Karl Popper

Sir Karl Popper was an Austrian-born British philosopher, one of the most influential thinkers about science and politics in the twentieth century. He argued that scientific theories can never be proved true, only tested and possibly refuted, so that what marks science out is that it could in principle be shown false. In politics he defended what he called the open society, a democracy that allows its policies to be criticised and changed, against the closed, planned societies promised by fascism and communism.

Popper was born on 28 July 1902 in Vienna, then capital of Austria-Hungary. His parents were of Jewish origin but had converted to Lutheranism; his father was a lawyer with a large library, his mother a musician. After the First World War Vienna was poor and politically divided. As a teenager Popper was briefly drawn to Marxism, but in 1919 he was shocked when several young socialists were shot during a demonstration, and began to doubt a theory that seemed to justify sacrifices in the name of a future history had supposedly already decided.

He worked as a cabinetmaker's apprentice and with Alfred Adler's child guidance clinics, studied at the University of Vienna and in 1928 took a doctorate in psychology under Karl Bühler. He qualified as a secondary-school teacher of mathematics and physics and taught while writing philosophy in the evenings. He was close to, but never a member of, the Vienna Circle, the group of logical positivists around Moritz Schlick, and his first book was largely an argument against them.

That book, Logik der Forschung, appeared in 1934; it was published in English, much expanded, as The Logic of Scientific Discovery in 1959. Popper attacked the old idea, going back to Francis Bacon and challenged by David Hume, that science proceeds by induction, piling up observations until a general law is confirmed. No number of white swans proves that all swans are white, but a single black swan refutes the claim. Science therefore advances by bold conjectures and attempted refutations: a theory is scientific if it forbids certain things and could be falsified by observation.

He drew the contrast vividly. Albert Einstein's general relativity made risky predictions about the bending of starlight that could have failed in 1919 and did not. By contrast, he argued, the theories of Karl Marx as his followers used them, and the psychoanalysis of Sigmund Freud and Adler, could explain any outcome at all, which made them look strong but in fact meant they were not science. Popper did not say such ideas were meaningless, only that they were not testable. This "criterion of demarcation" became one of the best-known ideas in the philosophy of science.

Fearing the rise of Nazism, Popper looked for work abroad. In 1937 he became a lecturer at Canterbury University College in Christchurch, New Zealand, shortly before the Nazi annexation of Austria in 1938. During the Second World War he wrote what he called his war effort, The Open Society and Its Enemies, published in 1945, and a series of essays later collected as The Poverty of Historicism (1957).

In these works Popper attacked "historicism", the belief that history follows laws that allow its course to be predicted. He traced this belief, and the authoritarian politics it encouraged, through Plato, who designed an ideal state ruled by philosopher-kings, Georg Wilhelm Friedrich Hegel and Marx. His treatment of Plato and Hegel was fierce, and many scholars thought it unfair, but the book's positive argument was widely admired. Since no one can know enough to plan a whole society, he argued, reform should be "piecemeal", testing policies and correcting mistakes, and the real test of a democracy is that citizens can remove their rulers without bloodshed. He also stated the "paradox of tolerance": a society that tolerates those who would destroy tolerance by force may lose it.

In 1946 Popper moved to the London School of Economics, where he became professor of logic and scientific method in 1949 and taught until his retirement in 1969. In October 1946 he had a famously heated encounter with Ludwig Wittgenstein at a Cambridge seminar, at which Wittgenstein is said to have waved a fireplace poker; accounts of what happened differ. Popper believed philosophy dealt with real problems, not puzzles about language.

His later books include Conjectures and Refutations (1963), Objective Knowledge (1972), which set out his theory of three "worlds" (physical things, mental states, and the products of the mind such as theories and arguments), and The Self and Its Brain (1977), written with the neuroscientist John Eccles, whom he had met in New Zealand. He developed an evolutionary account of knowledge, in which theories compete and the weaker are eliminated, and defended a "propensity" interpretation of probability. In the 1960s he took part in a long dispute with the Frankfurt School over the methods of the social sciences. He was knighted in 1965 and became a Companion of Honour in 1982.

Popper's account of science has been popular among working scientists, and his name is often invoked when a claim is called "unfalsifiable". Philosophers have been more critical. His former student Imre Lakatos and the historian Thomas Kuhn argued that scientists do not abandon theories at the first failed test, and that real science moves through longer programmes or paradigms. Others pointed out that any test depends on further assumptions, so a refutation is never as clean as Popper suggested. His student Paul Feyerabend went further and rejected the idea of a single scientific method altogether.

His political ideas had a wide influence beyond universities. The financier George Soros, who studied under him at the LSE, named his Open Society Foundations after the book, and in the last years of the Cold War Popper was read by dissidents in central and eastern Europe. He spent his last years in Kenley, on the southern edge of London, and died there on 17 September 1994, aged 92. He belongs in the history of analytic philosophy, though he always stood somewhat apart from it.