Kuhn versus Popper: Towards Critical Rationalism

Written by Dr Jim Petch

This is written in response to an earlier contribution by Derek Alker, which I criticized and I need to begin with an apology to Derek. My criticism was clumsy in its wording and could have been interpreted as arrogant in spirit. As far as I know Derek has no ill intent towards me and I have none towards him but I was surprised by my own apparent aggression when I saw the contribution on-line. This is important, though I withdraw none of the points of criticism I made, since the manner of criticism is an important issue in the community of science and in particular in the debates about climate change, which (as anyone will know who is familiar with the main blogs) are awash with insults and bad behaviour and even threats. I want to begin by correcting my own mistake in that respect by referring to a key aspect of Popper’s philosophy.

Popper and Kuhn

In the debate about the relative merits of Popper’s and Kuhn’s ideas about what I will call the dynamic of science, this matter of attitude is pivotal, though few commentators have analysed it. And this in spite of the repeated emphasis that Popper gave to it. It is captured in the often-quoted lines;

“I may be wrong and you may be right,

and by an effort, we may get nearer to the truth”

These lines encapsulate what Critical Rationalism means and they hold the secret to the whole of Popper’s approach. They have many layers of meaning; logical, methodological and social. They point to a logic of discovery that is based on criticism, to an attitude of mind that is non-authoritarian, to a culture of tolerance and to humility in the individual.

These ideas sit in contrast to implications of the model of Normal Science put forward by Kuhn (1962) as well as to the related model of Post-Normal Science advocated by Funtowicz and Ravetz (1993) that seems to have gained some traction in the Climate Change world especially through the writings of Mike Hulme, see for example Hume (2009). It is worth exploring these differences since the implications are important for the conduct of the debate on climate change and for how the scientific aspects of that debate should be understood.

In his many writings Popper provides a connected set of ideas about the conduct of science. First is his view of what science is all about; on the one hand it is about ideas and the growth of knowledge, while on the other it is about criticism and an openness of debate that allows criticism. He provides a principle of demarcation, which is based on deductive logic, which provides a basis for deciding whether or not a statement about ideas is scientific; that is that the statement must be capable of being falsified. This logical issue pivots on the asymmetry between deductive and inductive propositions that is summed up in the famous adage that no number of observations of white swans proves the theory all swans are white but a single observation of a black swan disproves it.

Popper’s model of the dynamic of scientific investigation, the testing of testable propositions, is captured in the simple schematic of conjecture and refutation;


This is a continuous process that has no beginning, but in a given instance it doesn’t start with observation as such but with a problem P1, which may or may not result from observation. The problem is one with an idea or theory, which fails to match reality. The investigator proposes a new, tentative theory TT which he proposes explains the anomaly as well as what the previous theory explained and he tests the theory in a process of error elimination through observation EE which in its turn provides new problems of explanation P2, P3 and so on and on.

An important elaboration of this idea, which has great relevance for the climate debate, is provided by the idea of Multiple Working Hypotheses formulated by Chamberlain (1890) who proposed what he thought was a better way of proceeding than examining a single hypothesis because of the risk of bias from those who wanted to protect their own ideas. It involves testing competing hypotheses against each other. The critical issue then is not to design experiments that test a single idea but to design experiments that discriminate between different possible explanations. This quite different challenge is clearly pertinent to the climate change debate.

Either way, we should note some characteristics of these ideas on science, which as we will see distinguish them from other, fundamentally different ideas of how science operates and that are accepted by many in the climate change field:

– The process is driven by ideas and by problems with ideas and not by observation

– It is based on criticism

– It is anti-positivist, that is, it doesn’t seek certain knowledge

– It does not seek authority, in fact it is anti-authority or at least neutral to it

– It demands invention and new observations

There are two key corollaries of this model of science, which have been expressed by Popper in a number of places, about authority and optimism. Their relevance for the current debates on Climate Change speaks for itself.

“..in my view the appeal to the authority of experts should be neither excused nor defended. It should, on the contrary, be recognized for what it is – an intellectual fashion – and it should be attacked by a frank acknowledgement of how little we know, and how much that little is due to people who have worked in many fields at the same time. And it should also be attacked by the recognition that the orthodoxy produced by intellectual fashions , specialization and the appeal to authorities is the death of knowledge, and that the growth of knowledge depends entirely on disagreement.” Popper (1994)


“…..we need not accept that there is…a historical tendency for things to become worse. The future depends on ourselves. It is we who bear all the responsibility.

For this reason, an important principle holds: It is our duty to remain optimists.


The future is open. It is not predetermined and thus cannot be predicted – except by accident.

The possibilities that lie in the future are infinite. When I say ‘It is our duty to remain optimists’, this includes not only the openness of the future but also that which all of us contribute to it by everything we do: we are all responsible for what the future holds in store.

Thus it is our duty, not to prophesy evil but, rather, to fight for a better world.” Popper (1994)

Turning to the models of Normal Science and of Post Normal Science we find a different picture. The Kuhnian model of normal science describes a model of science quite different in nature from the model of Popper. “In contrast to the classical and critical traditions of science which argue that science is essentially a rational activity, some believe that scientific attitudes and judgements are made relative to the beliefs of the scientist and of the scientific community. One such critic of the rationalist image of science is Kuhn. He views science in terms of alternating periods of stable, normal science, in which practices are dominated by some paradigm accepted by the scientific community, and periods of paradigm change. Central to it is the role of the scientific community in judging scientific work and knowledge. This is achieved within a disciplinary matrix of shared beliefs and thought exemplars or archetypal applications. These provide both the puzzles which scientists attack and the bases for judging their solutions. There is only a subjective basis for choice between competing theories or between competing paradigms. This is the thesis of relativism. This idea rests upon the incommensurability of different points of view or of different theories since if competing theories cannot be compared then the rational image of science cannot be supported.” Haines Young and Petch (1985)

Comparing Normal Science with Critical rationalism we can note the following in comparison with Critical rationalism:

  • It is driven by the need for consensus and takes its dynamic not from the clash of ideas but from ensuring conformity

  • It is positivist, it assumes certainty and seeks certainty in ideas

  • It is strongly authoritative and encompasses the social processes of authority

  • It is incompatible with radical invention

  • The idea of Post Normal Science of Funtowicz and Ravetz (1993) is based on Kuhn’s model but take things further in terms of the scenarios in which science is conducted and of the role and objectives of practitioners, the rationale is grows out of the recognition of severe uncertainty. The Wikipedia summary of what it is about is:

    “Post-Normal Science is a concept developed by Silvio Funtowicz and Jerome Ravetz, (1993) attempting to characterise a methodology of inquiry that is appropriate for cases where “facts are uncertain, values in dispute, stakes high and decisions urgent”. It is primarily applied in the context of long-term issues where there is less available information than is desired by stakeholders.

    According to its advocates “post-normal science” is simply an extension of situations routinely faced by experts such as surgeons or senior engineers on unusual projects, where the decisions being made are of great importance but where not all the factors are necessarily knowable. Although their work is based on science, such individuals must always cope with uncertainties, and their mistakes can be costly or lethal.

    Because of this, advocates of post-normal science suggest that there must be an “extended peer community” consisting of all those affected by an issue who are prepared to enter into dialogue on it. These parties bring their “extended facts”, that will include local knowledge and materials not originally intended for publication, such as leaked official information. A political case existsfor this extension of the franchise of science, but Funtowicz and Ravetz also argue that this extension is necessary for assuring the quality of the process and of the product.


    Physicist and policy adviser James J. Kay described post-normal science as a process that recognizes the potential for gaps in knowledge and understanding that cannot be resolved in ways other than revolutionary science. He argued that (between revolutions) one should not necessarily attempt to resolve or dismiss contradictory perspectives of the world, whether they are based on science or not, but instead incorporate multiple viewpoints into the same problem-solving process.” (http://en.wikipedia.org/wiki/Post-normal_science)

    Comparing Post-Normal Science with Critical rationalism we can note the following:

  • It is driven by social, political and practical needs

  • It is positivist and seeks certainty through consensus and aspects of quality improvement of data

  • It eschews traditional authority but seeks authority in the security of wide consensus, i.e. in the crowd

  • It approaches uncertainty by harvesting opinions and data and by endeavoring to assess quality of information

  • Looking at all this from the perspective of a critical rationalist, there is the classic issue of understanding the merits of these various approaches to the ‘growth of knowledge’. That issue is, what is the basis on which we know anything? While recognizing the importance and relevance of the social and political aspects in the practice, promotion and use of science, there remains the fundamental issue of rationality. Normal science and Post Normal Science, largely ignore what that means for argument in science. They make the mistake of seeking certainty through what each in its own way describes as method but paradoxically “the central mistake is the quest for certainty”(Popper, 1972) and so they avoid the method of critical discourse and enquiry. Consequently, they both seek authority and authority is central to whatever method they use; and that authority is the authority of the group or the expert. However, the only workable authority in science is the authority of one the idea over another based on empirical evidence and that evidence has to be gathered and used critically in order to test ideas.

    One way of making sense of all this so as not to dismiss what on the surface appear reasonable but conflicting positions that have been taken regarding the functioning of science is to realize that in each of these three ”camps” the word science has entirely different meanings. We might leave it at that and agree to differ, but the issue of rationality, deductive logic and the critical testing of ideas will not go away so easily.

    It is claimed that Post Normal Science is a new approach to science (see http://www.nusap.net/ ) but it is obvious that its science is defined differently from the Normal Science of Kuhn or the Critical Rationalist Tradition of science as described by Popper. There is great merit in seeking to deal with uncertainty and with seeking unconventional sources of information and ideas but why call it science? Ravetz, in a most readable and thought provoking set of contributions to the Watts Up With That blog (http://wattsupwiththat.com/2010/02/09/climategate-plausibility-and-the-blogosphere-in-the-post-normal-age/) considers Popper as an “idealist” with the clear implication that his ideas are in some way unimportant because they are detached from the real and messy world of modern, complex scientific problems. Even Kuhn’s model of normal science he considers unsuited to real-world complex problems.

    Hulme (2009) on the other hand effectively ignores these issues altogether in spite of writing on “What is Scientific Knowledge” (2009, p76 ). He has little of substance to say on rationality, logic or method. He chooses to skirt around the methodological bases for disagreement (which in a book on ‘why we disagree’ is, let us say, somewhat surprising) and instead focuses on the social and cultural aspects of judgment instead. Still the problem of rationality doesn’t go away.

    On these seemingly simple points rests the whole edifice of scientific reasoning whether we follow one camp or another. The edifices of Normal Science and Post Normal Science rest on the sand of consensus and no matter what the degree of consensus or the breadth and depth of opinion for or against a consensus they have no logical basis for claiming they meet the criteria of scientific reasoning as laid out by Popper and by many others.

    Advocates both of Normal Science and of Post Normal Science seem to forget that the methods of science and methods of reasoning developed and refined since the ancients are a response to exactly the sorts of problems they claim to face. In their different ways they claim to address issues of uncertainty and authority in special and new ways but they make two mistakes; first, thinking that what they proposed is different and new and secondly ignoring the logic of discovery and of the growth of knowledge. By ignoring these issues they risk missing the opportunities really to advance knowledge of climate change.


    Chamberlain T.C. 1890. The Method of Multiple Working Hypotheses. Science, XV,92-96.

    Funtowicz S.O. and Ravetz J.R., 1993. Science for a post-normal age. Futures, 25, 739-755.

    Haines-Young R.H. and Petch J. R., 1985. Physical Geography: its Nature and Methods. Harper and Row, London

    Hulme M, 2009. Why we Disagree about Climate Change. Cambridge University Press, Cambridge..

    Kuhn T, 1962. The Structure of Scientific Revolutions. 2nd Ed, University of Chicago Press. Chicago

    Popper K. R., 1972. Objective Knowledge. Oxford University Press, Oxford.

    Popper K. R., 1994. The Myth of the Framework: in Defence of Science and Rationality. Routledge, London.

    Jim PetchJim is a retired Physical Geographer, GIS educationalist and e-learning pioneer. Jim was formerly a Reader in Environmental Science at MMU and Head of Distributed Learning and co-Director of the e-Learning Research Centre, University of Manchester. He was founding Director of UNIGIS International masters programme, the world leader in its field. He has a career long interest in Critical Rationalism and its role in earth and ecological sciences as well as in the teaching of research methods and scientific writing at all levels. He is co-author of two books, one on Popperian ideas applied to Physical Geography,  and of over one hundred papers in a range of disciplines.


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    Comments (5)

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      Derek, thank you for your comments. I appreciate them.
      I agree that in applying Popper’s ideas many people fail but not so much in understanding but in applying to their own situations the key points of Popper’s arguments. They don’t drag themselves away from the seductive but misguided habits of confirming what they know through observation and of thinking that observation drives the growth of scientific knowledge.

      I would add to my earlier comments only one other idea which in fact finds a place in the opening section of Popper’s Logic of Scientific Discovery ( Chapter 1,2 on Elimination of Psychologism). Popper criticises the notion that ideas come from observations or that there is a logical method of having new ideas. Every development of ideas has a creative or arational element and it is this creativity that drives the development of knowledge based on the interplay of observation and ideas. Popper quotes Einstein; “There is no logical path tending to …. laws…they can only be reached by intuition, based on something like an intellectual love…of the objects of experience”.

      In the Climate Change field there is no shortage of ideas. And, looking around, I think it is fair to say that we can see that these ideas do not come from observation. There is a shortage however, of critically designed observations taken to test ideas and to distinguish between competing ideas.

      My view is that the preoccupation of climate scientists should be the design of observation programmes or the critical selection of existing observation data which test between competing theories, always recognising of course that for any phenomenon or situation one competing theory is the null hypothesis. For example the question they should be asking are, how can we tell that the changes we see in climate are not due to natural variability? What observation would distinguish between anthropogenic effects through the agency of CO2 and the natural cycles we know exist in climate? Very few analyses address this crucial type of question. And I have to say that very few analyses on either side of the debate do so.

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      Derek Alker


      Thank you Jim. But, in the end I deserved your response, because I had accepted something second hand that I had not checked for myself.

      I would note that no one has picked me up on this issue to date, and misinterpreting Popper is a pretty BIG issue, so your article must be news to many…

      I have used observation – hypothesis – experiment – theory – law as my intended approach for a long time now, and I feel comfortable with it. However, it is good to know it has weaknesses that I was not aware of as such, and that I need to keep in mind.

      I will reread your article, it may change my approach to science, but it most definitely has already left me better informed.
      Thank you.

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      Hi Pat, sorry to be slow replying to your kind comment but I have been away and preoccupied.
      I have no doubt that science will survive, since, based on my own experience, most scientists have integrity. They are however, too easily seduced by funding but in the case of climate science they are perhaps unaware of the deeper issues of politics and power behind the funding and hence are a little naive.
      Nevertheless, as Popper once said of democracy, “the price…. is eternal vigilance”. It isn’t a given that it will survive unscathed.
      And I am not sure I can form an adequate response to the point about engineering and philosophy. For me there is no absolute division between the three. As I see it, they feed off each other and the richness of each is bolstered by its relations with the other two.



    • Avatar

      Pat Obar


      Dr Jim Petch,
      Thank you for your well written and informitive article. Do you think that science, in particular physical science, will ever survive this attack? Must we only go back to engineering and phlosopy?

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