Karl Popper

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Sir Karl Raimund Popper (July 28 1902 – September 17 1994) was one of the most influential philosophers of science of the 20th century. He rejected the classical account of the scientific method, and proposed that empirical falsifiability should be the criterion for distinguishing science from non-science; he was also known for his defense of liberal democracy and the principles of social criticism which he thought was important for the "open society" to flourish.[1]

Popper wrote The Open Society and Its Enemies at the University of Canterbury (shown) in New Zealand, far from war-torn Europe

Popper's philosophy

Philosophy of science

Bold ideas, unjustified anticipations, and speculative thought are our only means for interpreting nature: our only organon, our only instrument for grasping her. And we must hazard them to win our prize. Those among us who are unwilling to expose their ideas to the hazard of refutation do not take part in the scientific game. (From "The Logic of Scientific Discovery)

Popper described his philosophy as critical rationalism, indicating his rejection of classical empiricism, and the observationalist-inductivist account of science that had grown from it. In his most influential book, the Logic of Scientific Discovery, he argued that scientific theories are universal, but can be tested only by the predictions that arise from them. He argued that scientific theory (and human knowledge generally) is always conjectural, generated by human imagination to solve problems that arise in particular historico-cultural settings. No number of positive outcomes of experimental tests can prove that a scientific theory is true, but just one counterexample is enough to prove it false. This logical asymmetry between verification and falsification led him to make falsifiability his criterion of demarcation between what is and is not scientific: a theory is scientific if, and only if, it is falsifiable.

Popper attacked psychoanalysis and Marxism as pseudoscientific because their explanations are not falsifiable. He also argued against the Copenhagen interpretation of quantum mechanics. He disagreed with Niels Bohr's instrumentalism and supported Albert Einstein's realist approach to scientific theories about the universe. 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, Popper tried to explain why our understanding of the universe seems to improve over time. If our theories can never be proved scientifically, how does the growth of science results in a growth in knowledge? In his view, knowledge advances in an evolutionary process characterised by:

In response to a given problem (), several competing theories (), are subjected to rigorous attempts at falsification. This process, error elimination (), performs for science what natural selection performs for evolution. Theories that better survive the process are not more true, but rather, more "fit"—more applicable to the problem situation at hand (). Consequently, just as a species' "biological fit" does not predict continued survival, no amount of testing will guarantee that a scientific theory will not be disproved in the future. Yet, just as biological evolution produces adaptive traits to deal with complex problems of survival, so the evolution of theories through the scientific method may reflect progress towards more interesting problems (). For Popper, it is in the interplay between the tentative theories (conjectures) and error elimination (refutation) that scientific knowledge advances, in a process akin to the interplay between genetic variation and natural selection.

Where does "truth" fit into this? In 1934 Popper wrote of the search for truth as one of the "strongest motives for scientific discovery". In Objective Knowledge (1972) he expressed concerns about the notion of truth as correspondence. Then came the semantic theory of truth formulated by the logician Alfred Tarski. Popper writes of learning in 1935 about the consequences of Tarski's theory, which met critical objections to truth as correspondence and 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, and the sentences themselves, are part of a metalanguage. For example, the sentence "Snow is white" is true if, and only if, snow is white. Popper refers to it as a theory in which "is true" is replaced with "corresponds to the facts". He bases this on examples such as the one described above which refer to two things: assertions and the facts. He identifies Tarski's formulation of the truth conditions of sentences as the introduction of a "metalinguistic predicate" and distinguishes between: "John called" is true, and "It is true that John called." The former belongs to the metalanguage while the latter is more likely to belong to the object language. Hence, "it is true that" has the logical status of a redundancy, while "Is true" is a predicate necessary for general observations such as "John was telling the truth about Phillip."

Popper went on to develop his notion of verisimilitude, the idea that scientific assertions can be measured by the amount of truth and falsity that they imply. By this, one theory can be evaluated as more or less true than another in a way independent of either "subjective probabilities" or "epistemic" considerations.

Popper gives a mathematical formulation of this in Conjectures and Refutations, where he defines it as:

where is the verisimilitude (or truthlikeness) of a, measures the content of truth of a, and measures the content of the falsity of a.

Knowledge, for Popper, was objective in that it is objectively true (or truthlike), and also in 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:

  1. World One, the phenomenal world, of direct experience;
  2. World Two, the world of mind, or mental states, ideas, and perceptions; and
  3. World Three, human knowledge expressed in all its forms, or the products of the second world made manifest in the materials of the first (–books, papers, paintings, symphonies, and all the products of the mind).

In his view, the influence of World Three on World Two is at least as strong as its influence of World One. In other words, the knowledge in an individual mind owes at least as much to the accumulated wealth of human knowledge as to direct experience. Thus, the growth of knowledge could be said to be a function of the independent evolution of World Three. Many contemporary philosophers have not embraced Popper's Three World conjecture, mainly because of its resemblance to Cartesian dualism.

Political philosophy

In The Open Society and Its Enemies and The Poverty of Historicism, Popper developed a critique of historicism and a defence of the 'Open Society' and liberal democracy. For Popper, the major question of political philosophy is not “Who should rule?”, but “How can we get rid of bad rulers?”. He saw democracy as the best answer to the second question, not the first, but if we are to get rid of unsatisfactory rulers through democratic processes, we must be able to criticize them, and that requires an open society.

Historicism, as Popper used the term, is the theory that history progresses by general laws towards a determinate end.[2]f Popper argued that this view underpins most forms of authoritarianism and totalitarianism, and that it is based on mistaken assumptions about the nature of scientific law and prediction. Because the evolution of human history depends on the growth of human knowledge, and because no society can predict its own future states of knowledge, he argued that it is not possible to make a predictive science from the study of human history.

Problem of induction

Among his contributions to philosophy is his answer to David Hume's Problem of Induction. According to Hume, just because the sun has risen every day, there is no rational reason to believe it will rise again tomorrow; we cannot know that a pattern will continue. Popper's response is that although we cannot prove that the sun will come up, we can theorize that it will. If it does not come up, then the theory will be falsified, but until then we can accept it. Thus, Popper's demarcation between science and non-science is an answer to an old logical problem as well. This approach was criticised by Peter Singer for masking the role of induction in empirical discovery.


Popper played a vital role in establishing the philosophy of science as an autonomous discipline within analytic philosophy. In 1946, he founded the Department of Philosophy, Logic and Scientific Method at the London School of Economics, where he influenced Imre Lakatos and Paul Feyerabend, two important philosophers of science in the next generation of analytic philosophy.

Popper had a long and close friendship with economist Friedrich Hayek, who also came to the London School of Economics from Vienna. In a letter to Hayek in 1944, Popper wrote, "I think I have learnt more from you than from any other living thinker, except perhaps Alfred Tarski." (See Hacohen, 2000). 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 also had long and influential friendships with art historian Ernst Gombrich, biologist Peter Medawar, and neuroscientist John Carew Eccles.

Among Popper's students at the London School of Economics was George Soros, who said that his investment strategies are modelled on Popper's understanding of the advancement of knowledge through falsification. Among Soros's philanthropic foundations is the Open Society Institute, a think-tank named in honour of Popper's The Open Society and Its Enemies, which Soros founded to advance the 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, as each is part of an environment of theories. Thus we might be able to say that the package of relevant theories has been collectively falsified, but cannot conclusively say which element must be replaced. For example when the motion of Uranus was found not to match the predictions of Newton's laws, Newton's laws themselves were not rejected; instead, the planet Neptune was postulated and the hypothesis that there are just seven planets in the solar system was rejected. Popper discussed this critique of naïve falsificationism in The Logic of Scientific Discovery. For Popper, theories that say more are preferred; the more generally applicable a theory, the greater its value. Thus Newton’s laws, with their wide general application, are preferred over the 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 that scientists followed a falsificationist methodology. 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 rejected any prescriptive methodology, and argued that the only universal method characterizing scientific progress was anything goes.

Popper had anticipated some of Kuhn's observations. In Conjectures and Refutations: The Growth of Scientific Knowledge, he wrote that "[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 using statistical criteria to evaluate a null hypothesis. More generally, it is not always clear that, when evidence contradicts a hypothesis, that this is a sign of flaws in the hypothesis rather than flaws in the evidence. However, this is a misunderstanding of what Popper's philosophy of science; in The Logic of Scientific Discovery he argues that the resolution of conflicts between hypotheses and observations can only be a matter of the collective judgement of scientists. [3]

Charles Taylor accuses Popper of exploiting his fame as an epistemologist to diminish the importance of philosophers of the 20th century continental tradition. According to Taylor, Popper's criticisms are baseless, but are received with an attention and respect that their "intrinsic worth hardly merits". [4]


Popper was born in Vienna (then in Austria-Hungary) in 1902 to middle-class parents of Jewish origins who had converted to Christianity. His father was said to have 10,000 volumes in his library at home.[5] He was brought up as a Lutheran, and studied at the University of Vienna.[6] He took a PhD in Philosophy in 1928, and taught in secondary school from 1930 to 1936.

In 1934 he published his first book, Logik der Forschung (The Logic of Scientific Discovery), in which he proposed his theory of potential falsifiability as 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 (at Christchurch). 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 president of the Aristotelian Society from 1958 to 1959. He was knighted in 1965, and was elected a Fellow of the Royal Society in 1976. He retired from academic life in 1969, but remained intellectually active until his death in 1994. He was invested with the Insignia of a Companion of Honour in 1982. Popper was a member of the Academy of Humanism; he described himself as an agnostic, but respected the moral teachings of Judaism and Christianity.

Popper won many awards and honours, 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 Cambridge. Austria awarded him the Grand Decoration of Honour in Gold.

Notes and references

  1. Watkins, J. Obituary of Karl Popper, 1902-1994. Proc Brit Acad 94:645–84
  2. This is what historians would call teleological determinism; they have a different definition of historicism.
  3. Popper, Karl (1934) Logik der Forschung, Springer. Vienna. Amplified English edition, Popper (1959)
  4. Taylor, Charles, "Overcoming Epistemology", in Philosophical Arguments, Harvard University Press, 1995
  5. Raphael, F The Great Philosophers London: Phoenix, p. 447
  6. Magee, Bryan The Story of Philosophy. New York: DK Publishing, 2001