|Name: Karl Raimund Popper
|Birth: July 28, 1902
|Death: September 17, 1994
|School/tradition: critical rationalism, fallibilism, evolutionary epistemology
|Epistemology, philosophy of science, social and political philosophy
|falsifiability, hypothetico-deductive method, open society
|Socrates (via Plato), Aristotle, Kant, Schopenhauer, G. W. F. Hegel, Albert Einstein, Ludwig Wittgenstein Vienna Circle, Alfred Tarski,
Otto Selz, Bertrand Russell, Donald T. Campbell
|Friedrich Hayek, Milton Friedman, Imre Lakatos, Paul Feyerabend, George Soros, David Miller, Joseph Agassi, William Warren Bartley, Ian Jarvie, Paul Levinson, Helmut Schmidt, Peter Munz, Bryan Magee, Konrad Lorenz, Jeremy Shearmur, Peter Medawar, Dimitris Dimitrakos, Hans Albert, Abdolkarim Soroush, and Ayaan Hirsi Ali
Sir Karl Raimund Popper (July 28, 1902 – September 17, 1994) was an Austrian and British philosopher and a professor at the London School of Economics. He is counted among the most influential philosophers of science of the twentieth century, and he also wrote extensively on social and political philosophy. Popper is perhaps best known for repudiating the classical observationalist-inductivist account of scientific method by advancing empirical falsifiability as the criterion for distinguishing scientific theory from non-science; and for his vigorous defense of liberal democracy and the principles of social criticism which he took to make the flourishing of the "open society" possible.
Popper was born in Vienna (then part of Austria-Hungary) in 1902 to middle-class parents of Jewish origins. He was educated at the University of Vienna. His father was a bibliophile who was rumored to have ten thousand volumes in his library at home. As a young man he was apprenticed to a cabinetmaker and learned the craft of furniture making; he made furniture for his own use later in life even after he had become a world-renowned academic and scholar. He took a Ph.D. in philosophy in 1928, and taught in secondary school from 1930 to 1936.
In 1934 he published his first book, Logik der Forschung (it was not translated and published in English until 1959, when it was entitled The Logic of Scientific Discovery), in which he criticized psychologism, naturalism, inductionism, and logical positivism, and put forth his theory of potential falsifiability being the criterion for what should be considered science.
In 1937, the rise of Nazism and the threat of the Anschluss led Popper to emigrate to New Zealand, where he became lecturer in philosophy at Canterbury University College of the University of New Zealand at Christchurch, New Zealand. In 1946, he moved to England to become reader in logic and scientific method at the London School of Economics, where he was appointed professor in 1949. He was knighted by Queen Elizabeth II in 1965, and was elected a fellow of the Royal Society in 1976. He retired from academic life in 1969, though he remained intellectually active until his death in 1994.
Popper won many awards and honors in his field, including the Lippincott Award of the American Political Science Association, the Sonning Prize, and fellowships in the Royal Society, British Academy, London School of Economics, King's College London, and Darwin College of the University of Cambridge. Austria awarded him the Grand Decoration of Honour in Gold.
Philosophy of Science
Popper coined the term "critical rationalism" to describe his philosophy. This designation is significant, and indicates his rejection of classical empiricism, and of the observationalist-inductivist account of science that had grown out of it. Popper argued strongly against the latter, holding that scientific theories are universal in nature, and can be tested only indirectly, by reference to their implications. He also held that scientific theory, and human knowledge generally, is irreducibly conjectural or hypothetical, and is generated by the creative imagination in order to solve problems that have arisen in specific historico-cultural settings. Logically, no number of positive outcomes at the level of experimental testing can confirm a scientific theory, but a single genuine counter-instance is logically decisive: it shows the theory, from which the implication is derived, to be false.
Popper's account of the logical asymmetry between verification (or confirmation by positive instances), which is based on induction and thus falls victim to David Hume's problems with inductive inferences, and falsification, which is based on the logically valid inference of Modus Tollens, lies at the heart of his philosophy of science. Popper claimed repeatedly and adamantly that he had solved Hume's problem; he had done it, he claimed, by replacing induction with successive applications of falsification. This insight about the logical difference between confirmation and falsification also inspired Popper to take falsifiability as his criterion of demarcation between what is and is not genuinely scientific: a theory should be considered scientific if and only if it is falsifiable. This led him to attack the claims of both psychoanalysis and contemporary Marxism to scientific status, on the basis that the theories enshrined by them are not falsifiable. His work in theory or philosophy of science was influenced by his study of quantum mechanics—he has written extensively against the famous Copenhagen interpretation—and by Albert Einstein's approach to scientific theories. Popper's falsificationism resembles Charles Peirce's fallibilism. In Of Clocks and Clouds (1966), Popper said he wished he had known of Peirce's work earlier.
In All Life is Problem Solving (1999), Popper sought to explain the apparent progress of scientific knowledge—how it is that our understanding of the universe seems to improve over time. This problem arises from his position that the truth content of our theories, even the best of them, cannot be verified by scientific testing, but can only be falsified. If so, then how is it that the growth of science appears to result in a growth in knowledge? In Popper's view, the advance of scientific knowledge is an evolutionary process characterized by his formula:
In response to a given problem situation (), a number of competing conjectures, or tentative theories (), are systematically subjected to the most rigorous attempts at falsification possible. This process, error elimination (), performs a similar function for science that natural selection performs for biological evolution. Theories that better survive the process of refutation are not more true, but rather, more "fit"—in other words, more applicable to the problem situation at hand (). Consequently, just as a species' "biological fit" does not predict continued survival; neither does rigorous testing protect a scientific theory from refutation in the future. Yet, as it appears that the engine of biological evolution has produced, over time, adaptive traits equipped to deal with more and more complex problems of survival, likewise, the evolution of theories through the scientific method may, in Popper's view, reflect a certain type of progress: toward more and more interesting problems (). For Popper, it is in the interplay between the tentative theories (conjectures) and error elimination (refutation) that scientific knowledge advances toward greater and greater problems; in a process very much akin to the interplay between genetic variation and natural selection.
Where does "truth" fit into all this? As early as 1934 Popper wrote of the search for truth as one of the "strongest motives for scientific discovery." Still, he describes in Objective Knowledge (1972) early concerns about the much-criticized notion of truth as correspondence. Then came the semantic theory of truth formulated by the logician Alfred Tarski (its first published form was in 1933). Popper writes of learning in 1935 of the consequences of Tarski's theory, to his intense joy. The theory met critical objections to truth as correspondence and thereby rehabilitated it. The theory also seemed to Popper to support metaphysical realism and the regulative idea of a search for truth.
According to this theory, the conditions for the truth of a sentence as well as the sentences themselves are part of a metalanguage. So, for example, the sentence "Snow is white" is true if and only if snow is white. Although many philosophers have interpreted, and continue to interpret, Tarski's theory as a deflationary theory, Popper refers to it as a theory in which "is true" is replaced with "corresponds to the facts." He bases this interpretation on the fact that examples such as the one described above refer to two things: assertions and the facts to which they refer. He identifies Tarski's formulation of the truth conditions of sentences as the introduction of a "metalinguistic predicate" and distinguishes the following cases:
1) "John called" is true.
2) "It is true that John called."
The first case belongs to the metalanguage whereas the second is more likely to belong to the object language. Hence, "it is true that" possesses the logical status of a redundancy. "Is true," on the other hand, is a predicate necessary for making general observations such as "John was telling the truth about Phillip."
Upon this basis, along with that of the logical content of assertions (where logical content is inversely proportional to probability), Popper went on to develop his important notion of verisimilitude.
In words, the intuitive idea behind verisimilitude is that the assertions or hypotheses of scientific theories can be objectively measured with respect to the amount of truth and falsity that they imply. And, in this way, one theory can be evaluated as more or less true than another on a quantitative basis which, Popper emphasizes forcefully, has nothing to do with "subjective probabilities" or other merely "epistemic" considerations.
The simplest mathematical formulation that Popper gives of this concept can be found in the tenth chapter of Conjectures and Refutations. Here he defines it as:
where is the verisimilitude (or truthlikeness) of a, is a measure of the content of truth of a, and is a measure of the content of the falsity of a.
Knowledge, for Popper, was objective, both in the sense that it is objectively true (or truth-like), and also in the sense that knowledge has an ontological status (i.e., knowledge as object) independent of the knowing subject (Objective Knowledge: An Evolutionary Approach, 1972). He proposed three worlds: World One, being the phenomenal world, or the world of direct experience; World Two, being the world of mind, or mental states, ideas, and perceptions; and World Three, being the body of human knowledge expressed in its manifold forms, or the products of the second world made manifest in the materials of the first world (i.e. books, papers, paintings, symphonies, and all the products of the human mind). World Three, he argued, was the product of individual human beings in exactly the same sense that an animal path is the product of individual animals, and that, as such, has an existence and evolution independent of any individual knowing subjects. The influence of World Three, in his view, on the individual human mind (World Two) is at least as strong as the influence of World One. In other words, the knowledge held by a given individual mind owes at least as much to the total accumulated wealth of human knowledge, made manifest, than to the world of direct experience. As such, the growth of human knowledge could be said to be a function of the independent evolution of World Three. Compare with Memetics. For whatever reason—possibly because of its resemblance to Cartesian dualism—contemporary philosophers have not embraced Popper's Three World conjecture.
In 1944-1945 Popper published The Poverty of Historicism—this was originally published in three parts in the journal Economica in London, and in later years published in book form. In 1945 his The Open Society and Its Enemies was published in two volumes. Popper referred to this—especially Open Society—as his contribution to the war effort.
In The Poverty of Historicism Popper argued against the view, prevalent at the time and still held by some, that history has a grand plan or movement behind it, that it develops inexorably and necessarily according to knowable general laws towards a determinate end, and that the proper stance or role for humans is to unite with and support this historical development or unfolding—a view derived from Hegel's philosophy and accepted and used by both the political left (Marx and Marxism) and right (various forms of fascism). Popper argued that this view is the principal theoretical presupposition underpinning most forms of authoritarianism and totalitarianism. He claimed that historicism is founded upon mistaken assumptions regarding the nature of scientific law and prediction. Since the growth of human knowledge is a causal factor in the evolution of human history, and since "no society can predict, scientifically, its own future states of knowledge," it follows, he argued, that there can be no predictive science of human history. For Popper, metaphysical and historical indeterminism go hand-in-hand.
In The Open Society and its Enemies Popper attempted to show that our civilization is still in its infancy, that it is "aiming at humaneness and reasonableness, at equality and freedom, and [that it] continues to grow in spite of the fact that it has been so often betrayed by so many of the intellectual leaders of mankind." In particular, it tried to deal with totalitarianism by applying the "critical and rational methods of science to the problems of the open society" Popper made a distinction between what he called "utopian social engineering" and "piecemeal social engineering," and thoroughly rejected the utopian stance and effort but supported the piecemeal one.
The first volume of this work, after an introductory chapter and one on Heraclitus, consisted of a long and close study of Plato. Popper accused Plato of being an anti-humanitarian, an enemy of human freedom and the open society, and a totalitarian. He says, among many other things, that Plato was "in the long run only too successful in his propaganda for the arrest and overthrow of a civilization which he hated." Volume 2 deals with Aristotle, Hegel, Marx, and the aftermath. His stance is that adoption of critical rationalism—the stance of true or genuine science—and tolerance are the road to an open and hence good society. Popper thus ties together his philosophy of science and his social philosophy, claiming that the proper scientific stance, as he understood it, is the antidote to political totalitarianism.
Problem of Induction
Among his contributions to philosophy is his answer to David Hume's Problem of Induction. In fact, Popper was adamant in stating that he had solved Hume's problem. Hume stated that just because the sun has risen every day for as long as anyone can remember, doesn't mean that there is any rational reason to believe it will come up tomorrow. There is no rational way to prove that a pattern will continue on just because it has before.
Popper's reply is characteristic, and ties in with his criterion of falsifiability. He states that while there is no way to prove that the sun will come up, we can theorize that it will. If it does not come up, then it will be disproven, but since right now it seems to be consistent with our theory, the theory is not disproven.
Popper's demarcation between science and non-science thus serves, he claimed, as an answer to an old logical problem as well. This approach was criticised by Peter Singer for masking the role induction plays in empirical discovery.
Popper as teacher
According to all accounts, Popper had a very strong influence on his students. He seems to have been both adulated and despised by them; in fact some of them—notably Imre Lakatos and Paul Feyerabend—moved from admiring him earlier in their association to severely criticizing and even denouncing him later on. Although he subscribed to what he called "critical rationalism," Popper did not brook criticism of his views from his students, even going to the extreme of banning them from his classes if they persisted. In the case of Lakatos, at one point Popper praised him as his best and most astute successor, and then later denounced him and said that Lakatos completely misunderstood and misrepresented him.
One commentator has written of what he called the "Popper Church" and said that it consisted of a "lightly disguised squabble of alley cats." Another told of the Popper class dynamics—when a guest came to make a presentation, Popper would gradually shove the guest aside and move to the center of the head table, also monopolizing all the pieces of chalk so that the guest could not write on the chalk board. But other students, notably the late W. W. Bartley III, continued working with and admiring Popper to the end; Bartley also wrote a denunciation of Lakatos. So, at this remove, it is difficult to know who to believe and whose voice or testimony about the internal dynamics and interactions between these powerful characters is most reliable.
In any event, Popper seems to have had a leonine and prickly personality that was quick to notice and respond to what he perceived as slights. He seems to have been especially quick to take offense and to think that he was not being accorded the respect and status that he saw other leading philosophers receiving.
By all accounts, Popper has played a vital role in establishing the philosophy of science as a vigorous, autonomous discipline within analytic philosophy, through his own prolific and influential works, and also through his influence on his own contemporaries and students—chief among them, Imre Lakatos and Paul Feyerabend, two of the foremost philosophers of science in the next generation of analytic philosophy. Lakatos' work drastically modified Popper's position, and Feyerabend's repudiated it entirely, but the work of both was deeply influenced by Popper and engaged with many of the problems that Popper set.
While there is some dispute as to the matter of influence, Popper had a long standing and close friendship with economist Friedrich Hayek, also from Vienna. Each found support and similarities in each other's work, citing each other often, though not without qualification. In a letter to Hayek in 1944, Popper stated, "I think I have learnt more from you than from any other living thinker, except perhaps Alfred Tarski." Popper dedicated his Conjectures and Refutations to Hayek. For his part, Hayek dedicated a collection of papers, Studies in Philosophy, Politics, and Economics, to Popper, and in 1982 said, "...ever since his Logik der Forschung first came out in 1934, I have been a complete adherent to his general theory of methodology."
Popper's influence, both through his work in philosophy of science and through his political philosophy, has also extended beyond the academy. Among Popper's students and advocates is the multi-billionaire investor George Soros, who says his investment strategies are modeled on Popper's understanding of the advancement of knowledge through falsification. Among Soros' philanthropic foundations is the Open Society Institute, a think-tank named in honor of Popper's The Open Society and Its Enemies, which Soros founded to advance the Popperian defense of the open society against authoritarianism and totalitarianism.
The Quine-Duhem thesis argues that it is impossible to test a single hypothesis on its own, since each one comes as part of an environment of theories. Thus we can only say that the whole package of relevant theories has been collectively falsified, but cannot conclusively say which element of the package must be replaced. An example of this is given by the discovery of the planet Neptune: when the motion of the planet Uranus was found not to match the predictions of Newton's laws, the theory "there are seven planets in the solar system" was rejected, and not Newton's laws themselves. Popper discussed this critique of naïve falsificationism in chapters three and four of The Logic of Scientific Discovery. For Popper, theories are accepted or rejected via a sort of “natural selection.” Theories that say more about the way things appear are to be preferred over those that do not; the more generally applicable a theory is, the greater its value. Thus Newton’s laws, with their wide general application, are to be preferred over the much more specific “the solar system has seven planets.”
Thomas Kuhn’s influential book The Structure of Scientific Revolutions argued that scientists work in a series of paradigms, and found little evidence of scientists actually following a falsificationist methodology. Although Kuhn does not put it in these terms, there is an implicit rejection in Kuhn's work of the old positivist "context of discovery" vs. "context of justification" distinction because Kuhn sees discovery and justification as going together (although he does not speak of justification in the way that the positivists did). But Popper clung to this distinction, saying that the process of coming up with a new theory is extra-logical, and only the process of falsification of that theory follows a logical procedure.
Popper's student Imre Lakatos attempted to reconcile Kuhn’s work with falsificationism by arguing that science progresses by the falsification of research programs rather than the more specific universal statements of naïve falsificationism. Another of Popper’s students, Paul Feyerabend, ultimately rejected any prescriptive methodology, and argued that the only universal method characterizing scientific progress was anything goes.
Popper seems to have anticipated Kuhn's observations. In his collection Conjectures and Refutations: The Growth of Scientific Knowledge, Popper writes:
[S]cience must begin with myths, and with the criticism of myths; neither with the collection of observations, nor with the invention of experiments, but with the critical discussion of myths, and of magical techniques and practices. The scientific tradition is distinguished from the pre-scientific tradition in having two layers. Like the latter, it passes on its theories; but it also passes on a critical attitude towards them. The theories are passed on, not as dogmas, but rather with the challenge to discuss them and improve upon them.
Another objection is that it is not always possible to demonstrate falsehood definitively, especially if one is using statistical criteria to evaluate a null hypothesis. More generally, it is not always clear that if evidence contradicts a hypothesis that this is a sign of flaws in the hypothesis rather than of flaws in the evidence. However, this is a misunderstanding of what Popper's philosophy of science sets out to do. Rather than proffering a set of instructions that merely need to be followed diligently to achieve science, Popper makes clear in The Logic of Scientific Discovery, his belief that the resolution of conflicts between hypotheses and observations can only be a matter of the judgment of scientists, in each individual case.
Other critics seek to vindicate the claims of historicism or holism to intellectual respectability, or psychoanalysis or Marxism to scientific status. It has been argued that Popper's student Imre Lakatos transformed Popper's philosophy using historicist and updated Hegelian historiographic ideas.
Charles Taylor accuses Popper of exploiting his fame as an epistemologist to diminish the importance of philosophers of the continental tradition. According to Taylor, Popper's criticisms are completely baseless, but they are received with an attention and respect that Popper's "intrinsic worth hardly merits."
Martin Gardner allegedly claimed Popper had "enormous egotism," and was motivated by an "intense jealousy of Rudolf Carnap."
A. A. Derksen examined the development of Popper's thought from The Logic of Scientific Discovery, through Conjectures and Refutations (1963) to Objective Knowledge (1972), and concluded that, instead of having a unified deductive system, "we are left with a disconnected theory, its parts in need of support." Falsifiability, according to Derksen, is not one thing, but several. Derksen's final conclusion is that Popper's deductivist system cannot stand, and that what emerges is a form of inductivism.
In 2004, philosopher and psychologist Michel ter Hark of Groningen, Netherlands, published a book, Popper, Otto Selz and the Rise of Evolutionary Epistemology, in which he claims that Popper got part of his ideas from his tutor, the German-Jewish psychologist Otto Selz. Selz himself never published his ideas, partly because of the rise of Nazism which forced him to quit his work in 1933, and the prohibition of referencing to Selz' work.
Popper wrote an enormous amount, and most of his writings have appeared in many different editions and reprints, from many publishers. The following is a list of his major works with the dates of their first appearance.
- Logik der Forschung, 1934 (in German)
- The Open Society and Its Enemies, 1945
- The Logic of Scientific Discovery, 1959 (Translation of Logik der Forschung)
- The Poverty of Historicism, 1957
- Conjectures and Refutations: The Growth of Scientific Knowledge, 1963
- Objective Knowledge: An Evolutionary Approach, 1972, Rev. ed., 1979
- Unended Quest; An Intellectual Autobiography, 1976
- The Self and Its Brain: An Argument for Interactionism (with Sir John C. Eccles), 1977
- Die beiden Grundprobleme der Erkenntnistheorie - aufgrund von Manuskripten aus den Jahren 1930-1933 (The Two Fundamental Problems of Epistemology - Based on Manuscripts from the Period 1930-1933) (Edited by Troels Eggers Hansen), 1979 (in German)
- The Open Universe: An Argument for Indeterminism, 1982
- Realism and the Aim of Science, 1982
- Die Zukunft ist Offen (The Future is Open) (with Konrad Lorenz), 1985 (in German)
- The Lesson of this Century, Interviewer: Giancarlo Bosetti, English translation: Patrick Camiller), 1992
- The World of Parmenides, Essays on the Presocratic Enlightenment, (Edited by Arne F. Petersen with the assistance of Jørgen Mejer)
- The Myth of the Framework: In Defence of Science and Rationality, 1994
- Knowledge and the Mind-Body Problem: In Defence of Interactionism, 1994
- Quantum Theory and the Schism in Physics, (From the Postscript to The Logic of Scientific Discovery)
- In Search of a Better World, a collection of Popper’s essays and lectures covering a range of subjects from the beginning of scientific speculation in classical Greece to the need for a new professional ethic based on the ideas of tolerance and intellectual responsibility
Other works and collections:
- The Philosophy of Karl Popper, 2 vols., in The Library of Living Philosophers series, ed. by Paul Arthur Schilpp. La Salle, Illinois: Open Court, 1974. Essential to Popper studies. Contains Popper's "Intellectual Autobiography," and "Replies to My Critics," as well as many essays on Popper and his work. Vol. 2 also contains an extensive "Bibliography of the Writings of Karl Popper," by Troels Eggers Hansen; this lists all the different published versions of Popper's work up to 1974.
- Critical Rationalism: A Re-Statement and Defence by David Miller, 1994
- Popper Selections (Text by Popper, selected and edited by David Miller)
- All Life Is Problem Solving (Text by Popper, translation by Patrick Camiller), 1999
- Science and Skepticism by John N. W. Watkins, 1984
- F. Raphael, Popper: The Great Philosophers (London: Routledge, 1991, ISBN 0415923913), 447.
- Karl Popper, The Open Society and Its Enemies (Princeton University Press, 2020, ISBN 978-0691210841).
- Malachi Haim Hacohen, Karl Popper - The Formative Years, 1902-1945: Politics and Philosophy in Interwar Vienna (Cambridge University Press, 2002, ISBN 978-0521890557).
- W. Weimer and D. Palermo (eds.), Cognition and the Symbolic Processes (Psychology Press, 1982, ISBN 978-0898590661).
- Karl Popper, Conjectures and Refutations: The Growth of Scientific Knowledge (Routledge, 2002, ISBN 978-0415285940).
ReferencesISBN links support NWE through referral fees
- Derksen, A. A. "The Alleged Unity of Popper's Philosophy of Science: Falsifiability as Fake Cement," Philosophical Studies 48 (1985), 313-336.
- Eby, Lloyd. Objective Knowledge and the Knowing Subject: The Popper - Kuhn Debate. Unpublished Ph.D. dissertation. Bronx, NY: Fordham University, 1988.
- Edmonds, D. and J. Eidinow. Wittgenstein's Poker. New York: Ecco 2001.
- Feyerabend, P. Against Method. London: New Left Books, 1975.
- Kadvany, John. Imre Lakatos and the Guises of Reason. Durham, NC: Duke University Press, 2001. ISBN 978-0822326601
- Hacohen, M. Karl Popper: The Formative Years, 1902 – 1945. Cambridge: Cambridge University Press, 2002. ISBN 978-0521890557
- Kuhn, T. S. The Structure of Scientific Revolutions. Chicago: University of Chicago Press, 1962.
- Lakatos, Imre, and Alan Musgrave (eds.). Criticism and the Growth of Knowledge. New York and London: Cambridge University Press, 1970, 1972, 1974.
- Magee, B. Popper. London: Fontana, 1977.
- Magee, B. Confessions of a Philosopher. Weidenfeld and Nicolson, 1997.
- O'Hear, A. Karl Popper. London: Routledge, 1980.
- Radnitzky, G. and W. W. Bartley (eds.). Evolutionary Epistemology, Rationality, and the Sociology of Knowledge. La Salle, IL: Open Court Press 1987. ISBN 0812690397
- Raphael, F. Popper: The Great Philosophers. London: Routledge, 1991. ISBN 0415923913
- Richmond, Sheldon. Aesthetic Criteria: Gombrich and the Philosophies of Science of Popper and Polanyi. Rodopi, Amsterdam/Atlanta, 1994. ISBN 905183618X
- Robinson, Dave and Judy Groves. Introducing Political Philosophy. Icon Books, 2003. ISBN 184046450X
- Stokes, G. Popper: Philosophy, Politics and Scientific Method. Cambridge: Polity Press, 1998.
- Taylor, Charles, "Overcoming Epistemology," in Philosophical Arguments, Harvard University Press, 1995.
- Weimer, W., and D. Palermo (eds.). Cognition and the Symbolic Processes. Psychology Press, 1982. ISBN 978-0898590661
All links retrieved October 5, 2022.
- Karl Popper Stanford Encyclopedia of Philosophy.
- The Karl Popper Web
- Sir Karl Popper – Influence on Friesian Philosophy
- A Skeptical Look at Karl Popper by Martin Gardner, 2001.
General Philosophy Sources
- Stanford Encyclopedia of Philosophy
- The Internet Encyclopedia of Philosophy
- Paideia Project Online
- Project Gutenberg
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