Karl Popper

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Karl Popper (28 July 1902 – 17 September 1994) was an Austrian-British philosopher who is generally regarded as one of the most influential philosophers of science of the 20th century. Karl Popper's most important epistemological works were The Logic of Scientific Discovery1, which was originally published in German as Logic der Forschung in 1935, and Conjectures and Refutations2 published in 1963.3 His work had three main objectives:

  • To solve Hume's problem of induction as a limitation on human knowledge
  • To allow theories to be objectively evaluated even before they were put to the test
  • To formulate a critical method for science which proceeded through cycles of conjecture, refutation, and correction of falsified theories.

Popper’s work directly opposed the positivist principle that inductive logic adequately separates empirical sciences from metaphysical and non-scientific knowledge. His critical rationalism stands as a unique variation from a lineage of competing interpretations of the mind’s limitations when attempting to prove the truth of general empirical claims, such as scientific theories. Popper’s approach focused on ascertaining criteria for a demarcation between science and non-science, and on debunking the opinion that science establishes truth beyond doubt. He defined a value for scientific progress beyond both the optimism of describing essences and realities hidden behind appearances, and the pessimism that relegates scientific discovery to instrumental, or heuristic utility.

Historical Context

Pyrrho’s skepticism is commonly associated with philosophical thought on the nature of things in themselves as necessarily separated from our experience of them. This problem of sensations leads to accepting a limitation to empirical knowledge. Such limits influenced the classical philosophy of the Enlightenment as well as contemporary philosophy of science, including Popper’s critical rationalism.

Empiricist theories such as Locke’s tabula rasa (the mind as a blank slate), Berkely’s phenomenalism, and Hume’s criticism of inductive reasoning (i.e. making generalizations from singular instances), correspond in their shared belief that human knowledge originates in the senses. On the other hand, rationalist philosophers such as Descartes, Spinoza, and Leibniz, hoped to escape empirical problems for epistemological ontology by containing the problem of sensations and inductive skepticism within a logically structured, a priori rationalization.

According to Kant, a world that did not abide by universal causation would not allow for the corroboration of Newton’s physics through testing and wide applicability. Popper maintained Kant’s sentiment that human thought arises from mental faculties relating phenomena and not from empirical observation. But Popper disagreed with Kant’s presupposed position on the knowledge of truth, and reproached him for not being sufficiently Socratic — for not having understood well enough that we do not know anything.

Just as deterministic interpretations of classical physics inspired Kant’s ontological epistemology, scientific theories of probabilistic indeterminism during the 20th century were an impetus for a change in philosophical discussion of science. Popper’s opinions on the attainment of truth and the demarcation of science were influenced by Marxism, psychotherapy, quantum physics, and logical positivism. Popper personally identified with Marxism as a youth. During his time in Vienna, Popper was introduced to psycho-analytic theories of Freud and Adler. He was also present for a lecture given by Einstein on his theory of relativity. He later became affiliated with the Vienna Circle and the logical positivist tradition.

Popper came to notice the main difference between Marxism, psychoanalytic theory, and Einstein’s theory of relativity, is that a principle of falsifiability is inherent to Einstein’s theory. In contrast, he found that theories like psychoanalysis and Marxism were insusceptible to refutation or experimental falsification. In his view this made theories like these pseudo-scientific, since tests which could falsify them were inconceivable. Weimar culture and a lineage of philosophers such as Thomas Hobbes, George Berkely, and Wittgenstein, who provided epistemological theories of sense-data, influenced the Vienna Circle’s nominalist and essentialist theories. The logical positivists held metaphysics as meaningless, consisting of nonsensical pseudo-propositions. Popper came to disagree with logical positivist theory and its fundamental claims, such as Rudolph Carnap’s position that probabilities of truth in general synthetic claims could be assessed by calculating the certainty of individual corroborating observations. Popper opposed Carnap’s reductionist position on the verifiability of universal synthetic claims largely on the basis of the Quine-Duhem thesis, which maintains that no individual theory, test, or observation can be separated from the taxonomy of thought which produces it. To Popper, an openness to being refuted by observation, testing, and criticism, was the criteria of demarcation for a claim of scientific value.

Major Contributions

Popper proposed that an openness to falsifiability for general synthetic claims determined their scientific value. Popper therefore made the normative assumption that individual corroborations for a theory should be open to criticism and the general claim always open to refutation regardless of the probability of verisimilitude attributed to the general claim and its corroborating evidence.

Popper on the Limits of Rationality

Popper builds upon the Humean problem of induction, and denies the classical empiricist position on the preeminence of ‘pure’ observation, since observations are particular and theory-laden. While early inductivists attempted to ignore problems concerning the ascertainment of truth from particular data to general theories, Popper’s critical rationalism provides a unique opposition. Popper commended Rudolph Carnap’s verifiability criterion for polishing the logic of anti-metaphysical theory, but maintained criticism against his analyses of confirmation and the demarcation of science from non-science. While Popper upheld the priority of falsifiability, Carnap proposed the calculus of observational truth content and meaning of language.

Popper on the Problem of Demarcation

Although science strives for absolute and objective truth, it must maintain a critical perspective insofar as no theory is justified as the truth with certainty. In the practice of science an infinite regress of testing is not practically tenable, but is normatively implied. The demarcation of science from non-science lies in that scientific theories remain open to critical testing. In his book The Logic of Scientific Discovery, Popper posits that a scientific theory must distinguish between testing in which basic observations can result in either (a) Potential Falsifiers, or (b) Corroboration. Thus, the growth of knowledge proceeds from attempting to solve problems by means of falsification and corroboration. As Popper states, “We can put this more briefly by saying: a theory is falsifiable if the class of its potential falsifiers is not empty”. It is in this form that Popper switches the primacy of empirical verification of induction associated with classical empiricism, rationalism, and logical positivism, to the primacy of empirical falsification.

Popper on Method of Theory Choice and Scientific Progress

Popper proposes that the empirical content of competing theories should first be compared for their degrees of falsifiability. The theory with more daring consequences, due to higher risk of falsification, contains more scientific value. According to Popper, this value is attributed because falsifiable theories have greater chance of resulting in further conjectures due to openness to refutation and therefore revision. If a critical experiment result corroborates predicted expectations, its value is taken as an encouragement to continue with the theory. But corroborations should not be taken as an element of proof for the verifiability of the theory and the unobservable. In case of a falsifying experimental result that stands against the prediction, we decide to consider the theory falsified, but only tentatively. Further consideration of how deeply the refutation effects the universal theory should lead to conjectures which might save the programme, or form a new theory to supersede the previous one.

Günter Wächtershäuser compares Popper’s critical rationalism with the metaphor of a carriage drawn by two horses: “The experimental horse is strong, but blind. The theoretical horse can see, but it cannot pull. Only both together can bring the carriage forward. And behind it leaves a track bearing witness to the incessant struggle of trial and error”. Popper’s theory is therefore received by some as a positive method for scientific thought, a normative guide for the demarcation of sciences from non-sciences, and for progress of the sciences in general.


There were two waves of reception for Popper’s Logic of Scientific Discovery. The first was from the publication of Logik der Forschung in 1934. Much early criticism of Popper’s anti-inductivism came from logical positivist philosophers such as Rudolph Carnap, Otto Neurath, and Hans Reichenbach. Neurath maintained that the principle of induction is unsound, but that Popper did not sufficiently show why a falsified theory should be discarded and not the falsifying evidence. Hans Reichenbach likened dismissing inductivism from scientific method as reducing the process of discovery to ‘divination’. This kind of criticism holds that induction is not a problem for science, but is essential in developing and testing theories.

The return of criticism and interpretation of Popper’s work was due to the English publication of Logic of Scientific Discovery. Many arguments took over the vein of criticism from members of the Vienna Circle: that Popper’s rules for theory choice and scientific rationality represented an ideal of research, which was too distant from the actual practice of science. Wesley C. Salmon, a pupil of Reichenbach’s, Imre Lakatos, and Joseph Agassi, who studied under Popper, all held that Popper’s anti-inductivism and non-justification of theories by corroborations were ambiguous.

The criticism of Popper’s work left room for new philosophers of science to build novel interpretations. For instance, Popper is considered by some to be a transitional figure between logical positivism and the Kuhnian tradition. The critical response to Popper’s normative appraisal of falsifiability criteria has placed Popper’s philosophy of science as the progenitor to the Kuhnian relationship between anomalies and paradigms.

Imre Lakatos attempted to save the core rationality of Popper’s methodology, while taking into account the wealth of criticisms it received. For example, Lakatos introduced the concept of ‘progressive and degenerating research programmes’ to resolve the contradistinction between Popper’s normative critical rationalism of science and Thomas Kuhn’s uncritical socio-psychological appraisal in Structure of Scientific Revolutions.

In the wake of Lakatos’ reaction to the Popper and Kuhn debates, another opposition to Popperian interpretation emerged between the more moderate philosophy of Laudan, and the more radical one of Feyerabend. Feyerabend coined the term “anything goes” in an anarchist interpretation that science does not follow, and never has been governed by, a precise method or even rationally defined precept. Feyerabend’s book Against Method (1975) rejected Popper’s original theory, as well as Lakatos’ corrections, with his anarchist opinions on humanity and science. In face of this radical anti-rationalism Larry Laudan reformulated Lakatos’ work on “research programmes” and Khun’s theories of paradigms and normal science with his alternative “research traditions”.

The panorama of influence from Popper’s work is expansive and includes epistemologists such as Agassi, Bartley, Watkins, Musgrave, Albert and Stegmuller. Scientists such as Bondi, Eccles, Medawar and the anthropologist Ian Jarvie, have all paid respect to Popper. Among his critics, Miller, Tichy, Ackermann, and Johannson consider themselves largely in agreement with Popper’s work, especially in contrast to the relativism and irrationalism of Feyerabend and Kuhn.

Popper’s critical rationalism can be seen as a watershed of philosophical thought on the limits of empiricism, rationalism, and positivism, and an opening of the flood-gates for questioning and contemplation within nearly every branch of philosophy.


Here are the works of Popper included in the bibliographic records of this encyclopedia:

  • Popper (2002b): Popper, Karl. (2002) Conjectures and Refutations: The Growth of Scientific Knowledge. Routledge.
  • Popper (2002a): Popper, Karl. (2002) The Logic of Scientific Discovery. Routledge.
  • Popper (1974): Popper, Karl. (1974) The Autobiography of Karl Popper. In Schilpp (Ed.) (1974), 3-181.
  • Popper (1972): Popper, Karl. (1972) Objective Knowledge. Oxford University Press.
  • Popper (1963): Popper, Karl. (1963) Conjectures and Refutations. Routledge.
  • Popper (1959): Popper, Karl. (1959) The Logic of Scientific Discovery. Hutchinson & Co.
  • Popper (1947): Popper, Karl. (1947) New Foundations for Logic. Mind 56 (223), 193-235.
  • Popper (1947b): Popper, Karl. (1947) Logic Without Assumptions. Proceedings of the Aristotelian Society 47, 251-292.
  • Popper (1946): Popper, Karl. (1946) Symposium: Why Are the Calculuses of Logic and Arithmetic Applicable to Reality? Proceedings of the Aristotelian Society, Supplementary Volumes 20, 20-60.
  • Popper (1945b): Popper, Karl. (1945) The Open Society and Its Enemies, Volume I: The Spell of Plato. George Routledge & Sons, Ltd..
  • Popper (1945c): Popper, Karl. (1945) Open Society and Its Enemies, Volume 2: The High Tide of Prophecy: Hegel, Marx, and the Aftermath. George Routledge & Sons, Ltd..
  • Popper (1944a): Popper, Karl. (1944) The Poverty of Historicism, I. Economica 11 (42), 86-103.
  • Popper (1944b): Popper, Karl. (1944) The Poverty of Historicism, II: A Criticism of Historicist Methods. Economica 11 (43), 119-137.
  • Popper (1945a): Popper, Karl. (1944) The Poverty of Historicism, III. Economica 12 (46), 69-89.
  • Popper (1940): Popper, Karl. (1940) What is Dialectic? Mind 49 (196), 403-426.

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Related Topics

Mechanism of Theory Acceptance
Demarcation Criteria


  1. ^  Popper, Karl. (1959) The Logic of Scientific Discovery. Hutchinson & Co.
  2. ^  Popper, Karl. (1963) Conjectures and Refutations. Routledge.
  3. ^  Thornton, Stephen. (2016) Karl Popper. In Zalta (Ed.) (2016). Retrieved from https://plato.stanford.edu/archives/win2016/entries/popper/.