Karl Raimund Popper was an Anglo-Austrian thinker who is generally considered to be one of the most influential philosophers of science of the 20th century. He was also a prominent libertarian, who offered a trenchant defense of the basic principles of liberal democracy and of market economics at a time when both were under global threat from fascism and communism. He first rose to prominence in the English-speaking world with the publication of The Open Society and Its Enemies (1945), a work that he described as his “contribution to the war effort.” The publication of later works, such as The Poverty of Historicism (1957) and The Logic of Scientific Discovery (1959), greatly enhanced his reputation as a thinker of originality and subtlety, whose radical views opened up new possibilities for our understanding of the nature of science. His name still arises regularly in discussions of scientific method.
Popper was born in Vienna, Austria, to middle-class parents of Jewish descent, and he completed his doctorate on the psychology of learning at the University of Vienna in 1928. He worked for a time as a schoolteacher but was also an active participant in the vibrant intellectual life for which Vienna was then world famous. Popper was greatly influenced by some of the most prominent figures in science, psychology, and philosophy at the time, including Sigmund Freud, Karl Marx, Alfred Adler, and Albert Einstein. He also greatly admired the famous Vienna Circle of logical positivists that formed around Moritz Schlick when he was appointed professor of the philosophy of inductive sciences at the University of Vienna in 1922. However, Popper was never offered membership in the intellectually prestigious circle, something he undoubtedly coveted, and he became increasingly critical of both the aims and methods of logical positivism. In later years, Popper was to claim that he had been responsible for the death of the movement.
The rise of Nazism led Popper to emigrate to New Zealand in 1937 for a position at Canterbury University College, and while residing there he published The Open Society and Its Enemies. He returned to Europe in 1946 to teach at the London School of Economics, where he was appointed professor of logic and scientific method in 1949. From there he exerted a very significant influence on the subsequent development of the philosophy of science, particularly after the publication of The Logic of Scientific Discovery (1959), which brought Popper's innovative ideas on the nature of scientific knowledge into prominence.
Short in stature, Popper was a brilliant but at times rather vain and pugnacious thinker whose principal concern was with the nature of human rationality as it manifests itself in science and in political life. In Britain, as in Vienna, he found himself largely relegated to the periphery of a philosophical movement (usually termed analytic philosophy) dominated by the figure of fellow Austrian émigré Ludwig Wittgenstein, whom Popper met only once but whom he viewed, nonetheless, as his life-long nemesis.
Popper served as president of the Aristotelian Society from 1958 to 1959 and was the recipient of many honors, including a knighthood, Fellowships from the Royal Society and the British Academy, the Austrian Grand Decoration of Honour in Gold, the Lippincott Award of the American Political Science Association, and the Sonning Prize for merit in work that had furthered European civilization. He retired from academic life in 1969, though he remained influential and intellectually active until his death in 1994.
Popper described his philosophical position as critical rationalism, thus placing it in the tradition of such thinkers as Descartes, Gottfried Leibniz, and, above all, Immanuel Kant on the key issue of the origins of human knowledge. The phrase also signaled his rejection of the classical empiricism of Francis Bacon, John Locke, David Hume, and the account of scientific knowledge that had grown out of it, termed inductivism or observationalism. Inductivists hold that knowledge is inferred from experimental observation by a process of inductive reasoning that supposedly leads to the verification of hypotheses and their transmutation into the universal laws of science—a view to which no less a scientific authority than Isaac Newton had subscribed.
Popper argued against this view on the grounds that (a) all observation presupposes theory, and is itself consequently theory laden. The “pure” or theory-free observations that the inductivist presupposes do not exist and cannot therefore function as the starting point in scientific investigation; (b) as Hume had argued, inductive reasoning is formally invalid because in such reasoning the conclusion goes beyond the evidence contained in the premises; and (c) it is in any case logically impossible to verify the universal statements of science.
On the positive side, Popper emphasized the role of the trial-and-error method in all of our learning from experience and considered that it was underpinned by a crucially important logical difference between verification and falsification. For although observation cannot establish that a universal or lawlike proposition is true, it can (Popper argued) prove it false by finding an exception. This “logical asymmetry” between verification and falsification was, for Popper, elementary but fundamental, and he accordingly proposed that a system of theories is scientific only if it is refutable or falsifiable.
Rejecting the role given to observation by the inductivists, Popper argued that the theoretical systems of science are not derived from experience at all. He also held that they cannot be confirmed by experience: The notion of strict proof in science is a fallacious extrapolation from pure mathematics. For Popper, by contrast, scientific theories are irreducibly conjectural or hypothetical and are generated by the creative imagination to solve problems; science, he famously held, like the rest of human life, is essentially problem solving. Popper did, however, acknowledge the functional importance of experience: The critical role of experimental testing in science, he held, is to show us which theories are false, not which theories are true, though a theory that has successfully withstood critical testing is thereby said to be “corroborated” and may be regarded as being rationally preferable to its falsified rivals.
Popper made a distinction between this, the logic of falsifiability, and its applied methodology. He recognized that no observation is free from the possibility of error, and that in actual scientific practice, theories are often retained in the face of conflicting empirical evidence. He accordingly professed the need to embed falsifiability in a methodology, a set of rules for assuring that scientific statements are falsifiable. Such methodological rules have a critically important role to play in science: They prohibit as unscientific ad hoc theory alteration designed exclusively to protect the theory from falsification. Here Popper did not wish to suggest that the alteration or modification of a theory cannot be permitted in science, as there are circumstances in which it is both necessary and desirable. But an alteration designed simply to preserve the theory from falsification is to be regarded as an unscientific “immunising strategy.”
This “theory of demarcation,” as Popper sometimes termed it, formed the basis for his rejection of the claims of both psychoanalysis and contemporary Marxism to scientific status. Psychoanalytic theory, Popper argued, is couched in such general terms that it precludes the possibility of falsification. The fact that any form or pattern of human behavior is apparently explicable in terms of the theory entails that it is not “prohibitive” (that is, it rules nothing out) and so not genuinely empirical or falsifiable. By contrast, Popper held that Marxism in its original form was predictive and therefore genuinely scientific; however, it had subsequently been reformulated in an ad hoc manner to make it consistent with facts that would otherwise have proven it false. In conducting this maneuver, contemporary Marxists had given a “conventionalist twist” to the theory; and by this stratagem they had destroyed its much-advertised claim to scientific status.
Popper saw the growth of human knowledge as a dynamic process in which rival or conflicting theoretical systems are put to the test and in which preference is given to those theories that best withstand such critical evaluation. It was, therefore, perhaps inevitable that he would come to view this in terms that were explicitly Darwinian, as a competitive struggle that eliminates unfit hypotheses— parallel to Darwin's idea of natural selection.
This led Popper to advance an evolutionary epistemology or theory of knowledge in which he represented human knowledge as an extension of the biological processes of adaptation. Like Socrates before him, Popper emphasized the importance of learning from error. For him, the falsification of even a deeply cherished scientific theory should not be viewed in a negative way or taken as grounds for skepticism. Rather, he argued, it should be seen as a positive contribution to human knowledge, which precipitates and guides the search for better, more explanatory theories.
In both The Open Society and Its Enemies and The Poverty of Historicism, Popper offered his defense of liberal democracy and identified “historicism” as the central theoretical assumption underpinning forms of political authoritarianism in both the ancient and contemporary worlds. By “historicism” he meant the view that history has, as it were, a “plan”; that historical processes have a logical structure and are moving inexorably toward a predetermined end, the so-called goal of history. This view he held to be perniciously harmful, and he believed it to be particularly clearly evident in the social and political theories of Plato, Georg W. F. Hegel, and Marx. Associated with it, he held, is the view that the principal task of the social sciences is predictive, the delivery of knowledge of future social and political events and processes.
Popper attacked both doctrines by arguing that they are founded on a mistaken view of the nature of prediction in natural science. On this mistaken view, the unconditional prediction of events such as eclipses is wrongly taken as typical, when in fact they are relatively rare and apply only to systems (such as our solar system) that are isolated from others and internally recurrent. Moreover, because historical processes are partially determined by the growth of human knowledge (as witnessed, for example, by the development of different forms of technology), and because it is impossible in principle to predict 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. On the positive side, Popper had a strong ethical commitment to negative utilitarianism (the minimization of harm) and—influenced in part by the economist Friedrich Hayek, who worked with him at the London School of Economics and who was a lifelong friend—was strongly affirmative of the ideals of individualism, of market economics, and of liberal democracy, in which the key right of citizens is to change the ruling order through the electoral process.
Popper has had no shortage of critics, ranging from classicists who contend that his interpretations of Plato and Aristotle are ill grounded, to those who seek to justify the claims of psychoanalysis or of Marxism to scientific status. However, perhaps the single most serious challenge to him comes from the thought of the American philosopher of science Thomas Kuhn. Kuhn argued that what he termed “normal science” is essentially a highly conservative activity, in which the scientist tries to understand nature by fitting into a number of conceptual boxes supplied by a preexistent “paradigm” or worldview. Normal science, Kuhn argued, can operate only if the validity of that worldview is assumed; accordingly, for him, Popper's idea that science operates through critical attempts to falsify theories is fundamentally mistaken.
Imre Lakatos attempted a reconciliation of Popper's position with that of Kuhn by arguing that scientific theories are abandoned, not as a result of falsification, but rather as a consequence of the research programs with which they are associated becoming unproductive or “degenerative.” Paul Feyerabend came to repudiate all rationalistic accounts of scientific methodology, championing the anarchistic slogan “anything goes.”
It is arguable that, within contemporary philosophy of science, the consensus would lie in the direction of Lakatos's approach: While generally conceding the logical significance of falsifiability, few thinkers would now give it the status attributed to it by Popper. However, even Popper's most severe critics would acknowledge the dominant, agenda-setting position occupied by him in the philosophy of science since the decline of positivism, and few would question his status, along with Bertrand Russell, as one of the 20th century's most committed and passionate rationalist thinkers.
Note: Portions of this entry were adapted from Thornton, S. (2006). Popper, Karl. In M. Cohen (Ed.), Essentials of philosophy and ethics (pp. 231-234). London: Hodder Arnold.
Deductive Logic, Inductive Logic, Kuhn, Thomas, Logical Positivism, Scientific Method
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