Karl Popper

Karl Popper an Austrian and later British philosopher of science (1902-1994), one of the most influential thinkers of the twentieth century, whose criterion of falsifiability and defence of the open society shaped both the philosophy of science and political theory. He taught for many years at the London School of Economics.

Life and Education

Karl Raimund Popper was born in Vienna in 1902 into a family of Jewish descent that had converted to Protestantism. He studied at the University of Vienna, where he took a doctorate in philosophy in 1928, and he trained as a schoolteacher. He was briefly drawn to socialist politics and to the ideas of the Vienna Circle, though he soon became a sharp critic of their logical positivism.

With the rise of Nazism he left Austria, taking a position in New Zealand at Canterbury University College, where he wrote his most famous book. After the war he moved to England and joined the London School of Economics, becoming professor of logic and scientific method. He was knighted in 1965 and died in London in 1994.

He was a formidable polemicist and a generous teacher, and his seminar attracted students from many countries. His career spanned the central debates of twentieth-century philosophy of science, and he engaged directly with Einstein, Bohr and many leading thinkers of his time.

Falsifiability and the Logic of Science

Popper's central contribution to the philosophy of science is the criterion of falsifiability. He argued that a theory is scientific only if it makes predictions that could, in principle, be shown to be false by observation. A theory that cannot be refuted by any possible evidence is, on his view, not scientific at all.

This led him to reject the traditional view that science proceeds by confirming hypotheses. No number of observations can prove a universal law, but a single counterexample can refute it. Science therefore advances by bold conjectures and severe attempts at refutation, a method he called critical rationalism.

He used the criterion to distinguish the theories of Einstein, which risked refutation, from the doctrines of astrology and from some forms of psychoanalysis, which seemed to explain everything and thus forbade nothing. The Logic of Scientific Discovery, first published in German in 1934, set out this programme in detail.

The Open Society and Its Enemies

The Open Society and Its Enemies, written during the Second World War, is Popper's major work of political philosophy. It traces the origins of totalitarian thinking to Plato, Hegel and Marx, arguing that each offered a historicist doctrine according to which the future is knowable and the present must be sacrificed to it. He called this the conspiracy against the open society.

Against historicism he defended the open society, in which rulers can be removed without violence and institutions are designed to correct error rather than to enshrine a single truth. He argued that political philosophy should abandon the question of who should rule and ask instead how bad rulers can be restrained and their mistakes corrected.

He advocated piecemeal social engineering, the gradual reform of institutions guided by experience, in place of utopian schemes that promise wholesale transformation. His critique of Plato has been much contested, but the book became a classic defence of liberal democracy and of the value of criticism.

Epistemology and Metaphysics

Popper developed a theory of knowledge without a knowing subject, in which the growth of knowledge is studied as a product of public criticism rather than of private mental states. He later introduced the idea of three worlds: the physical world, the world of mental states and the world of objective contents of thought, such as theories and problems.

He defended metaphysical realism and the common-sense belief in a world that exists independently of observers. Though metaphysical statements are not empirically testable, he held that they can be rationally discussed and that they often guide scientific inquiry. He also proposed a propensity interpretation of probability, treating chances as real tendencies in the world.

His epistemology emphasised the value of bold speculation and the openness to correction. For Popper, the mark of rationality was not the certainty of one's beliefs but the willingness to expose them to criticism and to revise them when they fail.

Influence and Legacy

Popper's ideas reshaped the philosophy of science and are still taught as a starting point for debates about method. His emphasis on falsification influenced working scientists, including several who cited him as an inspiration, and it remains a central reference in discussions of scientific evidence and pseudoscience.

His political thought, though written in wartime, proved durable. The Open Society became a rallying text for defenders of liberal institutions, and his arguments against historicism and utopian planning were taken up by critics of collectivism in economics and politics. His notion of the open society has entered general political vocabulary.

He was also a critic of his contemporaries, most famously of Thomas Kuhn, whose account of paradigm change he thought blurred the line between science and dogma. He insisted to the end that criticism, not consensus, is the engine of knowledge, and his defence of fallibility remains a lasting contribution to thought.

See also

• Falsifiability
• Open society
• Critical rationalism
• Philosophy of science