I see from a quick read of the Wikipedia article that falsifiability is argued by some philosophers to be the defining criterion of science, setting it apart from non-science (there's lots of talk about demarcation, etc.). That's fine, but my impression has been (perhaps wrongly) that falsificationists go further than this and say that if something isn't falsifiable (and so isn't science), it's therefore epistemically defective in some way; that it's irrational, unjustified, unwarranted, etc.

Do any falsificationist philosophers actually argue this, that if some theory isn't falsifiable then it cannot be justified? And if they do, then how do they justify this extra step (i.e. is this theory of knowledge falsifiable)? Or do falsificationists more modestly say it's how we draw the line between science and non-science, but that non-science can be legitimate means of acquiring knowledge too (albeit with a different method)?

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    There are many human activities that are non-falsifiable but still legitimate: according to the "demarcationist" point of view, they are simply not "effective" ways to acquire knowledge about the world we live in. Jun 6, 2019 at 19:23
  • @MauroALLEGRANZA Thank you. I'm going to edit "legitimate" to say "legitimate means of acquiring knowledge", just in case I'm unclear. I am indeed asking about acquiring knowledge, not just general human activities. Jun 6, 2019 at 19:31
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    "Some philosophers" are Popper and his derivatives, and even Popper admitted unfalsifiable "metaphysical programmes" as rational and indispensable aspects of science (he even initially saw the evolution theory as one). After all, the hypotheses to be falsified have to be generated somehow. Mathematics is not falsifiable either. As for acquiring knowledge, the issue is scientific knowledge, and that has to be corroborated by surviving potential falsifications. What else gets to be called "knowledge", and how it is justified, is really beside the point.
    – Conifold
    Jun 6, 2019 at 19:41
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    @Conifold Right, I agree. It's just that in discussions I've seen people accuse certain propositions of being unfalsifiable, as though that's somehow a knock-down refutation. My question is whether falsificationists have actually argued this, that not being falsifiable implies not being knowledge simpliciter. (I also get that "falsificationist" is a broad label, and that even within themselves there might not be agreement here, but I honestly don't know much about it.) Jun 6, 2019 at 19:52
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    Aside from crude pop-versions, I think it is taken as a knock down only of something that professes to be scientific/empirical, etc., although this may not be made explicit. For example, falsificationists used it to "debunk" astrology, psychoanalysis, and even economics as "pseudoscience". Beyond that the issue is purely verbal, some may want to reserve "knowledge" for scientific knowledge only. A stronger position was held by logical positivists, who saw non-scientific claims as nonsensical, but they were verificationists, not falsificationists, and that is almost universally abandoned now.
    – Conifold
    Jun 6, 2019 at 20:06

3 Answers 3


So far as I am aware the demarcation principle, in Popper at least, serves merely to distinguish science from non-science. A scientific theory is empirically falsifiable while pseudoscience (e.g. astrology) and metaphysics are not.

A certain confusion arises from the fact that Popper also inclines to regard the non-scientific, as specified by his examples, as not intellectually respectable (as 'irrational, unjustified, unwarranted') by any criterion. Astrology is a prime example; he also excludes psychoanalysis and Marxism.

What is unintentionally misleading about this is that there is no reason why on Popper's own grounds the non-scientific has to lack intellectual respectability across the piece. Indeed, take some of his own work - The Open Society and The Poverty of Historicism. These are in no sense scientific works yet Popper plainly regards them as intellectually respectable. As indeed they are with the exception of the chapters on Aristotle and Hegel in The Open Society.

Note on falsifiability

If the Quine-Duhem Thesis is right, then no hypothesis is definitively falsifiable.

The Quine-Duhem thesis suggests that it is always possible to accommodate any recalcitrant observational evi- dence to any theory provided that one tinkers around enough in adjusting the auxiliary hypotheses and ceteris paribus clauses involved in the test situation. (Robert Klee, 'In Defense of the Quine-Duhem Thesis: A Reply to Greenwood', Philosophy of Science, Vol. 59, No. 3 (Sep., 1992), pp. 487-491: 488.)


K.R. Popper, The Logic of Scientific Discovery, tr. from the German of Logik der Forschung with additional footnotes and appendices, London: Hutchinson, 1959; New York: Basic Books, 1959).

K.R. Popper, The Open Society and Its Enemies, 2 volumes: Volume I: The Spell of Plato; Volume II: The High Tide of Prophecy: Hegel, Marx, and the Aftermath (London: George Routledge, 1945; revised edition, Princeton: Princeton University Press, 1950).

K.R. Popper, The Poverty of Historicism (London: Routledge and Kegan Paul, 1957; Boston, MA: Beacon Press, 1957).

Nicholas Maxwell, Karl Popper, Science and Enlightenment, Published by: UCL Press. (2017).

Robert Klee, 'In Defense of the Quine-Duhem Thesis: A Reply to Greenwood', Philosophy of Science, Vol. 59, No. 3 (Sep., 1992), pp. 487-491,


Do any falsificationist philosophers actually argue this, that if some theory isn't falsifiable then it cannot be justified?

Definitely. Although, not with regard to just any "theory", but with regard to any claim to empirical knowledge.

Popper's rationale for falsificationism, as explained in his seminal essay "Science: Conjectures and Refutations", is based on an analysis of David Hume's criticism of induction as a form of justification of empirical knowledge. Popper accepted the negative part of Hume's criticism, but replaced the positive part of it with falsificationism. Therefore, the scope of falsificationism is the same as the scope of induction i.e. every claim to empirical knowledge.

At some places it is indeed spoken as if falsificationism were only related to the demarcation of science from pseudo science. As if science was just a self enclosed game, unrelated to other human activities. But of course it isn't so enclosed. Science is a paradigm of rationality in general (which is not to say that it is the only paradigm). There is accordingly alongside falsificationism an associated conception of rationality: Critical Rationalism.

Some of Popper's works aim to apply the principles of critical rationalism beyond science proper, even to politics and to social administration, defending the values of open rational criticism and of evolution rather than revolution. Such are The Open Society and Its Enemies and also The Poverty of Historicism.


The Church Turing thesis is a fundamental tenet of math, logic, computer science, metamathematics.

It claims equivalence between Turing computable and effectively computable. Most members in all these disciples accept it as true.

But it's not provable since effective computability is a fuzzy informal notion.

Even more piquantly P=NP is widely believed to be false though not proven or disproven. Proving it true would cause commotion and havoc in cryptography. And from there in wide swathes of international finance.

Doesn't stop most if us from using our credit cards on the net does it?

In Summary

Above two are notable examples where Popper's thesis : "falsifiability = scientific-ness" appear to be falsified!

Added later

It may be argued that I am mixing CTT and P=NP being falsifiABLE with them being not yet falsifiED (or truthified).

This is true logically; it's not true sociologically. ie scientists are using/relying on these as scientific facts on a large scale – absence of (dis)proof notwithstanding

  • How is tentatively assume $P\not=NP$ despite the possibility of a disproof any different from tentatively assuming (say) general relativity despite the possibility of falsification? They're both instances of a claim being well-supported by current knowledge and evidence being tentatively taken for granted. Of course math isn't Popperian as noted above, but I don't think your example actually addresses this. (Maybe you're treating $P\not=NP$ as a purely negative claim, in contrast with a positive assertion? But that's a dividing line that quickly disintegrates under inspection ...) Jun 10, 2019 at 5:41
  • (Of course, "tentatively" in the above is an understatement - the point is that falsification in either case is not unimaginable.) Jun 10, 2019 at 5:41

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