Karl Popper (1902–1994) was an Austrian-born British philosopher whose work addressed philosophy of science, epistemology, and political philosophy. He proposed falsifiability as a criterion distinguishing empirical science from non-science and developed critical rationalism, an approach emphasizing criticism rather than the establishment of certain foundations. His political writings defended the open society, connecting institutional freedom with the possibility of identifying and correcting errors. (lse.ac.uk)
Life and intellectual background
Karl Raimund Popper was born in Vienna on 28 July 1902. He studied at the University of Vienna and received his doctorate in 1928, with a dissertation concerning the methodology of the psychology of thinking. His early interests included mathematics, theoretical physics, and psychology. Although he interacted with members of the Vienna Circle, he did not belong to it and opposed important features of its philosophical program. (plato.stanford.edu)
In 1937, Popper moved to Christchurch, New Zealand, to teach at Canterbury University College. During World War II, he worked on the political writings that became The Open Society and Its Enemies. In 1946 he joined the London School of Economics; in 1949 he became professor of logic and scientific method. He was knighted in 1965 and retired in 1969, but continued writing and lecturing. He died on 17 September 1994. (teara.govt.nz)
Falsifiability and scientific demarcation
Popper’s demarcation problem concerns what distinguishes empirical scientific theories from other kinds of claims. His answer was not that scientific theories have been conclusively verified, but that they prohibit possible observations: evidence could, in principle, conflict with them. A theory compatible with every conceivable outcome lacks this empirical vulnerability. (iep.utm.edu)
The distinction rests on an asymmetry in logic. Observing many white swans cannot establish that every swan is white, whereas an accepted observation of a non-white swan contradicts that universal assertion. Falsifiability is therefore a property of a theory’s relation to possible evidence, not a claim that the theory has already been disproved. (iep.utm.edu)
Popper regarded Albert Einstein’s theory of relativity as exemplary because it exposed itself to demanding observational tests. He contrasted this with explanatory systems that, in his interpretation, could accommodate any outcome through reinterpretation. Nevertheless, non-falsifiability did not automatically mean meaninglessness or worthlessness: ideas in metaphysics could contribute to scientific inquiry without themselves qualifying as empirical science. (iep.utm.edu)
Induction and critical rationalism
Popper accepted David Hume’s challenge to the logical justification of inductive reasoning: observed regularities do not deductively establish universal laws or guarantee future occurrences. His response to the problem of induction was to deny that science requires such justification. Scientists propose tentative explanations and use deductive reasoning to derive consequences that can be tested. Knowledge can improve without becoming certain. (plato.stanford.edu)
Critical rationalism generalizes this approach. Rationality consists in exposing proposals to objections and comparing alternatives, rather than appealing to an unquestionable authority. It incorporates fallibilism: even well-supported beliefs and accepted observations remain open to correction. Criticism need not overturn everything simultaneously; inquiry proceeds using provisional background assumptions. (iep.utm.edu)
A successful test provides corroboration, Popper’s term for a theory’s performance under attempted criticism, rather than proof of its truth. Actual testing is also more complicated than the swan example suggests. Predictions depend on instruments, initial conditions, and auxiliary assumptions. An unexpected result therefore requires investigation of the testing arrangement as well as the theory. Popper opposed modifications introduced solely to protect a theory from criticism, while recognizing that revisions can be scientifically legitimate. (iep.utm.edu)
Objective knowledge and truthlikeness
Popper defended the idea that scientific inquiry seeks truth, although investigators may never know conclusively that they have attained it. His concept of verisimilitude, or truthlikeness, attempted to explain how one imperfect theory could nevertheless improve upon another. His original formal definitions encountered mathematical objections, but the problem of comparing theories’ proximity to truth remained influential. (plato.stanford.edu)
In his three-worlds theory, Popper distinguished physical objects and processes, subjective mental experiences, and objective intellectual contents such as arguments, theories, and problems. The third category was not simply the physical books containing statements or the experiences of their readers: intellectual products could possess implications not recognized by their creators. (plato.stanford.edu)
Political thought
The Open Society and Its Enemies (1945) and The Poverty of Historicism (1957) presented Popper’s opposition to historicism, understood as the belief that historical development follows discoverable laws permitting large-scale prediction. He argued that the growth of knowledge affects history, while future discoveries cannot themselves be predicted in advance. (iep.utm.edu)
His account of democracy emphasized institutions enabling rulers to be removed without violence, rather than the identification of uniquely qualified rulers. He favored limited, revisable reforms addressing specific problems over comprehensive attempts to reconstruct society according to a predetermined ideal. These positions extended his concern with error correction into political institutions. (iep.utm.edu)
Major works and reception
Logik der Forschung, issued in 1934 with a 1935 publication date, appeared in English as The Logic of Scientific Discovery in 1959. Other major works include Conjectures and Refutations (1963), Objective Knowledge (1972), and the intellectual autobiography Unended Quest (1976). (plato.stanford.edu)
Popper’s methodology became a reference point for debates involving Thomas Kuhn, Imre Lakatos, and Paul Feyerabend. Disputes concerned whether falsification adequately describes scientific practice, how auxiliary assumptions should be assessed, and whether corroboration can guide theory choice without inductive reasoning. His influence thus includes both the adoption of critical testing as an ideal and sustained challenges to its philosophical formulation. (iep.utm.edu)