability in any case renders science superior to metaphysics and pseudo-science. Imprecise theories, such as astrology and psychoanalysis, or theories that resort to continual correction to render them immune from failure, such as Marxism, can add nothing to our knowledge: if the search for counter-examples is not practical, then neither can we have any clue as to their provisional reliability. Further, even if one metaphysical theory or another should by some chance speak the truth, it would in any case be a truth that was static and without progress. To the contrary, it is the falsifiable aspects of theories, the refutations and successes in resisting the attempts at refutation, that are capable of contributing to progress towards the truth. These ideas, not yet clarified in Popper’s youthful work, would be elaborated, as we have seen, in the later development of his thinking.

Pavel Tichý, one of the logicians who criticised the Popperian definitions of verisimiltude, defines Popper’s mature conception as ‘optimistic scepticism’ [ 14 : p. 155]: the scepticism comes from the statement that we can never prove the truth of a scientific theory; the optimism derives from another of his statements, that our theories, presumably false, can be improved by approaching the truth. It was Popper himself who empowered these definitions of Tichý’s, speaking of his position as halfway between a pessimistic and an optimistic conception of scientific knowledge [ 12 : pp. 3–10], tending to be closer to optimism than to sceptical pessimism. This is because, while we might be sceptical about our capability to recognise the truth, we have no reason to be so towards the notion of approaching the truth and the fact that our science can grow and progress.

The middle road between scepticism and optimism sought by Popper seems problematical in any case: from the technical point of view there remains the problem of an acceptable formalization of the notion of verisimilitude; from the epistemological point of view Popper had to recognise that in order to justify his rediscovered optimism he needs a ‘whiff’ of inductivism [ 11 : p. 1193] to be able to state (taking the history of science as his point of departure) that the theories best corroborated are those closest to the truth. We can see that in this way Popper, after having kicked induction out the door, lets it back in through the window, and further that his inductive argument, weak nevertheless, needs notions that do not seem objective in the strict sense. Popper says that science progresses by means of risky predictions and results that are unexpected, surprising and sometimes spectacular; he also tells us that science seeks truths that are difficult and interesting. In either case, it appears that (aside from Popper’s intentions) other elements, of a subjective, psychological or perhaps aesthetic nature, must be introduced into the scientific enterprise. Recently some scholars of the problem of verisimilitude have begun to consider, albeit with a great deal of caution, the possibility of introducing a principle of aesthetic induction into the evaluation of scientific theories. Footnote 4 But this is another story.

Translated from the Italian by Kim Williams.

On the concept of truth as conformity with (or correspondence to) reality or as conformity with the “existing state of affairs”, see [ 13 ].

Among the essays of Popper’s optimistic phase, Imre Lakatos particularly remarks the ‘Addendum’ to The Open Society and its Enemies [ 10 ], even while reproaching Popper for not having ‘fully exploited the possibilities opened up by his Tarskian turn’ [ 2 : p. 159].

For a survey of the studies on verosimiltude, see [ 6 ].

The discussion on aesthetic induction is introduced in [ 3 ]. On the relationship between aesthetic induction and truth, see [ 1 ] and [ 4 ].

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Veronesi, C. Falsifications and scientific progress: Popper as sceptical optimist. Lett Mat Int 1 , 179–184 (2014). https://doi.org/10.1007/s40329-014-0031-7

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How we edit science part 1: the scientific method

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We take science seriously at The Conversation and we work hard to report it accurately. This series of five posts is adapted from an internal presentation on how to understand and edit science by our Australian Science & Technology Editor, Tim Dean. We thought you might also find it useful.

Introduction

If I told you that science was a truth-seeking endeavour that uses a single robust method to prove scientific facts about the world, steadily and inexorably driving towards objective truth, would you believe me?

Many would. But you shouldn’t.

The public perception of science is often at odds with how science actually works. Science is often seen to be a separate domain of knowledge, framed to be superior to other forms of knowledge by virtue of its objectivity, which is sometimes referred to as it having a “ view from nowhere ”.

But science is actually far messier than this - and far more interesting. It is not without its limitations and flaws, but it’s still the most effective tool we have to understand the workings of the natural world around us.

In order to report or edit science effectively - or to consume it as a reader - it’s important to understand what science is, how the scientific method (or methods) work, and also some of the common pitfalls in practising science and interpreting its results.

This guide will give a short overview of what science is and how it works, with a more detailed treatment of both these topics in the final post in the series.

What is science?

Science is special, not because it claims to provide us with access to the truth, but because it admits it can’t provide truth .

Other means of producing knowledge, such as pure reason, intuition or revelation, might be appealing because they give the impression of certainty , but when this knowledge is applied to make predictions about the world around us, reality often finds them wanting.

Rather, science consists of a bunch of methods that enable us to accumulate evidence to test our ideas about how the world is, and why it works the way it does. Science works precisely because it enables us to make predictions that are borne out by experience.

Science is not a body of knowledge. Facts are facts, it’s just that some are known with a higher degree of certainty than others. What we often call “scientific facts” are just facts that are backed by the rigours of the scientific method, but they are not intrinsically different from other facts about the world.

What makes science so powerful is that it’s intensely self-critical. In order for a hypothesis to pass muster and enter a textbook, it must survive a battery of tests designed specifically to show that it could be wrong. If it passes, it has cleared a high bar.

The scientific method(s)

Despite what some philosophers have stated , there is a method for conducting science. In fact, there are many. And not all revolve around performing experiments.

One method involves simple observation, description and classification, such as in taxonomy. (Some physicists look down on this – and every other – kind of science, but they’re only greasing a slippery slope .)

However, when most of us think of The Scientific Method, we’re thinking of a particular kind of experimental method for testing hypotheses.

This begins with observing phenomena in the world around us, and then moves on to positing hypotheses for why those phenomena happen the way they do. A hypothesis is just an explanation, usually in the form of a causal mechanism: X causes Y. An example would be: gravitation causes the ball to fall back to the ground.

A scientific theory is just a collection of well-tested hypotheses that hang together to explain a great deal of stuff.

Crucially, a scientific hypothesis needs to be testable and falsifiable .

An untestable hypothesis would be something like “the ball falls to the ground because mischievous invisible unicorns want it to”. If these unicorns are not detectable by any scientific instrument, then the hypothesis that they’re responsible for gravity is not scientific.

An unfalsifiable hypothesis is one where no amount of testing can prove it wrong. An example might be the psychic who claims the experiment to test their powers of ESP failed because the scientific instruments were interfering with their abilities.

(Caveat: there are some hypotheses that are untestable because we choose not to test them. That doesn’t make them unscientific in principle, it’s just that they’ve been denied by an ethics committee or other regulation.)

Experimentation

There are often many hypotheses that could explain any particular phenomenon. Does the rock fall to the ground because an invisible force pulls on the rock? Or is it because the mass of the Earth warps spacetime , and the rock follows the lowest-energy path, thus colliding with the ground? Or is it that all substances have a natural tendency to fall towards the centre of the Universe , which happens to be at the centre of the Earth?

The trick is figuring out which hypothesis is the right one. That’s where experimentation comes in.

A scientist will take their hypothesis and use that to make a prediction, and they will construct an experiment to see if that prediction holds. But any observation that confirms one hypothesis will likely confirm several others as well. If I lift and drop a rock, it supports all three of the hypotheses on gravity above.

Furthermore, you can keep accumulating evidence to confirm a hypothesis, and it will never prove it to be absolutely true. This is because you can’t rule out the possibility of another similar hypothesis being correct, or of making some new observation that shows your hypothesis to be false. But if one day you drop a rock and it shoots off into space, that ought to cast doubt on all of the above hypotheses.

So while you can never prove a hypothesis true simply by making more confirmatory observations, you only one need one solid contrary observation to prove a hypothesis false. This notion is at the core of the hypothetico-deductive model of science.

This is why a great deal of science is focused on testing hypotheses, pushing them to their limits and attempting to break them through experimentation. If the hypothesis survives repeated testing, our confidence in it grows.

So even crazy-sounding theories like general relativity and quantum mechanics can become well accepted, because both enable very precise predictions, and these have been exhaustively tested and come through unscathed.

The next post will cover hypothesis testing in greater detail.

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What is a scientific hypothesis?

It's the initial building block in the scientific method.

Hypothesis basics

What makes a hypothesis testable.

• Types of hypotheses
• Hypothesis versus theory

Additional resources

Bibliography.

A scientific hypothesis is a tentative, testable explanation for a phenomenon in the natural world. It's the initial building block in the scientific method . Many describe it as an "educated guess" based on prior knowledge and observation. While this is true, a hypothesis is more informed than a guess. While an "educated guess" suggests a random prediction based on a person's expertise, developing a hypothesis requires active observation and background research. 

The basic idea of a hypothesis is that there is no predetermined outcome. For a solution to be termed a scientific hypothesis, it has to be an idea that can be supported or refuted through carefully crafted experimentation or observation. This concept, called falsifiability and testability, was advanced in the mid-20th century by Austrian-British philosopher Karl Popper in his famous book "The Logic of Scientific Discovery" (Routledge, 1959).

A key function of a hypothesis is to derive predictions about the results of future experiments and then perform those experiments to see whether they support the predictions.

A hypothesis is usually written in the form of an if-then statement, which gives a possibility (if) and explains what may happen because of the possibility (then). The statement could also include "may," according to California State University, Bakersfield .

Here are some examples of hypothesis statements:

• If garlic repels fleas, then a dog that is given garlic every day will not get fleas.
• If sugar causes cavities, then people who eat a lot of candy may be more prone to cavities.
• If ultraviolet light can damage the eyes, then maybe this light can cause blindness.

A useful hypothesis should be testable and falsifiable. That means that it should be possible to prove it wrong. A theory that can't be proved wrong is nonscientific, according to Karl Popper's 1963 book " Conjectures and Refutations ."

An example of an untestable statement is, "Dogs are better than cats." That's because the definition of "better" is vague and subjective. However, an untestable statement can be reworded to make it testable. For example, the previous statement could be changed to this: "Owning a dog is associated with higher levels of physical fitness than owning a cat." With this statement, the researcher can take measures of physical fitness from dog and cat owners and compare the two.

Types of scientific hypotheses

In an experiment, researchers generally state their hypotheses in two ways. The null hypothesis predicts that there will be no relationship between the variables tested, or no difference between the experimental groups. The alternative hypothesis predicts the opposite: that there will be a difference between the experimental groups. This is usually the hypothesis scientists are most interested in, according to the University of Miami .

For example, a null hypothesis might state, "There will be no difference in the rate of muscle growth between people who take a protein supplement and people who don't." The alternative hypothesis would state, "There will be a difference in the rate of muscle growth between people who take a protein supplement and people who don't."

If the results of the experiment show a relationship between the variables, then the null hypothesis has been rejected in favor of the alternative hypothesis, according to the book " Research Methods in Psychology " (​​BCcampus, 2015). 

There are other ways to describe an alternative hypothesis. The alternative hypothesis above does not specify a direction of the effect, only that there will be a difference between the two groups. That type of prediction is called a two-tailed hypothesis. If a hypothesis specifies a certain direction — for example, that people who take a protein supplement will gain more muscle than people who don't — it is called a one-tailed hypothesis, according to William M. K. Trochim , a professor of Policy Analysis and Management at Cornell University.

Sometimes, errors take place during an experiment. These errors can happen in one of two ways. A type I error is when the null hypothesis is rejected when it is true. This is also known as a false positive. A type II error occurs when the null hypothesis is not rejected when it is false. This is also known as a false negative, according to the University of California, Berkeley . 

A hypothesis can be rejected or modified, but it can never be proved correct 100% of the time. For example, a scientist can form a hypothesis stating that if a certain type of tomato has a gene for red pigment, that type of tomato will be red. During research, the scientist then finds that each tomato of this type is red. Though the findings confirm the hypothesis, there may be a tomato of that type somewhere in the world that isn't red. Thus, the hypothesis is true, but it may not be true 100% of the time.

Scientific theory vs. scientific hypothesis

The best hypotheses are simple. They deal with a relatively narrow set of phenomena. But theories are broader; they generally combine multiple hypotheses into a general explanation for a wide range of phenomena, according to the University of California, Berkeley . For example, a hypothesis might state, "If animals adapt to suit their environments, then birds that live on islands with lots of seeds to eat will have differently shaped beaks than birds that live on islands with lots of insects to eat." After testing many hypotheses like these, Charles Darwin formulated an overarching theory: the theory of evolution by natural selection.

"Theories are the ways that we make sense of what we observe in the natural world," Tanner said. "Theories are structures of ideas that explain and interpret facts." 

• Read more about writing a hypothesis, from the American Medical Writers Association.
• Find out why a hypothesis isn't always necessary in science, from The American Biology Teacher.
• Learn about null and alternative hypotheses, from Prof. Essa on YouTube .

Encyclopedia Britannica. Scientific Hypothesis. Jan. 13, 2022. https://www.britannica.com/science/scientific-hypothesis

Karl Popper, "The Logic of Scientific Discovery," Routledge, 1959.

California State University, Bakersfield, "Formatting a testable hypothesis." https://www.csub.edu/~ddodenhoff/Bio100/Bio100sp04/formattingahypothesis.htm  

Karl Popper, "Conjectures and Refutations," Routledge, 1963.

Price, P., Jhangiani, R., & Chiang, I., "Research Methods of Psychology — 2nd Canadian Edition," BCcampus, 2015.‌

University of Miami, "The Scientific Method" http://www.bio.miami.edu/dana/161/evolution/161app1_scimethod.pdf  

William M.K. Trochim, "Research Methods Knowledge Base," https://conjointly.com/kb/hypotheses-explained/  

University of California, Berkeley, "Multiple Hypothesis Testing and False Discovery Rate" https://www.stat.berkeley.edu/~hhuang/STAT141/Lecture-FDR.pdf  

University of California, Berkeley, "Science at multiple levels" https://undsci.berkeley.edu/article/0_0_0/howscienceworks_19

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The Unfalsifiable Hypothesis Paradox

What is the unfalsifiable hypothesis paradox.

Imagine someone tells you a story about a dragon that breathes not fire, but invisible, heatless fire. You grab a thermometer to test the claim but no matter what, you can’t prove it’s not true because you can’t measure something that’s invisible and has no heat. This is what we call an ‘unfalsifiable hypothesis’—it’s a claim that’s made in such a way that it can’t be proven wrong, no matter what.

Now, the paradox is this: in science, being able to prove or disprove a claim makes it strong and believable. If nobody could ever prove a hypothesis wrong, you’d think it’s completely reliable, right? But actually, in science, that makes it weak! If we can’t test a claim, then it’s not really playing by the rules of science. So, the paradox is that not being able to prove something wrong can make a claim scientifically useless—even though it seems like it would be the ultimate truth.

Key Arguments

• An unfalsifiable hypothesis is a claim that can’t be proven wrong, but just because we can’t disprove it, that doesn’t make it automatically true.
• Science grows and improves through testing ideas; if we can’t test a claim, we can’t know if it’s really valid.
• Being able to show that an idea could be wrong is a fundamental part of scientific thinking. Without this testability, a claim is more like a personal belief or a philosophical idea than a scientific one.
• An unfalsifiable hypothesis might look like it’s scientific, but it’s misleading since it doesn’t stick to the strict rules of testing and evidence that science needs.
• Using unfalsifiable claims can block our paths to understanding since they stop us from asking questions and looking for verifiable answers.
• The dragon with invisible, heatless fire: This is an example of an unfalsifiable hypothesis because no test or observation could ever show that the dragon’s fire isn’t real, since it can’t be detected in any way.
• Saying a celestial teapot orbits the Sun between Earth and Mars: This teapot is said to be small and far enough away that no telescope could spot it. Because it’s undetectable, we can’t disprove its existence.
• A theory that angels are responsible for keeping us gravitationally bound to Earth: Since we can’t test for the presence or actions of angels, we can’t refute the claim, making it unfalsifiable.
• The statement that the world’s sorrow is caused by invisible spirits: It sounds serious, but if we can’t measure or observe these spirits, we can’t possibly prove this idea right or wrong.

Answer or Resolution

Dealing with the Unfalsifiable Hypothesis Paradox means finding a balance. We can’t just ignore all ideas that can’t be tested because some might lead to real scientific breakthroughs one day. On the other side, we can’t treat untestable claims as true science. It’s about being open to possibilities but also clear about what counts as scientific evidence.

Some people might say we should only focus on what can be proven wrong. Others think even wild ideas have their place at the starting line of science—they inspire us and can evolve into something testable later on.

Major Criticism

Some people criticize the idea of rejecting all unfalsifiable ideas because that could block new ways of thinking. Sometimes a wild guess can turn into a real scientific discovery. Plus, falsifiability is just one part of what makes a theory scientific. We shouldn’t throw away potentially good ideas just because they don’t fit one rule, especially when they’re still in the early stages and shouldn’t be held too tightly to any rules at all.

Another point is that some important ideas have been unfalsifiable at first but later became testable. So, we have to recognize that science itself can change and grow.

Practical Applications

You might wonder, “Why does this matter to me?” Well, knowing about the Unfalsifiable Hypothesis Paradox actually affects a lot of real-world situations, like how we learn things in school, the kinds of products we buy, and even the rules and laws that are made.

• Education: By learning what makes science solid, students can tell the difference between real science and just a bunch of fancy words that sound scientific but aren’t based on testable ideas.
• Consumer Protection: Sometimes companies try to sell things by using science-sounding claims that can’t be proven wrong—and that’s where knowing about unfalsifiable hypotheses helps protect us from buying into false promises.
• Legal and Policy Making: For people who make laws or guide big decisions, understanding this concept helps them judge if a study or report is really based on solid science.

Related Topics

The Unfalsifiable Hypothesis Paradox is linked with a couple of other important ideas you might hear about:

• Scientific Method: This is the set of steps scientists use to learn about the world. Part of the process is making sure ideas can be tested.
• Pseudoscience: These are beliefs or practices that try to appear scientific but don’t follow the scientific method properly, often using unfalsifiable claims.
• Empiricism : This big word just means learning by observation and experiment—the backbone of science and everything opposite of unfalsifiable concepts.

Wrapping up, the Unfalsifiable Hypothesis Paradox shows us that science isn’t just about coming up with ideas—it’s about being able to test them, too. Untestable claims may be interesting, but they can’t help us understand the world in a scientific way. But remember, just because an idea is unfalsifiable now doesn’t mean it will be forever. The best approach is using that creative spark but always grounding it in what we can observe and prove. This balance keeps our imaginations soaring but our facts checked, forming a bridge between our wildest ideas and the world we can measure and know.

September 7, 2020

The Idea That a Scientific Theory Can Be ‘Falsified’ Is a Myth

It’s time we abandoned the notion

By Mano Singham

Transit of Mercury across the Sun; Newton's theory of gravity was considered to be "falsified" when it failed to account for the precession of the planet's orbit.

Getty Images

J.B.S. Haldane, one of the founders of modern evolutionary biology theory, was reportedly asked what it would take for him to lose faith in the theory of evolution and is said to have replied, “Fossil rabbits in the Precambrian.” Since the so-called “Cambrian explosion” of 500 million years ago marks the earliest appearance in the fossil record of complex animals, finding mammal fossils that predate them would falsify the theory.

But would it really?

The Haldane story, though apocryphal, is one of many in the scientific folklore that suggest that falsification is the defining characteristic of science. As expressed by astrophysicist Mario Livio in his book Brilliant Blunders : "[E]ver since the seminal work of philosopher of science Karl Popper, for a scientific theory to be worthy of its name, it has to be falsifiable by experiments or observations. This requirement has become the foundation of the ‘scientific method.’”

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But the field known as science studies (comprising the history, philosophy and sociology of science) has shown that falsification cannot work even in principle. This is because an experimental result is not a simple fact obtained directly from nature. Identifying and dating Haldane's bone involves using many other theories from diverse fields, including physics, chemistry and geology. Similarly, a theoretical prediction is never the product of a single theory but also requires using many other theories. When a “theoretical” prediction disagrees with “experimental” data, what this tells us is that that there is a disagreement between two sets of theories, so we cannot say that any particular theory is falsified.

Fortunately, falsification—or any other philosophy of science—is not necessary for the actual practice of science. The physicist Paul Dirac was right when he said , "Philosophy will never lead to important discoveries. It is just a way of talking about discoveries which have already been made.” Actual scientific history reveals that scientists break all the rules all the time, including falsification. As philosopher of science Thomas Kuhn noted, Newton's laws were retained despite the fact that they were contradicted for decades by the motions of the perihelion of Mercury and the perigee of the moon. It is the single-minded focus on finding what works that gives science its strength, not any philosophy. Albert Einstein said that scientists are not, and should not be, driven by any single perspective but should be willing to go wherever experiment dictates and adopt whatever works .

Unfortunately, some scientists have disparaged the entire field of science studies, claiming that it was undermining public confidence in science by denying that scientific theories were objectively true. This is a mistake since science studies play vital roles in two areas. The first is that it gives scientists a much richer understanding of their discipline. As Einstein said : "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." The actual story of how science evolves results in inspiring more confidence in science, not less.

The second is that this knowledge equips people to better argue against antiscience forces that use the same strategy over and over again, whether it is about the dangers of tobacco, climate change, vaccinations or evolution. Their goal is to exploit the slivers of doubt and discrepant results that always exist in science in order to challenge the consensus views of scientific experts. They fund and report their own results that go counter to the scientific consensus in this or that narrow area and then argue that they have falsified the consensus. In their book Merchants of Doubt, historians Naomi Oreskes and Erik M. Conway say that for these groups “[t]he goal was to fight science with science—or at least with the gaps and uncertainties in existing science, and with scientific research that could be used to deflect attention from the main event.”

Science studies provide supporters of science with better arguments to combat these critics, by showing that the strength of scientific conclusions arises because credible experts use comprehensive bodies of evidence to arrive at consensus judgments about whether a theory should be retained or rejected in favor of a new one. These consensus judgments are what have enabled the astounding levels of success that have revolutionized our lives for the better. It is the preponderance of evidence that is relevant in making such judgments, not one or even a few results.

So, when anti-vaxxers or anti-evolutionists or climate change deniers point to this or that result to argue that they have falsified the scientific consensus, they are making a meaningless statement. What they need to do is produce a preponderance of evidence in support of their case, and they have not done so.

Falsification is appealing because it tells a simple and optimistic story of scientific progress, that by steadily eliminating false theories we can eventually arrive at true ones. As Sherlock Holmes put it, “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.” Such simple but incorrect narratives abound in science folklore and textbooks. Richard Feynman in his book QED , right after “explaining” how the theory of quantum electrodynamics came about, said, "What I have just outlined is what I call a “physicist’s history of physics,” which is never correct. What I am telling you is a sort of conventionalized myth-story that the physicists tell to their students, and those students tell to their students, and is not necessarily related to the actual historical development which I do not really know!"

But if you propagate a “myth-story” enough times and it gets passed on from generation to generation, it can congeal into a fact, and falsification is one such myth-story.

It is time we abandoned it.