Karl Popper
1902 CE – 1994 CE · Contemporary Era
“Science advances through falsification, not verification: and open societies require free criticism.”
Biography
Popper argued that the hallmark of genuine science is falsifiability, a theory must be testable and potentially refutable. Unfalsifiable theories (like Marxism or Freudianism, he argued) are not scientific. In political philosophy, he championed the open society against authoritarianism of all kinds.
Major Works
Key Arguments
Click “Philosophy 101” to read the full exploration of each argument.
Falsificationism
Popper argued that the hallmark of a genuinely scientific theory is not that it can be confirmed but that it can be falsified, that there is some possible observation that would prove it wrong. Einstein's general relativity predicted that light bends around massive objects; if the 1919 eclipse observations had shown no bending, the theory would have been refuted. That is what makes it scientific. By contrast, Popper argued, theories like Marxist history and Freudian psychoanalysis are structured so that every possible observation can be interpreted as confirming them, which means they are unfalsifiable and therefore not scientific. Science advances not by accumulating confirmations but through bold conjectures and rigorous attempts at refutation. A theory that survives many attempts to falsify it is 'corroborated', but never proven, because the next test might refute it.
Why it matters: Reshaped the philosophy of science and remains the starting point for every discussion of scientific methodology. Popper's demarcation criterion, falsifiability as the line between science and non-science, is taught in virtually every introductory philosophy and methodology course. While subsequent philosophers (Kuhn, Lakatos, Feyerabend) have complicated the picture, falsificationism remains the most influential single account of what makes science science.
The Open Society and Its Enemies
Popper argued that the great totalitarian movements of the 20th century, fascism and communism, have their roots in a philosophical tradition stretching back to Plato. Plato's vision of a perfect society ruled by philosopher-kings, Hegel's dialectical historicism claiming to know the direction of history, and Marx's 'scientific' prediction of inevitable communist revolution all share a fatal error: they claim to know the future course of history and are willing to sacrifice individuals to realize their vision. Popper called this 'historicism' and argued it is both intellectually bankrupt (we cannot predict the future course of human knowledge, and therefore cannot predict the future course of history) and morally dangerous. The alternative is the 'open society', one that relies on piecemeal social engineering, free criticism, democratic institutions, and the willingness to learn from mistakes rather than implementing utopian blueprints.
Why it matters: A major work of 20th-century political philosophy. Written during World War II, The Open Society and Its Enemies provided the philosophical foundation for liberal democratic resistance to totalitarianism. Its critique of utopian political thinking influenced Cold War liberalism, and its defense of incremental reform over revolutionary transformation remains central to democratic theory.
The Problem of Induction and Conjectures
Hume argued that induction, inferring general laws from particular observations, cannot be logically justified. Popper accepted Hume's conclusion but argued it does not matter, because science does not actually proceed by induction. Scientists do not start with observations and generalize; they start with problems, propose bold conjectures (hypotheses), and then try to refute them through observation and experiment. The conjectures come from imagination, not from data. The role of observation is not to confirm theories but to test them, to try to prove them wrong. Knowledge grows not by verifying hypotheses but by eliminating false ones. This 'conjectures and refutations' model solves Hume's problem by showing that science never needed induction in the first place.
Why it matters: Popper's solution to the problem of induction is an elegant move in 20th-century philosophy. By reconceiving the scientific method as conjecture and refutation rather than observation and generalization, he offered a way to preserve scientific rationality without the logical foundations that Hume had demolished. Whether the solution fully works remains debated, but it permanently changed how philosophers think about scientific reasoning.
Lasting Influence
Reshaped philosophy of science. His defense of open societies influenced democratic theory and scientific practice.
Your Reading Path
The Companion Guide
Seven eras of philosophy in one volume — reading lists, key terms, journal prompts · $19.99