Popper, Karl Raimund

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(b. Vienna, Austria, 28 July 1902; d. London, United Kingdom, 17 September 1994), philosophy, probability, logic, quantum theory, social theory, Darwinism.

One of the twentieth century’s leading philosophers of science, Popper is renowned for his contention that scientific theories are characterized by their falsifiability by evidence and for his rejection of inductivism in favor of an account of scientific progress through conjecture and refutation. In political philosophy Popper is remembered for his critique of historicism, which he saw present in the ideologies of both fascism and Soviet communism. Of wide-ranging intellectual interests, he also contributed to the axiomatization and interpretation of probability, the interpretation of quantum mechanics, the philosophy of logic, and the interpretation and application of Darwinian ideas.

Popper was born into a well-to-do Viennese family of Jewish descent, Lutheran practice, liberal sympathies, and cultured sensibilities. He left school at sixteen amid troubled times with the collapse of the Austro-Hungarian Empire. He worked as a manual laborer, was active in left-wing politics, undertook social work with deprived children, and worked with the psychoanalyst Alfred Adler. Initially as a nonmatriculated student, he took classes in history, literature, psychology, and philosophy, but mainly in mathematics and physics, at the University of Vienna. During this period he was apprenticed to a cabinetmaker, finishing that apprenticeship in 1924, not long after he had qualified as a primary school teacher. Always interested in music, Popper took piano lessons and was for a while a member of Schoenberg’s Verein für musikalische Privataufführungen, his sympathies lying fundamentally with an earlier period; on the strength of a fugue for organ he had composed, he was admitted to the Department of Church Music of the Vienna Konservatorium, but left, having decided he did not have the talent to be a musician.

In 1925 Popper enrolled in the newly founded Pedagogic Institute in Vienna, an institution, loosely connected to the university, whose purpose was to support education reforms then underway in Viennese schools. Popper undertook research in the psychology of thinking (Denkpsychologie), including some experimental work, submitting a dissertation on this work in 1927. In the following year he submitted a doctoral thesis, of a more methodological orientation, to the University of Vienna. In 1929 he qualified as a teacher of science in secondary schools with a third thesis on geometry. At the institute he met his future wife, Josefine Anna Henninger (known as “Hennie”). They married in 1930, the year Popper obtained his first teaching appointment.

In the period of his doctoral research Popper was influenced by the psychologist Karl Bühler, his supervisor, and the theories of the Würzburg school that Bühler represented, but he grew increasingly convinced that, rather than the psychology of thinking and intellectual discovery, the topic that concerned him was a logical issue— objective methodology, not the study of subjective thinking processes. By his own account, as far back as 1919 he had been impressed by Albert Einstein’s stated readiness to give up the theory of relativity should it fail certain experimental or observational tests. This Popper saw as in marked contrast to the psychoanalysts and vulgar Marxists who held it to be a virtue of their theories that there could be no conceivable counterevidence.

These issues came together in a long, and long unpublished, work, Die beiden Grundprobleme der Erkenntnistheorie (The two basic problems of the theory of knowledge). It was written at the instigation of Herbert Feigl, a member of the Vienna Circle of logical positivist philosophers, a group with whom Popper had begun to have contacts in 1929. The two basic problems dealt with the problem of induction and the problem of the demarcation of science. Popper proposed a single answer to both: scientific hypotheses are neither generated nor confirmed by any inductive process, rather they are tested with a view to falsifying them. It is the falsifiability of their hypotheses that distinguishes the sciences, and, contrary to David Hume, induction is not a logically indefensible

but psychologically necessary process. Rather, inductive reasoning has no part to play in learning from experience: What one learns is whether one’s currently entertained hypothesis has survived a test. Falsifiability is the cornerstone of Popper’s account of the methodology of the sciences and the growth of human knowledge. With that in place, a great deal remains to be elaborated, a task Popper took up in Die beiden Grundprobleme and in many subsequent publications.

In typescript, Die beiden Grundprobleme was much discussed by the members of the Vienna Circle. Rudolf Carnap in particular championed some of Popper’s views, although by and large the positivists failed to realize how strongly Popper was opposed to their central contentions such as their verifiability theory of meaning according to which metaphysics is meaningless. Popper was encouraged to publish a shorter work in part extracted from the typescript. This was Logik der Forschung, published in 1934. In addition to problems of induction and demarcation, it included discussions of quantum mechanics and of probability.

Popper proposed a statistical interpretation of quantum mechanics, arguing at length against Werner Heisenberg’s account of the uncertainty principle. He advanced a frequentist account of probability. The novelty of his version was his development of a testable notion of randomness, a topic on which he lectured to Karl Menger’s Mathematical Colloquium, prompting Abraham Wald to his work on the consistency of Richard von Mises’s notion of a collective.

As pointed out in one of the closing sections of the book, Logik der Forschung does not need the notions of truth and falsity. “Falsification” of a hypothesis occurs when consequences of the hypothesis are inconsistent with conventionally accepted basic statements (reports of experimental findings agreed by the scientific community). At the time of writing, philosophers were, by and large, suspicious of the notion of truth. In 1935 the Polish logician Alfred Tarski explained to Popper the essentials of his semantic conception of truth. In Popper’s view, Tarski’s work rehabilitated the correspondence theory of truth; subsequently Popper did not hesitate to use the notions truth and falsity and promoted Tarski’s ideas.

Popper remained a schoolteacher through the early and mid-1930s. As his star rose in philosophical circles, he took time off to travel to Britain, where he gave lectures and met, among others, Bertrand Russell, Erwin Schrödinger, and Friedrich von Hayek, and to Copenhagen, where he had discussions with Niels Bohr. With no prospect of a university position in Austria and the political situation worsening, Popper applied for and was appointed to a lectureship at Canterbury University College, Christchurch, a component college of the federal University of New Zealand. The Poppers set sail in early 1937.

In New Zealand Popper had to shoulder a heavy burden of university teaching. Additionally, he lectured for the WEA (Workers Educational Association) and to other groups. His teaching was popular and he came to stand out in quiet Christchurch. There being no other philosopher in the college, Popper’s closest academic acquaintances were mostly from the sciences.

Popper’s research in New Zealand initially focused on the axiomatization of the probability calculus, a topic to which he would return throughout the rest of his life. He was to propose axiomatizations that take conditional probability as fundamental, not as a derived notion as in the familiar Kolmogorov axiomatization. A remarkable feature of Popper’s axiomatization is that it does not presuppose any particular logico-algebraic structure on the domain of the probability function. Rather, one recovers a Boolean algebra from structure induced by a relation of probabilistic equivalence imposed by the probability function. (See The Logic of Scientific Discovery, Appendix *v.) Later, in work published in the years immediately after World War II, he turned to formal logic, developing his own natural deduction calculus, investigating classical and intuitionist logics in that framework, and exploring its philosophical implications.

At the same time, Popper’s work turned in quite another direction, toward what he would describe as his “war effort”: The Poverty of Historicism and the two-volume The Open Society and Its Enemies (whose bogeymen are Plato, G. W. F. Hegel, and Karl Marx). Popper labeled “historicist” any theory that discerns inexorable laws of historical development and took fascism and Marxism as exemplars: He believed historicist doctrines lead to totalitarianism. The fallibilism inherent in his general stand on knowledge encouraged him to reject any claim to knowledge of any such laws, and a commitment to indeterminism encouraged him to believe that there are no such laws. Popper saw the aim of social policy to be the reduction of suffering (negative utilitarianism) and thought this is best achieved through piecemeal social engineering in an “open society” that permits free and critical discussion. With its emphasis on criticism, Popper’s political philosophy was of a piece with his account of science and the demarcation criterion.

With Hayek’s assistance, “The Poverty of Historicism” was published as an article in three parts in Economica in 1944 and 1945. The Open Society was published in Britain in 1945 to some considerable success. Hayek had been instrumental in securing for Popper an invitation to take up a readership at the London School of Economics (LSE). He arrived in January 1946. In 1949 he was promoted to a professorship. Again his teaching proved popular. But over time he withdrew from active involvement with the school; he and Hennie moved from London to a rural retreat in Penn, Buckinghamshire, and Popper reduced to a minimum the occasions on which he had to be present at the LSE.

These years, the 1950s, were the years in which Popper’s reputation was to grow, albeit that he was at odds in doctrine and method with the Wittgensteinians and ordinary language philosophers in Oxford and Cambridge. He began to draw to him students who would identify themselves as Popperians. In 1959 The Logic of Scientific Discovery, a much expanded English translation of Logik der Forschung, was published. It was intended to publish alongside a volume referred to as the Postscript after Twenty Years that would detail the results of further work on the topics of Logik der Forschung. Edited by William W. Bartley III, the Postscript finally emerged in three volumes in 1982 and 1983.

Popper had turned back to problems in the philosophy of physics and to probability. He campaigned against (the early Heisenberg’s version of) the Copenhagen interpretation of quantum mechanics, developing the propensity interpretation of probability, an objective, dispositional understanding of probabilities that pertain to single instances. Popper maintained that all the mysteries of quantum mechanics stem from muddles about probability, a claim which, it has to be said, has gained few adherents and one that Popper himself was to weaken in the light of the Aspect experiments testing Bell’s Inequality.

Many of the lectures and essays that were collected in Conjectures and Refutations (1963) were first delivered or written at this time. While largely concerned with matters methodological, the topics vary widely from the pre-Socratics to political philosophy. One methodological topic that began to feature more prominently was verisimilitude, the truthlikeness of false theories. Scientific knowledge grows, according to Popper, by the severe, critical testing of boldly conjectured hypotheses which are discarded when refuted in experiment. To uphold a conception of scientific progress, one must be able to make sense of one false theory being closer to the truth than another (even if one cannot know when this is the case). To this end Popper gave formal definitions of verisimilitude. Unhappily, in the 1970s these definitions were shown to fail: No false theory could, by their lights, be any nearer the truth than another—and there was no quick fix for the definitions, as Popper acknowledged. Before then, verisimilitude had been one of the elements in Popper’s most public bust-up with a former colleague.

Imre Lakatos, who fled Hungary in 1956, had come to the LSE from Cambridge, where, in his Proofs and Refutations, he had applied Popperian ideas to mathematics. He was welcomed as a leading light among the Popperians but as he became more engaged in the philosophy of science tensions emerged. These came to a head in Lakatos’s contribution to the volume in the Library of Living Philosophers series dedicated to Popper, The Philosophy of Karl Popper. A theory that has survived severe attempts at refutation is said, in Popperian jargon, to be well corroborated. What link, if any, is there between corroboration and verisimilitude? Any claim to a strong link smacks of an inductive inference, yet exactly this is what Lakatos urged upon Popper. (Popper himself said that the degree of corroboration of a theory at a time is an indicator of its verisimilitude as it appears at that time and this, if it says anything significant, has struck many as dangerously close to an inductive principle.) Lakatos was an outcast from the Popperian fold; no reconciliation had occurred by the time of Lakatos’s death in 1974. The issue at stake between them here sometimes arises in the guise of the viability of a strictly noninductivist, falsificationist account of the technological application of scientific theories.

The 1960s saw Popper involved in a number of controversies. Firstly there was the so-called Positivismusstreit in German sociology in which Popper engaged with Theodor Adorno and Jürgen Habermas (among others) over the nature of social-scientific theorizing. Then there were disputes with Carnap over the form and function of inductive logic and with Thomas Kuhn over the nature of scientific revolutions.

Another strand to emerge in Popper’s thinking during the 1960s concerned Darwinism. His views on it changed, so that, for example, in Unended Quest, his intellectual biography from the 1970s, he is at pains to argue that Darwinism is metaphysical, because unfalsifiable— but metaphysical ideas can be important for the development of a science, and they can be subject to criticism. He recanted this view in 1977’s First Darwin Lecture, “Natural Selection and the Emergence of Mind,” given at Darwin College, Cambridge. Popper developed an evolutionary epistemology: He offered a somewhat speculative, gene-based account of problem-solving activity, and even spoke of the universe as a whole as being creative. It is the role of trial-and-error behavior that connects his evolutionary epistemology with his falsificationist methodology and political philosophy.

Popper retired from the LSE in 1969. Throughout his retirement he continued to publish across the wide range of topics already mentioned. He received many prizes and honors. He was elected a fellow of both the British Academy (1958) and the Royal Society (1976); knighted in 1965, he was appointed a Companion of Honour in 1982. Popper lived long enough to know of the fall of the Berlin Wall and to have the significance of The Open Society for those struggling against oppression in Eastern Europe acknowledged time and again.


Popper’s papers, correspondence, and other materials are archived at the Hoover Institution, Stanford University. His library is housed in the Karl-Popper-Sammlung at the Alpen-Adria-Universität Klagenfurt, Austria. The majority of Popper’s books in English have gone through a number of editions and impressions, often with significant changes. Mohr-Siebeck began publishing Popper’s collected works (in German) in 2001. A comprehensive list of works to 1974, compiled by Troels Eggers Hansen, is to be found in The Philosophy of Karl Popper, edited by Paul A. Schilpp. Manfred Lube has published a bibliography of Popper’s writings and writings on Popper to 2004.


Logik der Forschung. Vienna: Julius Springer, 1934 [dated 1935].

“The Poverty of Historicism.” Economica 11 (1944): 86–103, 119–137; 12 (1945): 69–89. In book form: The Poverty of Historicism. London: Routledge and Kegan Paul, 1957.

The Open Society and Its Enemies. London: George Routledge & Sons, 1945.

“New Foundations for Logic.” Mind 56 (1947): 193–235.

“The Propensity Interpretation of the Calculus of Probability and the Quantum Theory.” In Observation and Interpretation, edited by Stephan Körner. London: Butterworth, 1957.

The Logic of Scientific Discovery. London: Hutchinson, 1959. Conjectures and Refutations. London: Routledge and Kegan Paul, 1963.

Objective Knowledge: An Evolutionary Approach. Oxford: Clarendon Press, 1972.

Unended Quest. Glasgow: Collins/Fontana, 1976.

“Natural Selection and the Emergence of Mind.” Dialectica 32 (1978): 339–355.

Die beiden Grundprobleme der Erkenntnistheorie. Tübingen, Germany: J. C. B. Mohr (Paul Siebeck), 1979.

The Open Universe: An Argument for Indeterminism: From the Postscript to The Logic of Scientific Discovery. Edited by W. W. Bartley III. London: Hutchinson, 1982.

Quantum Theory and the Schism in Physics: From the Postscript to The Logic of Scientific Discovery. Edited by W. W. Bartley III. London: Hutchinson, 1982.

Realism and the Aim of Science: From the Postscript to The Logic of Scientific Discovery. Edited by W. W. Bartley III. London: Hutchinson, 1983.

A World of Propensities. Bristol, U.K.: Thoemmes, 1990.


Hacohen, Malachi Haim. Karl Popper: The Formative Years, 1902–1945. Cambridge, U.K.: Cambridge University Press, 2000.

Hark, Michel Ter. Popper, Otto Selz, and the Rise of Evolutionary Epistemology. Cambridge, U.K.: Cambridge University Press, 2004.

Jarvie, Ian, Karl Milford, and David Miller, eds. Karl Popper: A Centenary Assessment. Vol. I, Life and Times, and Values in a World of Facts. Aldershot, U.K.: Ashgate, 2007.

———, Karl Milford, and David Miller, eds. Karl Popper: A Centenary Assessment. Vol. II, Metaphysics and Epistemology. Aldershot, U.K.: Ashgate, 2007.

———, Karl Milford, and David Miller, eds. Karl Popper: A Centenary Assessment. Vol. III, Science. Aldershot, U.K.: Ashgate, 2007.

Keuth, Herbert. The Philosophy of Karl Popper. Cambridge, U.K.: Cambridge University Press, 2005.

Lakatos, Imre. “Proofs and Refutations.” British Journal for the Philosophy of Science 14 (1963–1964): 1–25, 120–139, 221–245, 296–342. In book form: Proofs and Refutations: The Logic of Mathematical Discovery, edited by John Worrall and Elie G. Zahar. Cambridge, U.K.: Cambridge University Press, 1976.

Lube, Manfred. Karl R. Popper: Bibliographie 1925–2004: Wissenschaftstheorie, Sozialphilosophie, Logik, ahrscheinlichkeitstheorie, Naturwissenschaften (Schriftenreihe der Karl Popper Foundation Klagenfurt, vol. 3). Frankfurt am Main, Germany, and New York: Peter Lang, 2005.

Miller, David. Critical Rationalism: A Restatement and Defence, Chicago and La Salle, IL: Open Court, 1994.

———. “Sir Karl Raimund Popper, CH, FBA.” Biographical Memoirs of Fellows of the Royal Society 43 (1997): 367–409. In book form as Chapter 1 of Miller, Out of Error. London: Ashgate, 2006.

O’Hear, Anthony, ed. Karl Popper: Philosophy and Problems. Cambridge, U.K.: Cambridge University Press, 1995.

Qureshi, Tabish. “Understanding Popper’s Experiment.” American Journal of Physics 73 (2005): 541–544.

Schilpp, Paul A., ed. The Philosophy of Karl Popper. La Salle, IL: Open Court, 1974. Two volumes with intellectual biography and replies by Popper.

Schroeder-Heister, Peter. “Popper’s Theory of Deductive Inference and the Concept of a Logical Constant.” History and Philosophy of Logic 5 (1984): 79–110.

Shih, Yanhua, and Yoon-Ho Kim. “Experimental Realization of Popper’s Experiment—Violation of the Uncertainty Principle?” Fortschritte der Physik 48 (2000): 463–471.

Watkins, John. “Karl Raimund Popper, 1902–1994.” Proceedings of the British Academy 94 (1997): 645–684.

Wettersten, John. “New Insights on Young Popper.” Journal of the History of Ideas 66 (2005): 603–631.

Peter Milne