Austrian-born British philosopher of natural and social science who subscribed to antideterminist metaphysics, believing that knowledge evolves from experience of the mind.

Although his first book, Logik der Forschung (1934; The Logic of Scientific Discovery), was published by the Vienna Circle of logical positivists, Popper rejected their inductive empiricism and developmental historicism. After studying mathematics, physics, and psychology at the University of Vienna, he taught philosophy at Canterbury University College, New Zealand (1937–45). In 1945 he became a reader in logic at the London School of Economics, and he served there as professor of logic and scientific method from 1949 until his retirement in 1969.

Popper's principal contribution to the philosophy of science rests on his rejection of the inductive method in the empirical sciences. According to this traditional view, a scientific hypothesis may be tested and verified by obtaining the repeated outcome of substantiating observations. As the Scottish empiricist David Hume had shown, however, only an infinite number of such confirming results could prove the theory correct. Popper argued instead that hypotheses are deductively validated by what he called the “falsifiability criterion.” Under this method, a scientist seeks to discover an observed exception to his postulated rule. The absence of contradictory evidence thereby becomes corroboration of his theory. According to Popper, such pseudosciences as astrology, metaphysics, Marxist history, and Freudian psychoanalysis are not empirical sciences, because of their failure to adhere to the principle of falsifiability.

Popper's later works include The Open Society and Its Enemies (1945), The Poverty of Historicism (1957), and Postscript to the Logic of Scientific Discovery, 3 vol. (1981–82). He was knighted in 1965.