Obituary: Professor Sir Karl Popper

Karl Raimund Popper, philosopher: born Vienna 28 July 1902; Senior Lecturer, Canterbury University College, Christchurch, New Zealand 1937-45; Reader in Logic and Scientific Method, London School of Economics 1945-49, Professor of Logic and Scientific Method 1949-69 (Emeritus); FBA 1958; President, Aristotelian Society 1958- 59; President, British Society for the Philosophy of Science 1959-61; Kt 1965; FRS 1976; CH 1982; Guest Professor in the Theory of Science, University of Vienna 1986-94; Senior Research Fellow, Hoover Institution, Stanford University 1986-94; author of Logik der Forschung 1934 (revised 1966, 1984), The Open Society and its Enemies 1945 (revised 1966), The Poverty of Historicism 1957, The Logic of Scientific Discovery 1959, On the Sources of Knowledge and of Ignorance 1961, Conjectures and Refutations 1963, Of Clouds and Clocks 1966, Objective Knowledge 1972, Unended Quest: an intellectual autobiography 1976, The Self and its Brain (with Sir John Eccles) 1977, Postscript to the Logic of Scientific Discovery (edited by WW Bartley) 1982-83, A Pocket Popper 1983, Popper Selections 1985, A World of Propensities 1990; married 1930 Josefine Anna Henninger (died 1985); died Croydon 17 September 1994.

KARL POPPER was one of the most widely read philosophers of the century. He achieved both professional eminence and vast popular esteem. Among philosophers he was known for his unremitting resistance to the simplistic reductionism of the Positivists. Among scientists he was admired for the clarity of his call for a rigorous rationality based on the unsentimental search for what would tend to prove one's pet ideas mistaken. But he was best known outside professional circles for his subtle and passionate defence of the democratic way against the tyranny of the state. His book The Open Society and its Enemies, published in 1945, became a modern classic.

Popper believed that no one could possibly know how societies will or even could develop, so that people should proceed in all practical affairs with the same cautious rationality as is employed in science; that a step-by-step tinkering with what we know already works is the best way to improve human affairs. In many ways Popper can be read as the champion of the individual against the massed forces of society. Rightly or wrongly, the more conservative cast of politicians have found inspiration in his philosophy and claimed him as their own.

Popper was Professor of Logic and Scientific Method at the London School of Economics from 1949 to 1969, and a visiting fellow or professor at countless institutions in the United States and elsewhere. Everyone who knew him was struck by the contrast between his somewhat abrasive manner in public and his hospitable amiability in private. He was small in stature, but his enthusiasm allowed him to dominate and enthral large audiences. He was an inspired teacher because of this passionate attachment to his ideas and his conviction of their importance. His most abiding legacy was to a generation of gifted pupils whose work, even if often animated by opposition to Popper himself, owed much to the stimulus of his doctrines and opinions. Among them was the late Imre Lakatos, who developed his own version of Popper's fallibilism which became an influential way of viewing the philosophy of scientific programmes.

Popper was born in Vienna in 1902. His father was a barrister, and the Popper family were part of the cultivated bourgeoisie which contributed so much to Viennese life and indeed to European civilisation. Alfred Einstein, the musicologist, was a close relative. Before emigrating to New Zealand in 1937, Popper held a variety of posts, serving for a while as an assistant to the psychologist Alfred Adler, who believed that there was more to the underpinnings of the human personality and its vagaries than the sexual impulse. Popper remained at Canterbury University as a Senior Lecturer in Philosophy until 1945. He returned to Europe to a post at the London School of Economics, where he remained until retirement. In his old age he spent some time in Vienna but eventually he ended his days in England, living in High Wycombe, in Buckinghamshire, and latterly in Purley, in Surrey. He was knighted in 1965, and had the honour, shared only with the historian Margaret Gowing, of being both a Fellow of the British Academy and of the Royal Society.

Popper's two most influential works, The Logic of Scientific Discovery (1959) and Objective Knowledge (1972), contain the essence of his philosophy, which is based upon two fundamental ideas. One is the assimilation of rationality to logic, and the other is that various kinds of human activities ought to be assessed by the degree to which they can be said to be 'scientific'. In both these respects, Popper's contributions to philosophy can be seen as having their roots in the traditional interests of those philosophers who flourished in Vienna in the first quarter of the 20th century, in particular Rudolf Carnap and Otto Neurath. Though Popper was never a member of the Vienna Circle and differed profoundly from them in one important respect, in his conception of rationality and in his belief in the possibility of cleaning up the Augean stables of human thought with the broom of science, his work stands very much in the same tradition.

The philosophers of the Vienna Circle, in endeavouring to define a formal test to judge whether an expertise should count as science and thus achieve scientific respectability, proposed the criterion of verification. If there were no way in which a claim could be empirically verified, then such a claim should be rejected as senseless.

Popper realised that there was a profound difficulty in applying this criterion within the strict framework of logic, a framework to which the members of the Vienna Circle were committed. If the claim under consideration purported to be a universal law, its scope was potentially infinite. Yet any positive evidence that could be gathered in its favour could never be enough for verification. Popper proposed that, instead of the criterion of verification, we should test hypotheses by looking for evidence against them. Only with a criterion of falsifiability can we select that which is truly worthy to be counted as 'scientific'. Positive evidence in favour of a hypothesis should be interpreted, from a logical point of view, as a failure to falsify. We can corroborate but not confirm scientific conjecture. It is easy to see the influence of a logical conception of rationality on this idea. Scientific conjectures are, according to Popper, nothing but guesses, scientific progress exists in the elimination of wrong guesses by finding evidence that contradicts some of their consequences. The method of science is 'conjecture and refutation'.

The idea of privileging negative evidence is an old one in the philosophy of science. It played an important part in Francis Bacon's methodology, just as much as in that of John Stuart Mill. However, Popper took his proposal much further in boldly claiming that his view of scientific method could solve at least one perennial philosophical problem, the problem of induction, particularly as it was formulated by David Hume. The problem is very simply stated. There can be no rational grounds for scientific knowledge, that is, general knowledge of the natural world, because we can have no positive evidence now that the world will not change in some fundamental way in the future, rendering all our present knowledge worthless. By shifting the focus of scientific method to the falsification of conjectures, Popper announced that he had resolved Hume's problem.

Unfortunately, the arguments that inductive sceptics used against drawing universal conclusions from locally valid scraps of positive evidence are equally telling against Popper's proposals for the use of negative evidence to advance the state of science. In thinking about the way in which we may justify the claim that limited positive evidence gives us ground for believing in the truth of a law of nature, philosophers have noticed that the reasoning could be made strictly deductive, and so conclusively, by adding an additional premiss. The premiss is the assertion that 'nature' is everywhere and at all times the same. Were we sure that nature was uniform, then the evidence we have accumulated here and now for a law would be enough to support its universal generalisation. But how can we justify our belief in the uniformity of nature? Perhaps we could gather some evidence to see if the hypothesis is true. But, to use that evidence in support of the truth of the claim would itself require the assumption of the uniformity of nature. So the argument is self-defeating.

However, Popper's methodology of conjecture and refutation, based upon the idea of the rationality of rejecting hypotheses which have been shown at a particular time and place to be false, also depends upon an assumption of a form of the uniformity of nature. In his case, it is the negative assumption that the universe will not change in such a way as to make what was disconfirmed today true tomorrow. Popper's methodology of conjecture and refutation makes no headway in the testing of that proposition. His claim to have solved the problem of induction must now be rejected.

There are other difficulties with Popper's attempt to construct a rational philosophy of science. Two spring to mind immediately. The first has to do with the important question of realism. There are various ways of expressing realist ideals for science, but they boil down to the idea that there is scientific progress - progress toward a better grasp of the way the world really is. Popper, unlike his positivistic Viennese compatriots, declared himself a realist. He was faced with the problem of showing how his methodology of conjecture and refutation could lead to a succession of theories each replacing a refuted rival that would lead inexorably to better knowledge of the natural world. He proposed various ways of defining increase in verisimilitude, as he called it. None was satisfactory. Indeed, it now seems that the idea upon which he based his conception of realism, namely, a correspondence between theory and the world, is a concept which is very hard to sustain.

In reflecting on the many aspects of Popper's philosophy of science which stem from his conviction that the underlying problems of philosophy were somehow rooted in logic, and his lifelong attempt to maintain a kind of scientific purism, one is puzzled by the way many distinguished scientists thought that Popper had provided them with the key to scientific method. It is indeed strange that a philosopher who denied the possibility of a rational and disciplined procedure for formulating theories, inspired by his adherence to the rigorous demands of traditional logic, should have appealed to physical and biological scientists whose ways of imaginatively penetrating nature and whose use of the discipline of models and analogies have been very thoroughly described by philosophers unsympathetic to Popper's conviction that rationality is exhausted by logic.

Not only did Popper write extensively on the philosophy of physical sciences, he also published two influential works which were concerned with the limits of methods of enquiry into the nature of human society. In The Poverty of Historicism (1957), he critically discussed a certain important strand in social science, while in The Open Society and its Enemies (1945; revised in 1966) he investigated the long history of attempts to formulate a theory of the state.

In both these works, the influence of Popper's Viennese background is evident. Like the members of the Vienna Circle, he was animated by a dislike of the views of Marx and of Freud. He detected a common flaw in the works of these two most influential thinkers. He believed that he could show that their hypotheses about hidden social and psychological processes were falsifiable and, consequently, unscientific. To declare a hypothesis unscientific was to condemn it to oblivion. But Popper went further. He saw in the enthusiasm for the idea of historical necessity and social laws, the roots of dangerously unworkable proposals for constructing blueprints for the reconstruction of society. Many such blueprints have, in our day, been shown to be not only unworkable but in the end morally untenable, however noble the intentions of their authors. Applying the same frame of thought to political and social matters as he had to the physical sciences, Popper attempted to show that the ideal of an open society could be expressed in much the same terms as that of the progress of science, namely by the piecemeal rejection of unsatisfactory hypotheses, or, as one might put it politically, of unworkable programmes.

Just as many criticisms have beset Popper's conception of the social sciences as his views about the physical have attracted. A major difficulty can be put in terms of a distinction which fits snugly against his criticism of Marx and Hegel. Their large-scale proposals for identifying lawful patterns of historical change presuppose the intelligibility of a collectivist view of society and of a social science, be it historical or contemporary, which purports to give reliable information about the nature, structure and development of such collectives. Agnostic on the question of whether there are collective social properties - he thought they might arise through a flux of unintended consequences of individual human actions - Popper nevertheless insisted upon a methodological individualism, that is, that all we could ever know about social processes would be the plans, projects, dispositions and beliefs of individual human beings. All the rest was speculation and thereby subject to his critical methodology of conjecture and refutation.

Karl Popper's philosophy was all of a piece. It involved the application of what was an essentially simple formula to a wide variety of cases. In this, it was vulnerable, as all such schemes are, if the general formula upon which the specific treatments are based is in some way defective. Looking back on the substantial corpus of work that Popper produced in accordance with his formula, one has a sense of disappointment. But I think the failure of almost all of Popper's projects nevertheless is a kind of success. He was the last of the great logicians, the most systematic, the most consistent and the most ruthless in pushing a logical programme of philosophical research through. The failure of Popper's projects shows us in a dramatic way the limitations of the rationalist ideal when it is worked out in terms of logic. Human beings do, I believe, use rational procedures but they have to be understood in relation to much richer patterns of thought and language than can be captured in the patterns of the traditional logics of truth and falsity.

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