Karl Popper

• 1. Introduction

  ›

  Sir Karl Raimund Popper was born in Vienna on the 28th of July 1902 and died on the 17th of September 1994. In between, he built a philosophy of science on an odd idea. The strongest theories, he argued, are the ones most exposed to being wrong. He called the result critical rationalism, and he described it as "the first non-justificational philosophy of criticism in the history of philosophy". In 1992 he received the Kyoto Prize in Arts and Philosophy for "symbolising the open spirit of the 20th century".

  Popper was not only a philosopher of laboratories and experiments. He also wrote a fierce defence of liberal democracy, and he opposed totalitarianism in nearly every form he met. So several questions run through his life. How can a theory deserve trust if nothing can ever establish that it is true? What does a rule for judging experiments have to do with resisting dictators? Why would a champion of tolerance insist that tolerance has limits? And how did a young man who briefly thought of himself as a communist end up as one of communism's sharpest critics? The first clues sit in a Viennese home crowded with books.

  1534912458

• 2. A Bookish Vienna

  ›

  Between 12,000 and 14,000 volumes filled the library of Simon Siegmund Carl Popper, a lawyer from Bohemia and the father of Karl. The son inherited both the books and his father's appetite for philosophy, the classics and social questions. Looking back, Karl called the atmosphere of his upbringing "decidedly bookish". His mother, Jenny Schiff, was an accomplished pianist of Silesian and Hungarian descent. His uncle, Josef Popper-Lynkeus, was a philosopher.

  By 1898 the Poppers had climbed rapidly through Viennese society. All of Karl's grandparents were assimilated Jews, and the family converted to Lutheranism before his birth, so he was baptised a Lutheran. Simon became a partner in the law firm of Raimund Grübl, the city's liberal mayor, and took over the practice after Grübl died. Karl received his middle name from the mayor. In his autobiography he mistakenly gave the mayor's first name as Carl. His parents were also close friends of Rosa Graf, the sister of Sigmund Freud.

  At the age of 16, Popper left the Realgymnasium and began attending university lectures as a guest student. His subjects ranged across mathematics, physics, philosophy, psychology and the history of music. In 1919 Marxism drew him in. He joined the Association of Socialist School Students and the Social Democratic Workers' Party of Austria, which then embraced Marxism fully. Then came the street battle in the Hörlgasse on the 15th of June 1919, when police shot eight of his unarmed party comrades. Popper abandoned the historical materialism of Karl Marx and remained a supporter of social liberalism for the rest of his life.

  Heavy labour in street construction proved too much for Popper. He then apprenticed as a cabinetmaker and finished as a journeyman, hoping the skill might help him open a daycare facility for children. After that he volunteered in one of the children's clinics run by the psychoanalyst Alfred Adler.

  In 1922 Popper passed his matura through a second-chance route and finally enrolled at the university as a regular student. By 1924 he had completed his examination as a primary school teacher and begun work at an after-school club for socially endangered children. In 1925 he moved to the newly founded Pädagogisches Institut to continue studying philosophy and psychology. Around then he began courting Josefine Anna Henninger.

  A doctorate in psychology followed in 1928, supervised by Karl Bühler, with Moritz Schlick as second chair of the committee. The dissertation was titled Zur Methodenfrage der Denkpsychologie, or On Questions of Method in the Psychology of Thinking. In 1929 Popper gained authorisation to teach mathematics and physics in secondary school, and he began doing so. In 1930 he married Henninger, a fellow teacher. Born in 1906 and known as Hennie, she would be his partner through exile and return.

  1026114212

• 3. Exile and Return

  ›

  In the early 1930s, fear of Nazism and the threat of the Anschluss drove Popper to write in the evenings and at night. He needed a book to win an academic post in a country that was safe for people of Jewish descent. The result was Die beiden Grundprobleme der Erkenntnistheorie, or The Two Fundamental Problems of the Theory of Knowledge. He never published that two-volume work. Instead he issued a condensed version with some new material as Logik der Forschung in 1934, later known in English as The Logic of Scientific Discovery.

  That 1934 book attacked psychologism, naturalism, inductivism and logical positivism. In their place it offered potential falsifiability as the line separating science from non-science. In 1935 and 1936 Popper took unpaid leave for a study visit to the United Kingdom.

  In 1937 Popper secured a position that allowed him to emigrate to New Zealand. He became a lecturer in philosophy at Canterbury University College in Christchurch, and there he wrote The Open Society and Its Enemies. In Dunedin he met the physiology professor John Carew Eccles, and the two formed a lifelong friendship. The events that followed the Anschluss in 1938 made his exile permanent.

  In 1946, after the Second World War, Popper moved to the United Kingdom as a reader in logic and scientific method at the London School of Economics. Three years later, in 1949, he was appointed professor. He served as president of the Aristotelian Society in 1958-59 and lived in Penn, Buckinghamshire. During the 1960s he took part in the positivism dispute over the methods of the social sciences, criticising the Frankfurt School of Theodor Adorno and Jürgen Habermas. He retired in 1969 but stayed intellectually active.

  In 1985 Popper returned to Austria so that his wife could have her relatives around her during her last months. She died that November. The Ludwig Boltzmann Gesellschaft then failed to install him as director of a newly founded branch for research on the philosophy of science. In 1986 he went back to the United Kingdom and settled in Kenley, Surrey.

  War in the early years of the Poppers' marriage led them to choose not to have children, a decision he later called "perhaps a cowardly but in a way a right decision". He died at the age of 92 from "complications of cancer, pneumonia and kidney failure". Until two weeks before, he had worked continuously, and he wrote his last letter just before he suddenly fell terminally ill. After cremation, his ashes were taken to Vienna and buried in the Lainzer cemetery beside the ORF Centre, where Hennie already lay.

  2644359140

• 4. Risky Theories

  ›

  Popper set Albert Einstein against Sigmund Freud and Alfred Adler. Einstein's theory of relativity set off a revolution in physics in the early 20th century, and Popper judged it highly "risky". Risky meant that consequences could be deduced from it that differed greatly from those of the dominant Newtonian physics. One such prediction, that gravity could deflect light, was verified by Eddington's experiments in 1919.

  Freud's theories looked different to Popper. Nothing, even in principle, could show the ideas of Freud and Adler to be false, so he thought they had more in common with primitive myths than with genuine science. Their apparent strength was really a weakness. They were crafted to rebuff any criticism and to explain every possible form of human behaviour. This verdict shaped his answer to the problem of demarcation. A theory that cannot in principle be falsified is not scientific.

  No number of successful experiments can confirm a theory, Popper argued, yet a single counterexample is logically decisive. This asymmetry between verification and falsification sits at the heart of his philosophy of science. He used the term critical rationalism for an approach that rejected classical empiricism and the inductivist account of science. Theories, he held, are abstract and can be tested only indirectly, through their implications. All human knowledge is conjectural, produced by the creative imagination to solve problems that arise in particular historical and cultural settings. The same test caught contemporary Marxism, which he said was compatible with any possible observation.

  Take the sun. Popper claimed to have solved the philosophical problem of induction with exactly this example. He agreed with David Hume that nothing in logic justifies expecting a sunrise merely because there have been sunrises before. In his words: "I approached the problem of induction through Hume. Hume, I felt, was perfectly right in pointing out that induction cannot be logically justified." Yet one can still state the theory that the sun rises every day. If it fails to rise on some day, the theory is falsified and must be replaced. Until then, tentative acceptance is reasonable, though never justified in an inductive sense.

  A fussier theory, that the sun rises until a certain day and then stops, is no more rational. It is harder to attack, because falsifying it requires the extra assumption that the stated day has not yet arrived. Popper held that the simplest, most easily falsifiable theory explaining the known facts is the one to prefer. That set him against positivism, which prefers the theory most likely to be true.

  A puzzle followed. If theories can only be falsified, how does knowledge grow? In All Life is Problem Solving, Popper answered with an evolutionary sequence. A problem situation gives rise to competing tentative theories. These face the most rigorous attempts at refutation, which he called error elimination, and the process ends in a new problem situation. Error elimination does for science what natural selection does for living species. Theories that survive are not more true, but more fit for the problem at hand. Survival so far guarantees nothing, since biological fitness does not ensure a species will last. Still, the process may carry science toward ever more interesting problems.

  Arithmetic raised a different worry. A statement such as "2 + 2 = 4" seems impossible to falsify, and if it is not falsifiable it cannot be scientific. Popper split the statement in two. In the pure mathematics sense, it is logically true and cannot be refuted. In the applied sense, it describes how apples behave, and then it can be tested. Put two apples in a container and add two more. If the count is not four, the theory is shown false. If it is four, the theory of numbers has proved applicable to reality.

  Popper's student W.W. Bartley III tried to radicalise this attitude and made the controversial claim that everything can be rationally criticised. Popper himself held that rational discussion of metaphysical ideas, moral values and even purposes is possible, because science is only a special case of the general method of criticism.

  Niels Bohr drew Popper's sharpest complaints in physics. Popper wrote extensively against the Copenhagen interpretation of quantum mechanics. He sided with Einstein's scientific realism over Bohr's instrumentalism, and he thought Bohr's view brought subjectivity into physics. Later in life he said Bohr was "a marvelous physicist, one of the greatest of all time, but he was a miserable philosopher, and one couldn't talk to him".

  Popper's falsifiability also resembles the fallibilism of Charles Peirce in the nineteenth century. In Of Clocks and Clouds, published in 1966, Popper remarked that he wished he had known Peirce's work earlier.

  3536339841

• 5. The Open Society

  ›

  Two books of the mid-1940s, The Poverty of Historicism and The Open Society and Its Enemies, answered the totalitarian ideologies dominating Central European politics. The failure of democratic parties to stop fascism in Austria in the 1920s and 1930s had traumatised him. His books defended democratic liberalism and criticised the philosophical assumptions beneath every form of totalitarianism.

  Historicism was his main target. He defined it as the belief that history unfolds necessarily, by knowable general laws, toward a determinate end. He saw it as the central theoretical assumption behind most authoritarianism and totalitarianism. His counterargument rested on knowledge. The growth of human knowledge affects the course of history, and "no society can predict, scientifically, its own future states of knowledge". Therefore, he argued, no predictive science of human history can exist. For Popper, metaphysical and historical indeterminism go hand in hand.

  For a few months in 1919, Popper considered himself a communist, and he worked as an office boy at the communist headquarters. Whether he ever joined the Communist Party is unclear. The episode left him wary of anyone who believed that spilling blood for a revolution was necessary. When human lives are at stake, he concluded, one must act with extreme prudence. He associated Marxist historicism with questionable means, and socialism with putting equality before freedom. He said he was a socialist for "several years" and kept an interest in egalitarianism. In the end he judged socialism a "beautiful dream" that, like egalitarianism, could not coexist with individual liberty.

  At first Popper saw totalitarianism as exclusively right-wing. Yet by 1945, in The Open Society, he was already describing Communist parties as weak opponents of fascism because they shared its historicism. Over time he came to view Soviet-type communism as a form of totalitarianism. He saw the main issue of the Cold War as democracy versus totalitarianism, not capitalism versus socialism. In 1957 he dedicated The Poverty of Historicism to the "memory of the countless men, women and children of all creeds or nations or races who fell victims to the fascist and communist belief in Inexorable Laws of Historical Destiny."

  In 1947 Popper co-founded the Mont Pelerin Society with Friedrich Hayek, Milton Friedman, Ludwig von Mises and others. He did not fully agree with its charter. He unsuccessfully recommended inviting socialists to take part. He also wanted the emphasis placed on a hierarchy of humanitarian values rather than on advocacy of the free market.

  An advocate of toleration, Popper still warned against unlimited tolerance. In The Open Society and Its Enemies he argued that it "must lead to the disappearance of tolerance". He did not mean that every intolerant idea should be silenced. As long as such philosophies can be met with rational argument and held in check by public opinion, suppression would be unwise. But a tolerant society, he wrote, should claim "the right not to tolerate the intolerant" if their followers refuse argument and answer with fists or pistols. Incitement to intolerance and persecution, in his view, should be treated as criminal, like incitement to murder, to kidnapping, or to the revival of the slave trade.

  Popper also criticised what he called the "conspiracy theory of society". This is the view that powerful people or groups, godlike in their efficacy, deliberately bring about all of society's ills. It cannot be right, he argued, because "nothing ever comes off exactly as intended." The philosopher David Coady notes that critics of conspiracy theories often cite Popper, and that his views remain an orthodoxy in some circles. Yet the philosopher Charles Pigden has pointed out that the argument touches only a very extreme kind of conspiracy theory.

  4550395253

• 6. Minds and Worlds

  ›

  In 1935 Popper learned of the consequences of the semantic theory of truth that the logician Alfred Tarski had published in 1933, and it delighted him intensely. The theory answered objections to the idea that truth means correspondence with facts, and so rehabilitated it. Its standard example holds that the sentence "Snow is white" is true if and only if snow is white. Many philosophers read Tarski as a deflationary thinker. Popper read him as supporting metaphysical realism and the search for truth.

  From this base Popper developed verisimilitude, or truthlikeness. The idea is that the hypotheses of scientific theories can be objectively measured by the amount of truth and falsity they imply. He insisted this has nothing to do with "subjective probabilities". His simplest formula, in the tenth chapter of Conjectures and Refutations, defines verisimilitude as the content of truth in a statement minus its content of falsity. His attempt to define an actual measure turned out to be inadequate. Still, it inspired a wealth of new attempts.

  Objective Knowledge: An Evolutionary Approach, from 1972, set out his three worlds. World One is the physical world. World Two is the private world of individual minds, with their ideas and perceptions. World Three is the public body of human knowledge, which he described as "scientific theories, ethical principles, characters in novels, philosophy, art, poetry, in short our entire cultural heritage". It takes material form in books, paintings, symphonies, cathedrals and particle accelerators.

  World Three, Popper argued, is the product of individual humans in the same way that an animal path in the jungle is created by many animals, though none of them planned or intended it. It therefore has an existence and an evolution independent of any single knower. Its influence on an individual mind, he thought, is at least as strong as the influence of the physical world. Daniel Dennett and other contemporary philosophers have not embraced the idea, mainly because it resembles mind-body dualism. Popper saw it instead as a pluralistic realism with more explanatory power than physicalism or dualism.

  For many years Popper and John Eccles speculated about free will, generally agreeing on an interactionist dualist theory of the mind. Popper was a body-mind dualist, yet he did not think the mind is a separate substance. He thought mental properties are distinct from physical ones. In 1965, in the second Arthur Holly Compton Memorial Lecture, he revisited quantum indeterminacy as a source of human freedom. Eccles had suggested that "critically poised neurons" might be influenced by the mind. Popper criticised Compton's idea of amplified quantum events shaping a decision.

  He also rejected a doctrine he traced from Hume to Schlick, that the only alternative to determinism is sheer chance. He called it "not only highly dogmatic (not to say doctrinaire) but clearly absurd". Instead he called for a mix of randomness and control: "freedom is not just chance but, rather, the result of a subtle interplay between something almost random or haphazard, and something like a restrictive or selective control."

  In The Self and its Brain, written with Eccles in 1977, he finally set this out as a two-stage model in time. New ideas resemble genetic mutations, he said. The brain offers a range of probabilistic proposals, and a selective procedure then eliminates those the mind finds unacceptable.

  3677555774

• 7. Darwin and God

  ›

  "Darwinism is not a testable scientific theory, but a metaphysical research program." With that sentence Popper handed both sides of the creation-evolution controversy a quotation, and even courts in their decisions invoked his criterion of falsifiability. Yet he added that the theory is "invaluable". He pointed to experiments with bacteria that adapt to penicillin. Natural selection, he said, suggests a mechanism of adaptation and lets researchers study it in detail.

  Popper later made what he called a recantation. He had once described natural selection as "almost tautological" and had treated it as a successful metaphysical research programme. Then he wrote: "I have changed my mind about the testability and logical status of the theory of natural selection; and I am glad to have an opportunity to make a recantation." Properly formulated, he said, the theory is far from tautological. It is testable, and it turns out not to be strictly universally true. Not all phenomena of evolution are explained by natural selection alone.

  Those famous passages are a small part of what he wrote on evolution. Popper also stressed that "the last thing I wish to do, however, is to advocate another dogma". What is called a science, and who is called a scientist, he said, must remain a matter of convention or decision.

  Popper's own view was the spearhead model, a form of genetic pluralism. Living organisms have goals and act on them, each guided by a central control. In humans this is the brain, and in far simpler species such as the amoeba it takes a cruder form. This control organ is the "spearhead of evolution". A mutation in the genes that shape it can change behaviour and preferences drastically without altering the organism's body. He called such a change a "hopeful behavioural monster". Small, quick bodily changes then follow to suit the new goals. By this reasoning, the eating habits of the giraffe must have changed before its long neck evolved.

  He named this view "evolution from within" or "active Darwinism". He contrasted it with the naturalistic "evolution from without", in which a hostile environment simply tries to kill a passive organism. He understood the universe as a creative entity that invents new things, including life, with no need for a god pulling strings from behind the curtain. Popper was also a key figure in encouraging the patent lawyer Günter Wächtershäuser to publish his iron-sulfur world hypothesis on the origin of life, and his criticism of the "soup" theory.

  On the controversy itself, Popper first quoted C.E. Raven in calling it "a storm in a Victorian tea-cup". After developing his own models, he revised that view. "I have to confess that this cup of tea has become, after all, my cup of tea; and with it I have to eat humble pie."

  At fifteen, after reading Spinoza at his father's suggestion, Popper later recalled gaining "a lifetime's dislike of theorizing about God". He kept no link to his Jewish ancestry and was not an observant Lutheran. In 1936 he applied to the Academic Assistance Council to leave Austria, describing himself as "Protestant, namely evangelical but of Jewish origin". Asked whether religious communities should be approached for him, he wrote "NO" opposite the Jewish Orthodox section and underlined it twice.

  In a 1969 interview, which he required to stay secret until after his death, he said he did not know whether God exists. Some forms of atheism, he said, are "arrogant and ignorant and should be rejected". He also said: "When I look at what I call the gift of life, I feel a gratitude which is in tune with some religious ideas of God." Organised religion, in his view, "tends to use the name of God in vain".

  He was Patron and Honorary Advisor of the Humanist Society of New Zealand, and a member of the Advisory Council of the British Humanist Association. His concept of the open society served as the "ideological backbone" of humanist campaigning across the 1960s and 1970s. In 1968 he contributed an essay to The Humanist Outlook. Yet he could not endorse "various humanist and secular offensives". "Although I am not for religion," he wrote in a letter, "I do think that we should show respect for anybody who believes honestly." Religion could not be rationally justified, he thought, but "because something isn't science, however, does not mean it is meaningless".

  6069668178

• 8. Critics and Heirs

  ›

  The discovery of Neptune is the standard illustration of the Quine-Duhem thesis. When the motion of Uranus failed to match the predictions of Newton's laws, astronomers rejected the claim that the solar system had seven planets. They did not reject Newton. The thesis holds that no single hypothesis can be tested alone, since each belongs to a web of theories. Popper discussed this critique of naive falsificationism in Chapters 3 and 4 of The Logic of Scientific Discovery.

  Thomas Kuhn, in The Structure of Scientific Revolutions in 1962, gave anomalous experiences a role like the one Popper gave to falsification. Kuhn denied the two were the same, and he doubted that theories could be falsified as Popper suggested. Popper claimed he had already recognised in 1934 that scientists develop their ideas within a theoretical framework. But he criticised what he saw as Kuhn's relativism, and that disagreement is the heart of the Kuhn-Popper debate. His student Imre Lakatos tried to reconcile the two by arguing that science advances through the falsification of research programs rather than of single statements.

  Carl Gustav Hempel pointed to a statement such as "for every metal, there is a temperature at which it will melt". No observation could falsify it, since a higher temperature always remains untested, yet it looks like a valid scientific hypothesis. The simplest reply is that accepted theories, such as atomic theory, imply it, so it counts as part of tentative scientific knowledge. Popper himself said that settling conflicts between hypotheses and observations can only be a matter of the collective judgment of scientists in each case.

  Other critics aimed at particular targets. Karl-Otto Apel, in Transformation der Philosophie in 1973, charged Popper with a pragmatic contradiction. Adolf Grünbaum argued in 1984 that psychoanalytic theories can in principle be falsified. In 1986 Roger Scruton held that Freud's theory of repression has strong empirical content, though he still judged Freudian theory unscientific. Charles Taylor accused Popper of using his fame as an epistemologist to belittle continental philosophers. John Gray argued that Popper's method would have blocked acceptance of Darwin and Einstein, a charge at odds with Popper's frequent use of relativity as a case study. Michel ter Hark wrote in 2004 that Popper borrowed ideas from his tutor Otto Selz, whose work was silenced after he was forced to quit in 1933.

  In 1946 Popper founded the Department of Philosophy, Logic and Scientific Method at the London School of Economics. There he influenced Imre Lakatos and Paul Feyerabend. Lakatos significantly modified Popper's position, while Feyerabend repudiated it entirely, yet both engaged with the problems Popper set. Popper had a close friendship with the economist Friedrich Hayek, who had also come to the LSE from Vienna. In a letter of 1944 Popper wrote: "I think I have learnt more from you than from any other living thinker, except perhaps Alfred Tarski." Popper dedicated Conjectures and Refutations to Hayek. In 1982 Hayek said he had been a complete adherent of Popper's general methodology ever since Logik der Forschung came out in 1934.

  Popper's other friends included the art historian Ernst Gombrich and the biologist Peter Medawar. Medawar called him "incomparably the greatest philosopher of science that has ever been". The German jurist Reinhold Zippelius uses Popper's method of trial and error in his legal philosophy. One of Popper's LSE students was George Soros, whose philanthropic foundations include the Open Society Institute, named for The Open Society and Its Enemies. At Central European University on the 28th of October 2009, Soros questioned his old teacher. Popper had assumed, he said, that the purpose of thinking is to understand reality, but manipulation might sometimes take precedence over understanding.

  The honours came in great number. Queen Elizabeth II knighted Popper in 1965. He was elected a Fellow of the Royal Society in 1976 and invested as a Companion of Honour in 1982. Vienna gave him its Ring of Honour in 1983, and Austria awarded him the Grand Decoration of Honour in Gold in 1986. In 1989 he was the first recipient of the Prize International Catalonia. Earlier prizes included the Lippincott Award of the American Political Science Association, the Sonning Prize and the Otto Hahn Peace Medal.

  After his death, his secretary and personal assistant Melitta Mew and her husband Raymond took charge of his estate. His manuscripts went to the Hoover Institution at Stanford University, partly in his lifetime and partly afterwards. In 1995 the University of Klagenfurt acquired his library, and the Karl Popper Archives was established there. It holds roughly 6,000 books, including his prized bibliophilic volumes, and is open to researchers. In October 2008 the university acquired the copyrights from the estate.

  8688949597