I can see how the idea of falsifiability (rather than some form of verificationism) could support a demarcation between science and pseudoscience. Here is the Wikipedia definition of falsifiability:

A statement, hypothesis, or theory has falsifiability (or is falsifiable) if it is contradicted by a basic statement, which, in an eventual successful or failed falsification, must respectively correspond to a true or hypothetical observation.

Does that "basic statement" have to be a reproducible basic statement? I don't think it does, but I don't know what Popper says about this.

For example, suppose we observe a dust cloud approaching Sagittarius A* where a black hole is supposed to exist. We predict, using our falsifiable gravitation theory, that the dust cloud will be absorbed in some way by the black hole at a certain time. We get only one chance to observe this. It is not a reproducible event. But I think that prediction of the dust cloud's absorption would still be considered a "basic statement" that could contradict the theory.

I am looking for quotes from Popper on this topic since I would like to read more about what he had to say and perhaps quote him later.

  • Funny... astronomy is a strange kind of lab in which experiments are carried-out by filtering rather than by creating. Commented Dec 13, 2018 at 22:02
  • IIRC Popper considered Eddington's eclipse observation as a paradigm of an experiment.
    – Dan Hicks
    Commented Dec 13, 2018 at 22:19
  • @elliotsvensson I imagine one assumes that if one could repeat them they would always do the same thing. But I wonder whether a statement that falsifies a theory has to be repeatable to be considered a basic statement. I suspect not. Commented Dec 13, 2018 at 23:03
  • @DanHicks That's what I would hope he says. I wonder if he discusses this somewhere in The Logic of Scientific Discovery. I have the book, but it is a paper copy and not easy to search. Commented Dec 13, 2018 at 23:07
  • ExperimentS must be reproducible; observations : not necessarily so. The obvious examples are astronomical ones, but also earthquakes. Commented Dec 14, 2018 at 9:39

4 Answers 4


“In so far as a scientific statement speaks about reality, it must be falsifiable: and in so far as it is not falsifiable, it does not speak about reality.” ― Karl Raimund Popper, The Logic of Scientific Discovery

In the same book, he also said

The point is that, whenever we propose a solution to a problem, we ought to try as hard as we can to overthrow our solution, rather than defend it. Few of us, unfortunately, praise this precept; but other people, fortunately, will supply the criticism for us if we fail to supply it ourselves. Yet criticism will be fruitful only if we state our problem as clearly as we can and put our solution in a sufficiently definite form - a form in which it can be critically discussed.

There are criticisms against Popper such as those from Sven Ove Hansson, who is a scientific sceptic, a professor of philosophy and the chair of the Department of Philosophy, and History of Technology at the Royal Institute of Technology (KTH) in Stockholm, Sweden.

To get a full grasp of what Hansson says, you need to read all three references (Hanson, 1996, 2008 & 2013), but in summary, Hansson said (Hansson, 2013)

Science (in the broad sense) is the practice that provides us with the most reliable (i.e. epistemically most warranted) statements that can be made, at the time being, on the subject matter covered by the community of knowledge disciplines (i.e. on nature, ourselves as human beings, our societies, our physical constructions, and our thought constructions).

Even Popper, in the book I have pointed to you said,

Those among us who are unwilling to expose their ideas to the hazard of refutation do not take part in the scientific game.

For more on Popper and his critics, see my answer in Psychology Meta about closure of questions about Freudian "pseudoscience" in psychology.


Hansson, S. O. (1996). Defining Pseudoscience. Philosophia Naturalis, 33(1): pp. 169—176

Hansson, S. O. (2008). Science and Pseudo-Science. In E. N. Zalta (Ed.), The Stanford Encyclopedia of Philosophy (Summer 2017). Metaphysics Research Lab, Stanford University. Retrieved from https://plato.stanford.edu/entries/pseudo-science

Hansson, S. O. (2013). Defining Pseudoscience and Science In:Philosophy of Pseudoscience: Reconsidering the Demarcation Problem Pigliucci & Boudry (Eds.) Chicago, IL: University of Chicago Press

  • Your defense of Freud as scientific is correct -- Freud based his theorizing on data, and did testing of it. Freudian psychology, however, like all highly elastic Research Programmes, is easily twisted to being unfalsifiable. Popper noted this, but overgenralized to condemn the entire movement, and not the search for confirmation bias that motivated many of the individuals within that movement. It was Quine's realization that ALL theory is underdetermined by evidence that doomed falsification. Popper in his latter years realized this, and treated theories as a family, not individually.
    – Dcleve
    Commented Jan 4, 2023 at 18:30
  • Hansson's SEP entry is quite good, except for its endorsement of CSI, and its entries on psi. CSI engaged in data fraud, for decades: discord.org/lippard/kammann.html His three references include one that dismisses evidence in favor of theory: departments.bloomu.edu/philosophy/pages/content/hales/articles/…, a second that is a series of smears and confirmation biases: jamesgmatlock.com/wp-content/uploads/2013/12/…, and a third that relies upon three false claims to dismiss psi philpapers.org/rec/FLEPSO
    – Dcleve
    Commented Jan 4, 2023 at 18:44
  • @Dcleve - Maybe I am misreading here, but, does including the Committee for Skeptical Inquiry in the "Other Internet Resources" provide any real endorsement? Surely, it is just including them for others to review. Thanks for your links in criticism against CSI. I will read them thoroughly when I have some time on my hands. While academic psychology does not yet accept the existence of psi, it is an area of investigation that interests me. Commented Jan 5, 2023 at 8:06
  • Chris Rogers -- The article notes that pseudoscientists such as vaccine deniers and creationists, have their own conferences and publications -- but that a key difference is they don't do peer review. They also pre-set their conclusion. Both of these conditions apply to the organized skeptic movement as well, which pre-declares physicalism as a precondition for science. Note that Hansson founded the Swedish skeptic society, and based it on CSICOP, which was at that time engaged in a purge of the whistleblowers who objected to data fraud.
    – Dcleve
    Commented Jan 5, 2023 at 19:41
  • CSI is an ideological advocacy organization, not a science organization, its purpose is to argue for a particular ideological view. Likewise for the Skeptic's Dictionary -- see its discussion of ESP in dogs -- Wisemans' "tests" had varying methodology and "goodness criteria" for each test, and his final one (first time dog on window ledge) was a clearly non-useful criteria. While Sheldrakes tests (more tests, with replications, multiple conditions) used a far better "fraction of time period on windowsill" which was relevant for a hyperactive dog, and showed the effect definitively.
    – Dcleve
    Commented Jan 5, 2023 at 20:02

The term 'reproducible' doesn't mean that we must reproduce a specific event: a technical impossibility, since every event is in fact unique. 'Reproducible' means two things:

  1. That we can identify a particular and well-defined class of events to which our theory applies, and...
  2. That we specify a rigorous and well-defined methodology for investigating this class of events.

1° is important because it prevents data-fudging, meaning we cannot pick only those events that conform to our theory and excluding events that disprove our theory, not when those events all properly belong to the same class. 2° provides accountability: everyone knows exactly what we did to get our results, and anyone can repeat exactly what we did for themselves. Any research that is designed that way is reproducible: we have a defined class of events to study, and a defined system for studying them. This leads to falsifiability: if we:

  • Choose an event correctly, and...
  • Apply methodology correctly, but...
  • Do not get the expected result,

...something is wrong with our theory.

In the case you cite, we will obviously never see that particular dust cloud approach that particular (presumed) black hole again. But there are myriad black holes and myriad dust clouds in the universe, and we can treat any dust cloud that approaches any black hole as an event of the same class, and apply our rigorous methodology to every case. If we ever see one behave unexpectedly (according to Popper) we have falsified our theory.

Now, there are a lot of problems with Popper's theory, and Popper is generally viewed as a defunct theorist in the philosophy of science, noted only for historical purposes. This is particularly true of his views on pseudoscience and demarkation, though those ideas have a vocal supporters in certain circles. The problem is that Popper himself never managed to create a proper definition of the class of events he considered 'scientific', nor did he outline a comprehensive methodology for studying such events. Scientific investigation generally involves a suspension of disbelief for the duration of experimentation — giving a theory the benefit of the doubt until it receives a falsifying result, and then allowing for modification and revision of the therefor subsequent cycles of investigation — the term 'pseudoscience' is almost always applied as an a priori assessment accompanied by strict skepticism. Not to say that the term is useless, but as it is generally applied it fails to distinguish itself from both bad science and non-science. Be careful using it.

  • You said, "Scientific investigation generally involves a suspension of disbelief for the duration of experimentation" I have always been told that for valid scientific study, you must try and disprove the theory. This is mirrored in the 2nd quote from Popper I provided in my answer. How are you trying to disprove it if you believe it to be true? Commented Jan 5, 2023 at 10:55
  • @ChrisRogers: A scientist generally tries to create science that is functional: that consistently produces the desired results. Anticipating/discovering failures is an important part of that process — one wants to rework theory to deal with such failures better — but someone whose only goal is to disprove theory isn/t doing science. S'he is doing 'science critique' (like art critique, but with lab coats). Suspension of disbelief means we accept a theory as true knowing that the real world might not work that way. We can always un-suspend our disbelief and go back to the drawing board. Commented Jan 5, 2023 at 14:53
  • @ChrisRogers -- Ted's answer is one I agree with, with the exception of his put downs of Popper. Falsifiability and attempted falsification is implicit in his bullet 3 under how to adapt the concept of verification to non-lab sciences (astronomy geology, anthropology, economics, environmental science, climatology -- experimental controls are basically impossible in these observational sciences). Falsificaiton and falsifiability are never absolute, but are part of an attitude of searching for, accepting, and working to accommodate any negative results.
    – Dcleve
    Commented Jan 5, 2023 at 16:30
  • @ChrisRogers -- Lakatos's adaptation of Popper, to look at Research Programmes more so than a specific family of hypotheses, accepts that one can have falsifications, which one sets aside to address in future theory tweaks and testing. But one only does so, if the Probgramme is producing new SUCCESSFUL test results as well. The Programme must be progressive. Useful to increase understanding, and suggest new interesting experiments or observations, and generally pass them.
    – Dcleve
    Commented Jan 5, 2023 at 16:36

Popper's views on reproducibility can be found in "The Logic of Scientific Discovery" Chapter 1, Section 8:

Kant was perhaps the first to realize that the objectivity of scientific statements is closely connected with the construction of theories— with the use of hypotheses and universal statements. Only when certain events recur in accordance with rules or regularities, as is the case with repeatable experiments, can our observations be tested—in principle—by anyone. We do not take even our own observations quite seriously, or accept them as scientific observations, until we have repeated and tested them. Only by such repetitions can we convince ourselves that we are not dealing with a mere isolated ‘coincidence’, but with events which, on account of their regularity and reproducibility, are in principle inter-subjectively testable.

Every experimental physicist knows those surprising and inexplicable apparent ‘effects’ which in his laboratory can perhaps even be reproduced for some time, but which finally disappear without trace. Of course, no physicist would say in such a case that he had made a scientific discovery (though he might try to rearrange his experiments so as to make the effect reproducible). Indeed the scientifically significant physical effect may be defined as that which can be regularly reproduced by anyone who carries out the appropriate experiment in the way prescribed. No serious physicist would offer for publication, as a scientific discovery, any such ‘occult effect’, as I propose to call it — one for whose reproduction he could give no instructions. The ‘discovery’ would be only too soon rejected as chimerical, simply because attempts to test it would lead to negative results. (It follows that any controversy over the question whether events which are in principle unrepeatable and unique ever do occur cannot be decided by science: it would be a metaphysical controversy.)

If you want to know what Popper said and assess his ideas you should read his writings yourself instead of relying on secondary sources like the ones advocated in other answers.

  • Popper considered there to be a very soft boundary between science and philosophy, and welcomed critique of, and adaptation to his philosophic ideas relative to how science can and should work. Over his long life, he did modify his views based on these critiques -- basically extending "test, falsificaiton, and modification of hypothesis" to his philosophy of science, where testing was challenge by peers. His entire body of work is needed to understand how he addressed critics, and how they affected his views on reproducibility.
    – Dcleve
    Commented Jan 5, 2023 at 17:23

As one astutue member, on another thread, remarked, scientific claims are universal statements (all/no). To falsify a universal statement, we require only one what is called a counterexample. It took but one black swan in Terra Australis to falsify the assertion that all swans are white. To get right to the point, reproducibility is not an critical requirement for falsification, but there are criteria for what counts as a good experiment/observation.

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