Karl Popper was one of the twentieth century’s preeminent philosophers of science. He was an avowed realist who was dedicated to the correspondence theory of truth. In his seventh decade of life, after fifty years of demonstrating professional excellence, he acknowledged that while correspondence to an unobservable reality describes the concept ‘truth’ very well, certainty about our inferences (even those interpretations which are based on operational measures, let alone those which aren’t) is never warranted.

That move (described in his book Objective Knowledge: an Evolutionary Approach) acknowledged that the presumptions of Modern (that is, Pre-Kantian) philosophy have been superseded by more coherent (Postmodern) perspectives.

“[W]hile Popper is a realist who holds that scientific theories aim at the truth…he does not think that empirical evidence can ever provide us grounds for believing that a theory is either true or likely to be true. In this sense, Popper is a fallibilist who holds that while the particular unfalsified theory we have adopted might be true, we could never know this to be the case. For these same reasons, Popper holds that it is impossible to provide justification for one’s belief that a particular scientific theory is true.” Garrath Williams, Stanford Encyclopedia of Philosophy

Kant’s insights into the limits of human understanding revolutionized the field of philosophy, but they haven’t yet filtered through social institutions to the level of inclusion in many educational curricula outside of philosophy departments. Most people who seek better understandings believe that it's appropriate for people to be absolutely certain about some of our inferences about complex subjects (events, processes, people...).

Was Kant mistaken? His insight has been widely acclaimed by professional philosophers for two centuries…no refutation of his work has been widely accepted as cogent.

Was Popper mistaken? If so, why? If not, is it more productive when engaging in philosophical inquiry to publicly examine the justifications for our beliefs than to insist or to imply that they’re indisputable?

What are the reasons for applying fallibilism? What are the reasons to believe that certainty is a better approach?

  • It is always prudent to inspect one's justifications rather than to take things on faith (although some things have to be so taken for practical reasons). I am not sure who are the "most people" who believe in absolute certainty. It was always an invention of philosophers, the folk are intuitively fallibilistic. Not only fallibilism, but even relativism and skepticism, "filtered through social institutions" pretty thoroughly, and perhaps too much, witness the post-truth and "alternative facts". For arguments for/against fallibilism see e.g. IEP
    – Conifold
    Commented Apr 11, 2019 at 19:10
  • fwiw, few would doubt kant's importance, but i was told no-one believes everything he claimed
    – user38026
    Commented Apr 12, 2019 at 0:54
  • @Conifold Thanks. I appreciate these comments. …"the folk are intuitively fallibilistic"...I agree up to a point, until I somehow manage to imply a question about something which they believe very strongly and they become so unwilling to reconsider that they demonize me in order to avoid thinking beyond their presumptions...have you noticed that type of behavior? You're "mystery" person; have you studied psychology? I interpret this syndrome as toxic certainty.
    – Rortian
    Commented Apr 12, 2019 at 8:58
  • I agree with @conifold up to a point, that is, about what people might say about what they think, but it still seems to me that most folks I meet (nonacademics nowadays) never need or want to think or talk about philosophy or about certainty. I wonder what other philosophy doers have noticed about that...
    – Rortian
    Commented Apr 12, 2019 at 9:00
  • @another_name "few would doubt kant's importance" lol Who would need to?...."no-one believes everything he claimed "...Who might need to? lol...thx!
    – Rortian
    Commented Apr 12, 2019 at 9:05

4 Answers 4


It is hard to correlate Kant and Popper, at least I find it so, since their enterprises were so different. Kant's major epistemological concern was with what might be termed psychological epistemology. The forms of intuition (our sense of space and time) and the categories of the understanding (causality, quantity, quality, plurality, limitation, possibility and the rest) were for Kant the fixed framework of thought without which there could be no coherent experience.


Popper did not accept the Kantian categories, or his understanding of space and time. These did not in Popper's view have the priori necessary validity that Kant attributed to them. I am not sure how far Kant would or need have been disturbed by revisions to the forms of intuition or the categories. What principally mattered to Kant was to show that whatever we sense or think can never be conceptually uncontaminated but must always be processed - structured - by the human cognitive apparatus. Only thus can there be the everyday and scientific - 'phenomenal' - world that we experience. On the details of that apparatus he could have been, whether he would actually have been or not, flexible. Newton or Einstein - it makes no matter. Space and time become space/ time. Kant could have accommodated that revolution in physics.

If I am right, then Kant counts in an admittedly extended sense as a fallibilist.

That contestable point aside, Kant is definitely a fallibilist as regards the individual's representations of phenomena. The individual can and does make all the errors that Descartes tabulates in the First Meditation. If e.g. causation is presupposed to coherent experience, I can still make mistakes about what causes what. The same goes, to take another example, for quantity. If coherent experience were impossible without the category of quantity, I can still make false judgements or form erroneous beliefs about specific quantities.


If Popper was a realist and Kant and idealist, Kant's philosophy posits a world of experience that is 'empirically real' but 'transcendentally ideal'. Kant's empirically real world of experience lends itself to Popper's hypothetico-deductive methodology.


Though Kant did not believe that his forms of intuition or his categories were products of evolution, there is nothing in his philosophy to prevent him from acknowledging evolution any more than there is in principle to prevent his recognising revolutions in science. Such revelations may briefly have suspended his famous daily walk but I can see no more serious consequence.

  • Very nicely put, thank you. We can take the most coherent bits from variant philosophical stories and recontextualise them in a perspective which is both broadly coherent and also pragmatically applicable for educational (or other) purposes.
    – Rortian
    Commented Apr 12, 2019 at 14:38
  • +1 @Rortian - This seems exactly the right approach. I have criticised Kant and Popper but they were brilliant thinkers who understood the problem and had useful things to say.
    – user20253
    Commented Apr 13, 2019 at 14:03
  • @PeterJ, Thank you for this assent. I appreciate the support.
    – Rortian
    Commented Apr 13, 2019 at 14:06

I believe this is one of the most important question in philosophy. It questions whether philosophy is worth doing.

The solution to the problem lies in seeing that Kant and Popper deny the possibility of certain knowledge, as does their tradition of thought. Thus the question of truth for them is confusing. This denial of the possibility of knowledge suffuses the thinking of our local tradition of philosophy and we see it in some of the comments here.

They speak of truth in terms of correspondence or coherence, or as requiring 'justified true belief', which must be the most muddled idea in philosophy. They argue quite rightly that we cannot claim the truth of a philosophical or scientific theory since a scientific or metaphysical theory is just a likely story, as one ancient Greek notes. It may be the best theory and it may be rigorous, but it makes no sense to say it is true in an absolute sense. We can know that Popper's Realism is not endorsed by metaphysics but is shown to be logically indefensible, but this does not prove it is false.

These issues can mostly be bypassed if we say that truth is knowledge and that we cannot know truth unless we know it is truth. Thus for the mystical epistemologist truth is what we know is true. This approach considerably simplifies the issues.

The arguments of Kant and Popper work well for scientific and metaphysical theories. These are never true or false. But their arguments say nothing about whether a person can know metaphysical truths. It is simply taken for granted that they cannot.

The physicist Paul Davies has a more informed view. From his analysis he concludes that Popper and Kant et al are correct as to the limits of theoretical knowledge and its inability to ever become truth or certain knowledge, so takes the view that if there is a way to know metaphysical truths it could only be by way of the methods described in mysticism. Their arguments do not apply to direct unmediated knowledge of Reality so although he is sceptical he does not have an argument to defeat mysticism's knowledge claims. This makes his Mind of God an unusual and brave book.

Was Kant mistaken? His insight has been widely acclaimed by professional philosophers for two centuries…no refutation of his work has been widely accepted as cogent.

I'm not sure what insight you mean. Is it that we cannot know the 'thing-in-itself'? This is said in Upanishads. The knower cannot know the knower but can only be it. Who is there to know the knower or understand the understander?

Kant does not prove we can know nothing of the thing-in-itself (even if he proved there is one) so his insight is a conjecture.

Was Popper mistaken? If so, why? If not, is it more productive when engaging in philosophical inquiry to publicly examine the justifications for our beliefs than to insist or to imply that they’re indisputable?

Beliefs as we usually define them are always disputable. They are never truth or knowledge. For truth and knowledge one has to go beyond beliefs. Popper is correct, it seems to me, in most of what he says, but he has a limited view of knowledge that means his arguments only work for physical and metaphysical theories. He says nothing about whether knowledge of truth is possible thus leaves us free, as Davies notes, to speculate that we can know truths and know we know them.

Russell's tradition is muddled on this issue, as we see from his confession that he is unable acquire any knowledge from metaphysics and has no idea how human beings know things. This problem is shared by his tradition and it therefore has a confused approach to knowledge and truth.

What are the reasons for applying fallibilism? What are the reasons to believe that certainty is a better approach?

If fallibilsm states that certainty is impossible then it is a speculative theory with no proof or evidence to support it and much counter-evidence. If it is the view that we should never be dogmatic about our speculations then it is just good philosophcial practice.

The search for certainty requires that we acknowledge the fallibility of our speculative theories and seek more certain knowledge, so the two approaches are complementary to some extent. In this sense fallibilism is just Cartesian doubt. But why would anyone do philosophy unless they think they can reach certainty?

If it is possible for human being to know metaphysical truths in the way mysticism teaches then the arguments of Popper and Kant would still be sound in respect of our speculative theories. That is, they are sound if we believe knowledge of truth is impossible. They are not global arguments though, since they do not rule out the possibility of such knowledge.

For this reason I see Davies as having a better grasp of the issues than either of them. He knows that he doesn't know whether such knowledge is possible.

Truth is knowledge. As far as we'll ever know there is no such thing as truth in the absence of knowledge. A truth is what we know to be true. 'Justified true belief' does not qualify, and coherence or correspondence are just useful conditions for building theories.

Meanwhile, it is perfectly possible to know relative truths. For instance, we can know that a speculative theory is never a known truth.

This is off-the-cuff so is not well organised but perhaps it outlines some of the issues that arise for the question.


Some epistemologists have taken fallibilism to imply skepticism, according to which none of those claims or views are ever well justified or knowledge. In fact, though, it is fallibilist epistemologists (which is to say, the majority of epistemologists) who tend not to be skeptics about the existence of knowledge or justified belief.

Emphasis mine (I'm sure!). One issue with fallibilism could be that the ideal of certainty, about our beliefs, might work better than acknowledgment of its universal impossibility.

Does this mean that "toxic certainty" is a desirable state? Probably not.


@PeterJ, thank you for this excellent explanation (despite the self-deprecating conclusion which of course must be untrue!).

Is certainty all-or-none, as truth is supposed to be? If so it’s theoretically unobtainable, but in practice only the wiser people realise this; the rest of us are busy arguing about who’s right or wrong about being in possession of true knowledge. Knowing doesn’t work like that, and those who think it does are arguing about castles in the sky.

Perhaps levels of certainty ought to be associated with levels of coherency? The more (reliable and relevant) evidence that we have which supports our assertions, the more coherent they are and the more certain we should be about them. Isn't that how coherency is supposed to work?

If the term ‘knowledge’ refers only to true knowledge then it’s a rather scarce commodity. Meanwhile there’s subjective knowledge and cultural lore; those phenomena also called types of knowledge; they're what we actually discussing!

So why do people focus only on true knowledge as ‘knowledge’? So that they can claim to be in possession of the best knowledge, superior knowledge, better knowledge than anyone who dares to disagree? It may be because they just don’t know better than to be absolutely certain about their favourite beliefs. They believe for certain that the best knowledge is true knowledge.

But why would anyone do philosophy unless they think they can reach certainty?

Thanks for this question. Cognitive and motivational psychology is my other interest.

We’re not always aware of our motives. Sometimes we do things intentionally, for a particular purpose of which we’re aware because we’ve thought about it and we believe it’s important. The rest of the time we’re on automatic pilot. We do what we must; we do what we feel like doing; we don’t always focus on purposeful action.

Everyone is driven to do stuff. We can only do things on purpose if we think about why we ought to do this or that but not something else. We’re more purposeful if we’re more thoughtful about why we do what we do.

My theme is better thinking produces better living. The better that I’ve learned to understand things (such as sciences and morality) the more productive and satisfied I’ve learned to become. I understand that I can’t know the absolutely certain truth about unobservable things or about complex systematic processes, but I do philosophy to develop more coherent discourses, in order to figure out how to apply myself more and more effectively to achieving my most important goals. In addition, I also intend to apply my understandings to support other people in figuring out what works optimally for them.

Another reason that people do philosophy is to know things better than other people. We feel better about ourselves if we think that we’re smarter than someone else. We’d rather win arguments than appear to be ignorant or less knowledgeable than others. We want to feel good, we want to be smart; that’s better than being dominated by people who know more and better than we do. If someone asks our reasons for believing what we believe or doing what we did, it’s much better to produce coherent reasons which apply to manifesting particular (beneficent) purposes (or to make up something that sounds good) than to admit that we behaved thoughtlessly.

We can adopt complete certainty about philosophical or scientific claims, but that's unwarranted and maladaptive.I was much more arrogant before I learned that I’d been fooled about this. Since I gave up believing that my beliefs are true I’m much more openminded, a bit more humble, and much more fulfilled. That's why I apply the value of fallibilism to my perspectives.

  • I can agree with all of this except your claim that certain knowledge is not possible,.If you're right then all of religion and mysticism is nonsense and all the great sages and teachers were deluded or lying. Many people believe this but nobody has proved it. You'll notice that all the people who claim certain and absolute knowledge is impossible cannot make sense of metaphysics. This should indicate a problem with their view. Pity we can't chat over a pint. .
    – user20253
    Commented Apr 13, 2019 at 14:15
  • @PeterJ, we can chat, and we can imbibe at will, and we could do both at the same time. I'm willing, and my chat room here is called Higher Order Thinking (an idea which I define as the generation of highly coherent sets of ideas about complex subjects). I'll chat if you will.
    – Rortian
    Commented Apr 13, 2019 at 15:24
  • Could you post a link or tell me how to find it?
    – user20253
    Commented Apr 13, 2019 at 18:08
  • voila The list of rooms is available through the "Chat" at the bottom left of these pages. We could schedule a mutually convenient time.
    – Rortian
    Commented Apr 13, 2019 at 18:28
  • @PeterJ...I dropped a file in there for visitors who are interested in discussing philosophy...
    – Rortian
    Commented Apr 13, 2019 at 19:09

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