The Debate over Science’s Essence: Hume, Popper and Kuhn

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Hume’s ‘Problem of Induction’

In our everyday life, we place tremendous faith in inductive reasoning. We intuitively seem to know that the sun will rise tomorrow just like it has risen today and since the day our ancestors started keeping track of time. While making a left turn we don’t expect vehicles to veer to the right. Yet one would have found it impossible to convince the Scottish philosopher David Hume(1711-1776) that the belief in inductive reasoning is rationally justified.  Hume nonchalantly dismissed induction as a custom or animal habit that is devoid of rational justification and he was right in this regard.

Hume identified the source of our habitual inductive reasoning in how we pin faith on ‘uniformity of nature’. We believe in a uniform universe where we expect everyday objects that we see and use to behave the same in the future. We expect Newton’s universal gravitation principle will apply to celestial bodies in the distant galaxies even though Newton never examined them.

Hume argued it is very much possible to imagine a non-uniform universe where objects behave in a random and erratic way. A possible non-uniformity renders the uniformity principle untenable. Moreover, if we are to defend ‘uniformity of nature’, the best we could do is to argue how natural laws up until now behaved uniformly. We can point out how inductive reasoning not only landed astronauts on the moon but also stops people from jumping off a cliff. But the premise of our defence will be the same inductive reasoning that we are trying to defend in the first place. As a result,  any defence of ‘uniformity of nature’ ends up being the logical fallacy of ‘circular reasoning’.

Hume’s insoluble problem gave rise to fundamental questions in the field of scientific inquiry. What makes a truth claim scientifically valid and where lies the boundary between scientific statements and non-scientific metaphysical claims? Is there an objective scientific truth that is eternally valid? Philosophers of science deal with these perplexing questions which many mainstream scientists find too troublesome to ponder upon. As we will see in the following paragraphs, Karl Popper and Thomas Kuhn, two twentieth-century philosophers of science introduced two radically different approaches in their attempts to solve these quandaries.


Popper’s Falsifiability

The Austrian-British philosopher Karl Popper(1902-1994) made significant contributions both in the fields of philosophy of science and political philosophy. His influential ‘falsification principle’ postulates that a hypothesis must be falsifiable to be scientifically valid. Any claim of a phenomenon that is not observable or does not leave any traces for detection is not a valid scientific statement.   In his hypothetico-deductive method,Popper argued that scientists use deductive reasoning to reach a scientific statement.  Once a hypothesis is formulated, scientists try to prove that hypotheses wrong by looking for contradictory evidence to the predictions made by the hypothesis. One black swan is enough to prove the ‘all swans are white’ hypothesis wrong. The scientist testing the hypothesis will have to look for one.

Falsification turned out to be an innovative solution to Hume’s ‘problem of induction’. It was also an attempt to solve the problem of demarcation between scientific theories and non-scientific and metaphysical claims. For Popper, the falsification of a hypothesis, albeit negative, contributes to the progress of human knowledge. We get rid of one false assumption at a time. In other words, progression in scientific knowledge is achieved through an eternal process of conjecture formulations and refutations.

Popper repudiated Sigmund Freud’s “Psychoanalytic Theory” and the Marxist “Class Struggle” narrative of history and branded them as pseudo-sciences. He argued that both theories lack the caveat of falsifiability. Any example of people behaving not in the thrall of their unconscious wishes could also serve examples of unconscious motivations. Freud’s theory is thus incompatible for testing against any clinical data. Similarly, when western capitalist societies didn’t pave the way for socialism and ultimately communism, the Marxist cognoscenti introduced many ‘ad-hoc’ theories as rationales for the apparent delay of, for example, the emergence of the welfare states.  Such hermeneutic justifications made the caveat of falsifiability to wither away, Popper contended.

It transpired that the very same practice of using ad-hoc theories that irked Popper helps scientists to make remarkable scientific discoveries. When the orbit of the planet Uranus didn’t oblige by Newtonian gravitational theory’s predictions, scientists didn’t rush to discard Newton’s theory. Rather in its defence, they proposed an ad-hoc theory which claimed that the gravitational force from an undiscovered planet was responsible for Uranus’s wayward behaviour. Later the discovery of the planet Neptune justified the provision of the ad-hoc theory. Moreover, it is nearly impossible for a scientific theory not to ignore a few anomalies in data. However, as scientists eventually found out, when anomalies start to stack up and successive ad-hoc theories aren’t enough to explain them, it often leads to shifts in scientific paradigms.


Kuhn’s Revolutionary Theory  

In his seminal book titled The Structure of Scientific Revolutions (1963), the American physicist, science historian and philosopher Thomas Kuhn (1922-1996) attacked the logical positivism school of scientists who were dominant in that period. The logical positivists believed in the apparent objectivity of the scientific method and in a ‘linear progression’ in scientific knowledge.  They delineated the historical process of ‘context of discovery’ as insignificant where scientists make subjective judgements on how to approach a scientific problem. They demarcated it from the ‘context of justification’ which they argued involves scientists using a standard set of rules to formulate a scientific theory. Positivists contented that unlike the former process, the ‘context of justification’ is very much pertinent to the scientific method.

Kuhn’s views were contrarian to logical positivism’s negligence towards history.  As a historian of science, he was rather interested in the revolutionary periods in the history of science when the advent of groundbreaking theories such as the Copernicus’s heliocentric model and Darwin’s evolution theory shifted the edifice of the scientific approach. He argued such upheavals indicate that the progression in scientific knowledge is rather chequered.

Kuhn coined the term ‘normal science’, which denotes the quotidian activities of scientists that do not stir a pandemonium. The scientists share a particular paradigm, which makes sure they stick to a basic set of rules or outlook on how to approach a scientific problem, the methods of solving the problem and setting the boundaries of their research. Scientists abstain from questioning the authenticity of the paradigm and ignore any contradictory anomalies, treating them as technical faults of their own.

For example, the eighteenth-century scientific paradigm classified the planet Uranus as a star. Even after detailed observations, scientist William Herschel sticking to the existing paradigm classified the planet as a comet. Moreover, political turmoils can profoundly influence the scientific faculties. The early 20th-century nationalist euphoria across the western world convinced many scientists to jump on the bandwagon of state-sponsored research in race sciences.

But when anomalies begin to accumulate, a fissure rips through the paradigmatic consensus. The confounded scientists start to lose confidence in the dominant paradigm. Thus begins the period of ‘revolutionary science’ Kuhn thought; a paradigm shift that forces scientists to look for better alternatives.  The classification of Uranus as a planet changed our perception of the solar system and enabled scientists to discover a number of distant planets in the later years. Despite the sporadic revivals of ‘race science’ in recent decades, there is a growing consensus in the scientific community on race being a social construct.

Kuhn further contended, the adoption of a new paradigm also involves the act of leap of faith on the scientist’s part and peer pressure also plays a role in it. In other words, a paradigm gains credibility when it satiates a significant number of scientists. Given that Kuhn’s proposition omitted the caveat of scientists using objective reasoning when they choose between competitive paradigms, many of Kuhn’s critics were dismayed as one critic flippantly accused Kuhn of reducing theory choice to the level of ‘mob psychology’.


Incommensurability and Theory-ladenness of data

Furthermore, much to his critics’ chagrin, Kuhn inoculated the concepts of ‘incommensurability and ‘theory-ladenness of data’. Through ‘incommensurability’ Kuhn argued that the adoption of a new paradigm renders it incomparable to the substituted paradigm. A conceptual framework common to both paradigms that can enable scientists to draw a comparison between the two is absent.

Once a paradigm shift takes place, proponents of the old and new paradigms will find it impossible to find a common standard for evaluating the two paradigms. Incommensurability thus makes scientific advancements directionless. In this scenario, newer paradigms are neither superior nor inferior to their predecessors.

Kuhn’s critics argued that although paradigms like the Ptolemaic and Copernican models of the solar system are at odds, the Newtonian and Einsteinian physics do not conflict in the same ways. Einstein’s theory states that an object’s mass depends on its velocity, Newton’s theory says it doesn’t.  But the Newtonian concept of ‘mass’ has a very different connotation when contrasted with Einsteinian physics.  Scientists, therefore, attempt a comparison between the two. It’s because unlike the Ptolemaic model, the Newtonian gravitational theory is still an integral part of modern physics.

In response, Kuhn exercised restraint with his incommensurability approach. He agreed it is possible for a conversation between the proponents of two different paradigms. But Kuhn thinks they still may disagree on the standards of evaluation, which will lead to an ‘incommensurability of standards’. ‘Each paradigm will be shown to satisfy the criteria that it dictates for itself and to fall short of a few of those dictated by its opponent’, he argued.

By ‘theory-ladenness of data,’ Kuhn maintained that theory-neutral data do not exist. All data he argued is more or less contaminated with residuals of theoretical assumptions. For a scientist, it means that she would first have to choose one of the competing theories. It’s because the validity of a data stems from theoretical assumptions which are again very much subjective. To put it succinctly, Kuhn’s ‘theory-ladenness of data’ contends that a scientific truth depends on the paradigm it clings to and therefore is ultimately relative.

Kuhn’s ‘theory-ladenness of data’ renders the quest for an objective truth as a forlorn cause. But by that same logic, Kuhn’s proposition of a paradigm reliant truth is itself paradigm-relative. A denial would mean that Kuhn’s proposition is the only objective truth out there, which would contradict the very premise it is built upon. An acceptance, however, makes an objective truth possible. The quest for an objective truth isn’t futile yet.

Kuhn was very unhappy with the intense criticism his theory met. He rebuked the claims of his theory being non-rational but still maintained ‘there is no algorithm for theory choice in science’. The positivist emphasis on rationality concerning theory choice he argued is too demanding and irrational. In reality, the approach is a moderate one that always has a scope for non-algorithmic subjective judgements.

The irony is that the cultural relativists who deny the existence of universal human values are enthralled by Kuhn’ ideas. Although Kuhn was strongly pro-science, his enthusiasts in the humanities and social sciences are very much anti-science as they often consider many irrational beliefs and cultural practices as equally respect worthy.

Nonetheless, thanks to Kuhn, philosophers of science today treat the contexts of discovery and justification equally and refrain from drawing a sharp line between them. Even his critics acknowledge that Kuhn’s radical ideas had a positive impact after all. His doctrines help us better understand the different mechanisms behind the established scientific theories.



So, how do contemporary philosophers of science address this issue? Some of them advocate for a multidisciplinary approach they call ‘triangulation’ where scientists from different disciplines of science would attempt to solve a single problem from their respective perspectives. The advantage of ‘triangulation’ is that a range of investigative methods is employed instead of a single one. Each method has its own advantages and disadvantages. But the multiple methods based on their formal basis provided by the philosophy of science will lead us much closer to the truth. Take Darwin’s theory of Evolution for example. Its credibility not only stems from palaeontology but also from other scientific disciplines such as geology, developmental biology, genetics and molecular biology.

Nobel winning physicist Richard Feynman once jested, ‘philosophy of science is as useful to scientists as ornithology is to birds’. But the current age of ‘post-truth’ politics when cherry picking the ‘facts’ is on the rise means that Feynman was wrong. The essence of science depends on its intellectual history and philosophical foundations, which make the works of Hume, Popper and Kuhn more relevant today than ever.





  1. Okasha, Samir(2002), Philosophy of Science: A very short introduction, Oxford University Press
  2. Hume, David(1748), An Enquiry Concerning Human Understanding, Project Gutenberg
  3. Popper, Karl(2002), The Logic of Scientific Discovery, Routledge
  4. Popper, Karl(1968), Conjectures and Refutations: The Growth of Scientific Knowledge, HarperCollins Publishers
  5. Kuhn, Thomas(1996), The Structure of Scientific Revolutions, University of Chicago Press
  6. Warburton, Nigel(2014), Philosophy: The Classics, Routledge
  7. Warburton, Nigel(2011), A Little History of Philosophy, Yale University Press
  8. Munafo, Marcus & Smith, George(2018, January 23). Robust research needs many lines of evidence. Retrieved April 21, 2018, from


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