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Popper's Demarcation

In Bacon’s Novum Organum (1620), he distinguishes between a true method and a false method. The true method, termed ‘interpretatio naturae’, literally translates as the ‘interpretation of nature.’ So it is, according to Bacon, that the pursuit of true knowledge relies on studying and interpreting the world around us; the study of science. Philosophers of science have long struggled over their most important problem: that of what, precisely, it is that separates science from non-science, true knowledge from false. This fundamental desire to make a binary distinction is known as the demarcation problem, and is one that lies at the heart of the philosophy of science.


 The demarcation problem troubled Karl Popper from an early age; indeed, he claims to have grappled with the problem “when should a theory be ranked as scientific?” (Popper, 1963:33) from the age of seventeen. The rise of Nazism and Stalinism troubled Popper, and in turning to science, he found a clarity of thought that was lacking in many of the political movements of the day.  The fact that many political leaders had begun to describe their methods as ‘scientific’ fuelled his desire to correctly demarcate between theories. The problem Popper set for himself, the one which would arguably be his most important and define the direction of his future work, was that of  “drawing a line […] between the statements,  or systems of statements, of the empirical sciences, and all other statements” (Popper, 1963:39).  Popper wasn’t the first scholar to consider this problem and, as we will see, his suggestions flew in the face of what were the accepted views surrounding the problem at the time. Here, we seek to discover just why Popper’s demarcation was so important.


The predominant school of philosophical thought in science up to the early 1900’s was that of positivism, superseded during Popper’s youth by its natural predecessor, logical positivism. Both of kthese philosophies are at their core based around the Baconian principle of induction. They promote a scientific method where direct observations (or in the case of logical positivism, observation statements) are studied, and from these a theory developed. Furthermore, they state that only knowledge gathered in this way is scientific. This constitutes the positivist method of demarcation, then  – that theories may be admitted as scientific only if they are derived from experience (or, for the logical positivist,  observation statements).


The major problem with this method rests on the belief that the use of induction in science is valid. If induction is invalid, then this method of demarcation falls down. A naïve argument against induction runs thus. If I grow up on a road where everybody drives a red car, I may infer that ‘all cars are red’. However, when I then see a blue car drive down the road, my theory is shown to be false; all cars are not red. To explain this in a more refined manner, we turn to Hume. Now, “the only basis we have for inductive inference is that the future will resemble the past” (Ladyman, 2002:40). Hume posited, though, that the only possible justification for stating that the future will resemble the past is that of past experience, that is to say, through induction, and induction itself is what is being questioned (Hume, 1740:Part III, Section vi).  Hence, Hume argues, induction is circular – “an infinite regress” as Popper agrees (Popper, 1963:42) – and as such there is no justification for the use of induction in science.  Despite this, Hume himself sees a need for demarcation, which he reveals in the final lines of his An Enquiry Concerning Human Nature, asking of a written volume (Hume, 1748:98)

“Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: for it can contain nothing but sophistry and illusion.”


Popper “found Hume’s refutation of inductive inference clear and conclusive” (Popper, 1963:42), but if we require further assurance of the invalidity of induction as a means of demarcation, we can turn to Wittgenstein’s criterion of meaningfulness. He suggested that every meaningful proposition must be logically reducible to elementary propositions (Wittgenstein, 1922:Section 5). As Popper points out, Wittgenstein’s criterion of meaningfulness “coincides with the inductivists criterion of demarcation, provided we replace their words ‘scientific’ by ‘meaningful’” (Popper, 1959:36). However, we might argue that natural scientific laws cannot be logically reduced to elementary propositions, as scientific laws can never be deduced from a finite number of observations. As such, the lofty ambition of the physicist – the search for natural laws – is rejected as meaningless. In this sense, Wittgenstein and the positivists reject natural science and fail to draw a line between scientific and non-scientific system; their attempt at demarcation fails, or, as Popper puts it, (Popper, 1959:37) “instead of eradicating metaphysics from the empirical sciences, positivism leads to the invasion of metaphysics into the scientific realm.”


Popper squarely states (Popper, 1959:34) that his “main reason for rejecting inductive logic is precisely that it does not provide a suitable ‘criterion of demarcation’.” So it was that, in agreeing with Hume and disagreeing with the established positivists (Popper, 1963:42), Popper accepted his most important challenge: to devise a system of demarcation that could accurately draw a line between scientific and non-scientific theories. Crucially, though, this was not a “problem of meaningfulness or significance, nor a problem of truth or acceptability” (Popper, 1963:39), as metaphysical theories that are sought to be excluded are not necessarily meaningless or nonsensical as the positivist would have us believe - they just cannot be backed by empirical evidence. In what follows, I attempt to provide a short discourse on Popper’s reasoning behind this, his most important work.


If we assume Popper’s stance on induction – namely, that “there is no such thing” (Popper, 1959:40) – then to infer theories from observations is logically impossible. The upshot of this is that it is impossible to completely verify a theory empirically, as to do so would require an infinite number of observations. In order to avoid excluding the natural laws of science through demarcation, Popper’s criterion has to accept statements that cannot be conclusively verified. Popper does, however, state that even though a theory cannot be completely verified, he (Popper, 1959:40) “shall certainly admit a system as empirical or scientific only if it is capable of being tested by experience.” The combination of these ideas – that a scientific theory is never entirely verifiable but must be testable through experience – leads him to suggest that it is the falsifiabilty of a theory which should be considered as a criterion for demarcation. This idea resonates with Popper, as it rules out as scientific those theories that find confirmatory evidence in every observation of the world around us, such as Alderian and Freudian psychology (Popper, 1963:35).


The basic method of demarcation that Popper proposes, then, is that a theory may be deemed to be scientific if it can be falsified.  That is to say that a theory must contain an inherent testability; it must be able to make predictions that can be accessed through experimentation. In this sense, the statement ‘eggs crack when dropped’ is scientific but ‘eggs may or may not crack when dropped’ is not. Popper’s method of demarcation does not require that a theory can ever be shown, once and for all, to be true, but it does require that through empirical testing a theory can be shown to be false. A negative experimental outcome falsifies a theory, which in turn should lead to its rejection as invalid. On the other hand, a positive outcome obviously agrees with the theory. For the positivists, this would have been seen as confirmatory of the theory as correct. In Popper’s eyes, however, “confirmation is […] really just unsuccessful falsification” (Ladyman, 2002:70); a failed attempt at showing a theory is incorrect. However, even though it is not possible to ultimately confirm a theory, those that stand up repeatedly to severe attempts at falsification are corroborated, and as such are the ones that are retained by the scientific community. These are the theories that are, at a given point in time, the closest to the truth that we have. On this, Popper says that (Popper, 1959:317) “we must not look on science as a ‘body of knowledge’, but rather as a system of hypotheses […] with which we work as long as they stand up to tests.” He highlights his view that a good scientist should take a critical attitude to the current system of hypotheses in use. More generally, the concept of falsification suggests that the same scientist should, in the pursuit of falsifying their theories, seek empirical evidence that is both “precise and wide in scope” (Ladyman, 2002:89). In doing so, they open themselves up to having to admit that their theories are incorrect, but find themselves in a position where they can learn from their mistakes. Popper’s criterion promotes a method of ‘best practice’ in science, lacking in previous examples.

Popper believes falsificationism offers a way for scientists to put forward bold conjectures which stand a high risk of being refuted; in fact he goes so far as to say that he would “much prefer an attempt to solve an interesting g problem by a bold conjecture, even (and especially) if it turns out to be false […] because [he] believe[s] this is the way in which we can learn from our mistakes.” It follows from Popper’s criterion, in fact, that some theories will be more falsifiable than others, as some theories claim more than others. Chalmers puts this succinctly when he states that “a good theory will be one that makes very wide-ranging claims about the world, and which is consequently highly falsifiable” (Chalmers, 1999:65). This is to say that a theory that claims more about the world than another is more falsifiable and, as a result, preferable. Take, for example, the theories ‘all eggs breaks when dropped’ and ‘all hen eggs break when dropped’. Clearly, an experiment which demonstrates that a hen egg does not break when dropped falsifies both theories, but an experiment showing that a quail egg does not break when dropped falsifies only the first; the first theory is more falsifiable than the second. Since the ambition of natural scientists is to discover those natural laws which govern the world around us, wider reaching, more falsifiable theories are to be preferred. It follows directly that the best scientists, according to Popperian demarcation, are those that develop bold, highly falsifiable theories. In thinking about theories creatively, radical new avenues of experimentation and observation may be explored. In the best case, bold theories are successful and help explain the world around us; in the worst, scientists learn from their mistakes; regardless, the Popperian notion of what a scientist should be leads to great advances within the field.


All this is not to say that Popper’s criterion of demarcation is without fault. On the contrary, serious criticisms have been leveled at the method of falsificationism. Perhaps chief amongst these is that high-level scientific theories can neither be directly verified nor falsified through experimentation. Duhem states ‘an experiment in physics can never condemn an isolated hypothesis but only a whole theoretical group’ (Duhem, 1906:183). Popper concedes this point, but argues that as long as there exists an intersubjective agreement amongst the scientific community as to how experiments are performed (Popper, 1959:80), then  experimental evidence is “conclusive as far as the practice of science is concerned” (Ladyman, 2002: 80).  Alongside this, there are some aspects of science that seem unfalsifiable, which Popper’s criterion must reject; however, most scientists would be disgruntled at the idea of rejecting theories such as the conservation of energy, or the second law of thermodynamics , both of which turn out to be unfalsifiable. Finally, Popperian philosophy struggles to explain our expectations about the future, due to Popper’s flat refusal to accept as possible the method of induction. What is apparent in most of the arguments leveled at Popper is that it seems impossible to fully explain a scientific method or criterion of demarcation without some element of recourse to induction. Regardless of criticism, Popper’s criterion is still highly regarded by scientists and philosophers alike.


Karl Popper tackled one of the fundamental problems in the philosophy of science, that of demarcation, and devised a criterion that defied conventional views at the time. His suggestion of using falsification, over the accepted confirmation, as a means of demarcation was inspired and on the whole successful; it opened the eyes of the scientific community to a new way of thinking. Popper’s work was undoubtedly hugely important to the progress of science in the 20th century. It drew attention to the fact that a good scientist should be critical and probing of received wisdom; that they should demand exact and wide ranging experimentation; and that they should choose to solve problems with bold and novel conjectures. Above all, a good scientist in the Popperian sense should not be precious about their theories, but attempt at every point to falsify them, such that they can develop better ideas in the future. Despite the fact that Popper drew his share of criticism, his ideas remain crucial to modern day science. Popperian thinking may idealise the scientist as a moral, noble crusader, but science can only benefit from this. His concept of demarcation turned the world of 20th century philosophy on its head, and this is what makes Popper crucial to modern science.




Bacon, F. (1620, tr. 1994) The Novum Organum, Illinois: Open Court

Chalmers, A.F. (1999) What is this thing called Science?, Maidenhead: Open University Press

Duhem, P. (1906, tr. 1962) The Aim and Structure of Physical Theory, New York: Athenum

Hume, D. (1740) A Treatise of Human Nature, New York: Dover

Hume, D. (1748) An Enquiry Concerning Human Understanding, New York: Dover

Klee, R. (1997) Introduction to the Philosophy of Science, Oxford: Oxford University Press

Klee, R. (1999) Scientific Inquiry: Readings in the Philosophy of Science, Oxford: Oxford University Press

Ladyman, J. (2002) Understanding Philosophy of Science, London: Routledge

Popper, K. (1934, tr.1959) The Logic of Scientific Discovery, London: Hutchinson

Popper, K. (1963) Conjectures and Refutations, London: Routledge

Wittginesin, L. (1922, tr. 1961) Tractatus Logico-philosophicus, London: Routledge