Karl Popper

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Popper, Karl 1902-1994


Sir Karl Raimund Popper was a leading twentieth-century philosopher. His first major work, Logik der Forschung (The Logic of Scientific Discovery, 1935), was a methodology of the physical sciences that dispensed with induction. His second major work, in two volumes, The Open Society and Its Enemies (1945), was a democratic manifesto that burst out of broad yet incisive discussions of the philosophy of history, society, and politics. Slightly less influential was his slim, sober Poverty of Historicism (1957).

David Humes (17111776) critique of induction that universal statements of science never follow deductively from particular statements describing experience inadvertently undermined the rationality of science. His own response to his critique was to base inductive inferences on habit rather than on rationality. Immanuel Kants (17241804) response was to base inductive inferences on the principle of induction (or of simplicity, as he called it), whose status is that of extralogical truth known with no recourse to experience: synthetic a priori knowledge. Most philosophers find Humes response a retreat to irrationalism and Kants response a retreat to dogmatism. They sought a different way around Humes critique. Thus, the problem of induction, the search for a satisfactory response to Humes critique, became a central concern of rationalist philosophy. Popper reworded the problem: how is theoretical learning from experience possible? To this, he had a new solution: learning from experience is deductive; it advances by the refutation of bold conjectures; and the bolder the better. The view of learning from experience by refutations is impervious to Humes critique: a statement of experience conflicts with a theory. Thus, contrary to Kant, Popper viewed science as no knowledge, much less as a priori knowledge. Science then is the search for explanatory conjectures and for ways to test them. He thus shifted traditional epistemology from the positive to the negative, to via negativa : groping in the dark and learning from error. He likewise turned traditional positive epistemology into Socratic negative epistemology: we know that we do not know, we know the limitations of some theories. Our pursuit of knowledge engenders our best and most interesting errors.

Poppers view aimed to account for the progress of science, not for its alleged reliability. On this he made three very important comments. First, absolute reliability is impossible and conditional reliability is question-begging. Second, most applied theories and the most frequently applied onesGalileos (15641642) and Isaac Newtons (16421727)are refuted. Third, current efforts to answer Hume must be hopeless and useless. In particular, reliability is not probability; to the extent that it is possible it is the elimination of some dangerous applications of science achieved by severe tests. Following probability is caution, whereas scientific thinking is bold and so are attempts to apply it. Yet because theories are hypotheses that invite critical discussion, a testable theory of induction may be welcome, but most proposed solutions of the problem of induction are untenstable and so they are pseudoscientific.

Poppers contribution to social and political philosophy accords with his negative philosophy of science. It is more significant, both politically and intellectually. He argued that any regime that safeguards the means for peaceful corrections of government errors deserves respect as a democracy. Politics invites criticism aimed at improvements, he observed, and seeking improvements is superior tomore fruitful thanthe traditional theory of the sovereign that is a futile search for the best regime. This is the central message of Poppers philosophy, both theoretical and practical, regarding science, government, and anything else worthwhile: improving is preferable to legitimating. He took particular aim at intellectuals, arguing that they had moral responsibilities commensurate with their privileges and that they had a long history of falling short. As examples, he expounded the ideas of two of his greatest intellectual heroes, Plato (427347 BCE) and Karl Marx (18181883). He claimed that their devotees glossed over the defects of their philosophies, especially their illiberalism. His doctrine that great men make great mistakes from which we should learn was more challenging than his negative epistemology.

Popper deemed unscientific the doctrines of historical inevitability or historical destiny (historicism in his jargon) of Plato and of Marx. He said that no doctrine of this kind can be worded in a manner clear enough to be put to the test of experience, because if such a theory were to cohere with known facts, it must be vague. (Irrefutable versions of historicism are easy to invent: the simplest is the purely existential historical destiny exists.) Ingeniously, Popper found a way to refute all reasonable versions of historicism, despite its inherent vagueness. The argument deploys two intuitive premises: future science is in principle unpredictable, and its impact on society is tremendous. Hence, no large-scale theory of the future development of society can possibly yield significant or interesting predictions.

Poppers mode of thinking leads to his greatest and most significant idea. It is his replacement of the theory of rationality that characterizes Western philosophy. Most people take for granted the idea that the best culture is their own. The first to reject this idea as too complacent were the leading ancient Greek philosophers. They deemed problematic all cultures, and what cultures espouse as true they declared to be nonbinding, conventional truths. They deemed binding only universal, absolute truthstruths by nature. Proof is required to show that an assertion is true by nature.

Popper challenged this doctrine: applying the Socratic maxims to science, he declared its rationality to rest neither on proof nor on surrogate proof but on willingness to engage in critical debate. This willingness creates a balance between tradition and science, between conservative and radical politics, between the given and the hoped for: it is a plea for reformist democracy, a view of scientific progress as an approximation to the truth and to freedom and justice, a view that applies to all walks of life, a remarkable and exciting move towards a synthesis and a challenge to push it forward. As the root of rationality is willingness to debate, reform is secondary to the replacement of both traditionalism and radicalism with the advocacy of individual autonomy.

SEE ALSO Philosophy of Science


Agassi, Joseph. 1988. Sir Karl Popper in Retrospect: The Positive Power of Negative Thinking. In The Gentle Art of Philosophical Polemics, ed. Joseph Agassi, 479501. LaSalle IL: Open Court.

Bunge, Mario, ed. 1964. The Critical Approach to Science and Philosophy. London and New York: Free Press.

Kekes, John. 1977. Popper in Perspective. Metaphilosophy 8:3661.

Levinson, Paul, ed. 1982. In Pursuit of Truth: Essays in Honour of Karl Popper on the Occasion of his 80th Birthday. Atlantic Highlands, NJ: Humanities Press.

Magee, Bryan. 1985. Philosophy and the Real World: An Introduction to Karl Popper. Chicago: Open Court.

Magee, Bryan. 1985. Popper. 3rd ed. London: Fontana.

Miller, David, ed. 1985. Popper Selections. Princeton, NJ: Princeton University Press.

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

Popper, Karl. 1945. The Open Society and Its Enemies. 2 vols. London: Routledge and Kegan Paul.

Popper, Karl. 1957. The Poverty of Historicism. London: Routledge and Kegan Paul.

Popper, Karl. [1935] 1959. The Logic of Scientific Discovery. London: Hutchinson. (Originally published as Lokig der Forschung.)

Popper, Karl. 1963. Conjectures and Refutations: The Growth of Scientific Knowledge. London: Routledge and Kegan Paul.

Schilpp, Paul Arthur, ed. 1974. The Philosophy of Karl Popper. 2 vols. La Salle, IL: Open Court.

Joseph Agassi

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Karl Raimund Popper (1902–1994) was a philosopher of science and politics best known for advancing falsifiability as the criterion for distinguishing science from non-science and for a defense of what he termed the open society. Born in Vienna on July 28, Popper received his Ph.D. in philosophy from the University of Vienna in 1928. After teaching secondary school from 1930 to 1936, he fled the rise of Nazism and the impending Anschluss by emigrating to New Zealand, where he lectured in philosophy at Canterbury University College. In 1946 he moved to England, and three years later became professor at the London School of Economics, which he developed into a leading center for philosophy of science. He was knighted by Queen Elizabeth II in 1965 and elected fellow of the Royal Society in 1976. Popper remained active as a writer and lecturer until his death in Croydon, Surrey, on September 17.

Philosophy of Science

Popper's philosophy of science emerged in the context of Vienna Circle logical positivism, which held that scientific and therefore all meaningful statements are of two kinds, with their truth or falsity accordingly verifiable in one of two ways. Analytic statements (for example, Triangles are three-sided plane figures) are true or false simply on the basis of their conceptual and logical structure; synthetic (empirical) statements (such as The tree is green) are verifiable insofar as they can be tested by positive sense experience. Any statement that did not fit into one of these categories could not be counted as part of science and was considered cognitively meaningless.

Like the logical positivists, Popper was interested in distinguishing science from nonscience, but rejected its verification theory of meaning. Like others, he wanted to assess the theories of physics, Marxism, and psychoanalysis scientifically, but recognized that for abstract or general synthetic statements in physics (for example, The electron has a negative charge or F=ma) as much as in Marixism or psychoanalysis, it was often difficult to specify their direct derivation from sense experience. But upon hearing a lecture on the theory of relativity by Albert Einstein, Popper, then 17 years old, recognized a unique epistemic feature of Einstein's work, namely, that his theory clearly made some unexpected predictions that, if not observed, would falsify it. This contrasted with the theories of Karl Marx and Sigmund Freud, which, despite many positive confirmations, were not subject to any straightforward falsification.

Thus in his first book, Logik der Forschung (The logic of scientific discovery) (1934), Popper argued that no number of positive confirmations at the level of empirical observation could establish a theory as true or probably true, although a single genuine counterinstance could refute a theory. This asymmetry between verification and falsification—one that could never be definitive, the other that could—provided the basis for a clear demarcation between science and nonscience, and became central to Popper's philosophical analysis of scientific rationality. While recognizing the meaningfulness of nonscientific statements in ethics and metaphysics in ways that logical empiricists refused to do, Popper nevertheless emphatically rejected Marxist and psychoanalytic theories as pseudoscience because he found them nonfalsifiable.

Without verification through confirmation, however, it was difficult to explain how scientific knowledge can accumulate or grow. But, for Popper, a "theory is comprehensible and reasonable only in its relation to a given problem-situation" (Popper 1963, p. 139). His proposed metric of scientific progress was that "the best tentative theories (and all theories are tentative) are those which give rise to the deepest and most unexpected problems" (Popper 1972, p. 286). Thus a rationally acceptable theory is one that can withstand criticisms as a proposed answer to questions posed by a problem-situation shared by members of a scientific discipline.

In short, Popper's response to issues regarding the growth of knowledge was this: Because a theory may be false, the appropriate rational response is to look for its weaknesses in order to get rid of them. Science progresses by the conjecture of bold (more general and falsifiable) theories proposed as solutions to the problems identified in prior theories. This analysis was a major influence on subsequent work in the philosophy of science, especially the turn toward philosophical analyses of the history of science by Thomas Kuhn and others.

Closed and Open Societies

Popper's problem-solving model led him to develop an evolutionary epistemology that accepted true theories and useful technologies as dual aims of science, while denying that either truth or utility can ever be determined definitively. In his effort to lay out the framework in which this evolutionary problem solving takes place, Popper developed a three-world ontology. World 1 is constituted by physical objects, world 2 by subjective experience, and world 3 by objective experience that presents in science, art, ethics, and politics. Popper argued that the world of science, which bears on world 1, evolves in ways analogous to organic evolution.

Popper further contrasted the growth of theory, which tended toward unifying explanations, and technology, which advanced through increased differentiation and specialization. This distinction enabled Popper to extend his critical thinking on theory and praxis to technics, and to balance the judgment that "the critique of technology ... is urgently necessary," often from the outside, with the insight that it would be dogmatic and irresponsible "to attack science and technology as a whole, when they alone permit the necessary corrections to be made" (Popper 1999, p. 101).

This ability to criticize science and its applications is, for Popper, the central feature of an open society where knowledge is freely available to all. Liberal democracy protects the identity and agency of individuals and allows for the peaceful removal of leaders. It is founded on critical rationalism, in that individuals are free to critique systems of thought and work incrementally through democratic processes toward better conditions.

"This is why rationalism is closely linked to the political demand for practical social engineering—piecemeal engineering, of course—in the humanitarian sense, to the demand for the rationalization of society, to planning for freedom, and to the control of freedom by reason. Such societal goals are not governed by science, or by a Platonic, pseudorational authority, but by Socratic reason that is aware of its limitations, and that therefore respects others and does not aspire to coerce anyone—not even into happiness." (Popper 1962, vol. 2, p. 238).

Popper believed that society is no more or less than the aggregate of individuals, and that history is indeterminate because it is driven by the consequences of individual choices rather than intrinsic laws. Thus the link between Popper's philosophy of science and social philosophy is fallibalism. Just as scientific progress is made by subjecting theories to critical scrutiny, so too the open society can be sustained only if individuals are free to critically evaluate government decisions and technological change and to modify each in light of such evaluation. Just as in scientific communities, differences in the open society should be resolved by critical discussion rather than force.

By championing the open society, Popper was primarily refuting the dangerous presuppositions at the heart of closed (totalitarian or authoritarian) societies rather than defending a libertarian ideology. As he argued in both The Open Society and its Enemies (1945) and The Poverty of Historicism (1961), the closed society is predicated on the related postulates of holism and historicism. Holism is the belief that societies are greater than the sum of their members and that society inexorably influences individuals to shape the course of history. Historicism, in Popper's usage, is the belief that history develops according to certain intrinsic principles toward a determinate end. The most significant implication of historicism is that a scientific method can be used to study history and formulate theories to predict social and political developments.

Popper believed historicism to be theoretically erroneous and socially dangerous. History, he contended, is unavoidably indeterminate and not amenable to predictive theories that can lead to falsifiable claims. Yet the view of history as the unfolding of an internal and knowable logic inevitably leads to totalitarian, centralized regimes. These governments feel justified in carrying out massive social engineering programs in order to fulfill a logic of history. Popper's position is that science must be demarcated from nonscience not only to guarantee the growth of knowledge, but also to guard against a tyrannical regime and the authority it could derive from an erroneous interpretation of history as scientific. For Popper, "The fact that we predict eclipses does not, therefore, provide a valid reason for expecting that we can predict revolutions" (Popper 1963, p. 340). Popper's political philosophy shows that the theoretical task of demarcating and limiting the sphere of science and its influence on human affairs is just as ethically important as the physical and political restraint of dangerous technologies.

Popper also derides the absurdity of a "scientific ethics" that would construct "a code of norms upon a scientific basis, so that we need only look up the index of the code if we are faced with a difficult moral decision" (Popper 1962, p. 237). Setting up scientific criteria of ethics relieves human beings of responsibility and therefore all ethical concerns. Thus scientific ethics (which includes ethical naturalism and its attempt to define human nature or the good) is actually an escape from the urgent problems of the moral life. The escape from personal responsibility is compounded and made more dangerous by the tendancy of tyrants to utilize some concept of scientific ethics (i.e., a knowable, natural law) to develop sociological laws and enforce programs of social engineering based on them. For Popper, then, it is crucial for the open society that moral laws remain distinct from natural laws. Only in this way will human choice, freedom, and rationality be entitled to enter the political realm.

Assessment and Extension

Popper's work has been a major stimulus for ongoing discussions regarding the philosophy of science and political philosophy. Popper's students Imre Lakatos (1922–1974) and Paul Feyerabend (1924–1994) became leading philosophers of science. The former defended Popper's critical and cumulative rationalism against the challenges of Kuhn's historically discontinuous paradigms by interpreting paradigms as research programs. The latter repudiated Popper's critical rationalism in the name of an epistemological anarchism that, he argued, was an extension of Popper's own creative openness. In political philosophy, Popper's historical interpretations of Plato, Hegel, and Marx have been hotly contested, but his overall influence has been salutory in its promotion of democracy and the critical assessment of technology.

One interpreter, Paul Levinson, has sought to bridge Popper's philosophy of science and political philosophy by means of the philosophy of technology. For Levinson, Popper's world 3 is too limited. In Levinson's technomaterialist reformulation of Popper's three-world ontology, the human mind (world 2) acting in and on the material world (world 1) forges technology (world 3). Technology thus "enjoys a unique ontological status commensurate with its unique role in the universe: with the execption of humans themselves, nothing is as special ... or as different from all other things" (Levinson 1988, p. 80). The practical criticism and revision of technology is for Levinson a material parallel to critical rationalism in science.


SEE ALSO Incrementalism; Liberalism; Social Engineering.


Levinson, Paul. (1988). Mind at Large: Knowing in the Technological Age. Greenwich, CT: JAI Press.

Popper, Karl. (1934). Logik der Forschung [The logic of scientific discovery]. Vienna: Julius Springer Verlag. English translation, The Logic of Scientific Discovery. London: Hutchinson, 1959.

Popper, Karl. (1961). The Poverty of Historicism, 2nd edition. London: Routledge. Revised book version of original articles from 1944.

Popper, Karl. (1962). The Open Society and Its Enemies, 2 vols. Originally published London, 1945.

Popper, Karl. (1963). Conjectures and Refutations: The Growth of Scientific Knowledge. London: Routledge. The basic statement of Popper's principle of falsifiability.

Popper, Karl. (1972). Objective Knowledge: An Evolutionary Approach. Oxford: Clarendon Press.

Popper, Karl. (1999). All Life is Problem Solving. London: Routledge. An autobiography.

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Philosopher of science, political theorist, b. Vienna, July 28, 1902; d. London, Sept. 17, 1994. The son of a leading Jewish lawyer who had converted to Protestantism, Popper studied science, philosophy, and music at the University of Vienna, earning in 1928 a doctorate for a thesis on methodological issues in the psychology of discovery. He qualified in 1929 as a schoolteacher in mathematics and physics, a career he followed until his departure from Austria. Popper's masterpiece, Logik der Forschung (The Logic of Scientific Discovery ) was published in 1934. In opposition to the doctrines of the Vienna Circle, it elaborated a radically new approach to the methodological problems of natural science and also articulated thorough-going objectivist interpretations of probability and quantum mechanics. For many years anxious for the safety of democracy in central Europe, and for their own safety in a totalitarian state, Popper and his wife eventually left Austria in 1937 for New Zealand, where he had been offered a lectureship at Canterbury University College. There he wrote The Open Society and Its Enemies, a rousing defense of democracy and of rationality, published in 1945.

Shortly after the end of the World War II, Popper returned to Europe to take up a readership at the London School of Economics. He was a frequent visitor to the United States and continental Europe beginning in 1950, the year he gave the William James Lectures at Harvard. That year Popper remained in London as professor of logic and scientific method, a post he held until his retirement in 1969. He was knighted in 1965, and made a Companion of Honour in 1982. He published scores of papers and several books on an extraordinary range of subjects: ancient Greek philosophy, the formalization of logic, the axiomatics and interpretation of probability, quantum mechanics, thermodynamics, indeterminism, evolutionary biology, logical problems in both natural and social science, political theory, the theory of knowledge, the body-mind problem, and more.

Philosophy. Central to Popper's philosophy is the Socratic maxim that there is no easy road to understanding the world we live in, and that any wisdom we have resides in acknowledging our lack of knowledge. This is not skepticism, since he insists that the irremediable unsureness of our ideas need not prevent some of them from being preferable to others and, perhaps, true. The achievement of Logik der Forschung was to show that scientific knowledge, usually thought to be the best founded of all our knowledge, consists not of laws established by the traditional empiricist methods of observation, experiment, and induction, but of a network of hazardous speculations whose pretensions to truth we probe constantly by empirical tests. Inductive inference, says Popper, has no role to play in science: hypotheses are not inferred, but invented; and the only inferences needed are the deductive ones used to derive testable predictions from them. More generally, according to Popper, rational argument has been universally misconstrued as a form of justification or proof, and not seen to be simply a method for uncovering mistakes. His methodological view of rationality, critical rationalism, restores some sense after more than 2,000 years of unsuccessful responses to skeptical and mystical assaults on man's claims to rational knowledge.

For Popper the methodological question, "How can we detect, and eliminate, mistakes?" replaces the unanswerable epistemological question "How do we know?" Likewise a question of social engineering, "How can we set up institutions that stop our rulers from doing too much damage?" replaces the authoritarian question, "Who should rule us?" that has dominated political philosophy since Plato. There is no authority that we may not challenge, in either intellectual or political affairs; though we may hope for the best, for enlightening thoughts and enlightened rulers, we should also prepare for the worst. Democracies are special not for the way in which they appoint good leaders, for they seldom do, but for the way they are able to dismiss bad ones without bloodshed.

Popper is widely recognized as one of the most important philosophers of science and as a social and political thinker of courage and imagination, but the impact of his revolutionary epistemological ideas on most traditional philosophical problems has been oddly underrated by the philosophical profession. His influence on the general public, and on those who value a philosophy that is both rational and humane, has been profound.

Bibliography: Principal works. Logik der Forschung (Vienna 1934), trans. as The Logic of Scientific Discovery (New York 1959); The Open Society & Its Enemies (Princeton 1950, 1966); The Poverty of Historicism (New York 1977); Conjectures & Refutations (New York 1968); Objective Knowledge (New York 1972, 1979); Unended Quest (La Salle, Ill. 1976, 1984); The Self & Its Brain [with j. c. eccles] (Heidleberg 1977); The Postscript (Realism & the Aim of Science, The Open Universe, Quantum Theory & the Schism in Physics) (Totowa, N.J. 1982, 1983). d. w. miller, ed., Popper Selections (Princeton, N.J. 1985). Studies. p. a. schilpp, ed., The Philosophy of Karl Popper (La Salle, Ill. 1974). b. magee, Philosophy & the Real World: An Introduction to Karl Popper (La Salle, Ill. 1985) [previously published as Popper (New York, 1973)]. h. keuth, Die Philosophie Karl Poppers (Tübingen 2000).

[d. w. miller]

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Sir Karl Raimund Popper

The Austrian philosopher Sir Karl Raimund Popper (1902-1994) offered an original analysis of scientific research that he also applied to research in history and philosophy.

Karl Popper was born in Vienna on July 28, 1902, the son of a barrister. He studied mathematics, physics, and philosophy at the University of Vienna. Though not a member of the Vienna Circle, he was in sympathy with some, if not all, of its aims. His first book, The Logic of Scientific Discovery (1935), was published in a series sponsored by the Circle. In 1937 Popper accepted a post in New Zealand as senior lecturer in philosophy at Canterbury University College in Christchurch.

At the end of World War II, Popper was invited to the London School of Economics as a reader, and in 1949 he was made professor of logic and scientific method. Popper then made numerous visits to the United States as visiting professor and guest lecturer. In 1950 he gave the William James Lectures at Harvard University. In 1965 Popper was knighted by Queen Elizabeth II.

Foundations of Popper's Theory

Popper's first book laid the foundations for all the rest of his work. It offered an analysis of the procedure to be used in scientific work and a criterion for the meaning of the statements produced in such work. According to Popper, the researcher should begin by proposing hypotheses. The collection of data is guided by a theoretical preconception concerning what is relevant or important. The examination of causal connections between phenomena is also guided by leading hypotheses. Such a hypothesis is scientific only if one can derive from it particular observation statements that, if falsified by the facts, would refute the hypothesis. A statement is meaningful, therefore, if and only if there is a way it can be falsified. Hence the researcher should strive to refute rather than to confirm his hypotheses. Refutation is real advancement because it clears the field of a likely hypothesis.

Understanding History and Society

Popper later applied his analysis of knowledge to theories of society and history. In The Open Society and Its Enemies (1945) he attacked Plato, G. W. F. Hegel, and Karl Marx as offering untenable totalitarian theories that are easily falsifiable. The Open Society is often considered one of Popper's most influential books of this century. It also was responsible for the prevalent use of the term "open society." Critics argue that Popper succeeded in this book and in its sequel, The Poverty of Historicism (1957), in formulating a deterministic theory about general laws of historical development and then refuting it. A lively controversy ensued on the issue of which philosophers, if any, held the doctrine Popper refuted. Popper found himself embroiled in a decade of polemics, particularly with partisans of Plato. Popper was thus credited with a convincing logical refutation but one misdirected in its targets.

Popper's later works Objective Knowledge (1972) and The Self and Its Brain (1977) combined his scientific theory with a theory of evolution. In the 1980s, Popper continued to lecture, focusing mainly on questions of evolution and the role of consciousness. Karl Popper died of complications from cancer, pneumonia, and kidney failure on September 17, 1994 at the age of 92.

Further Reading

A work in progress edited by Paul A. Schilpp, The Philosophy of Karl Popper, will contain a biographical sketch, critical essays, and Popper's replies and should become the definitive work. In the interim the best study is Mario Bunge, ed., The Critical Approach to Science and Philosophy (1964), a Festschrift for Popper's sixtieth birthday containing essays by distinguished scholars and a bibliography complete to 1964.

Additional Sources

O'Mear, Anthony, Karl Popper, Cambridge University Press, 1995.

Popper, Karl Raimund, Unended Quest: An Intellectual Autobiography, Open Court, 1978.

Honderich, Ted, ed., Oxford Companion to Philosophy, Oxford University Press, 1995.

The New York Times, September 18, 1994. □