Karl Popper was an Austrian-British philosopher whose account of scientific method dominated the mid-twentieth century and still shapes how scientists describe what they do.

The demarcation problem

His starting question was not what makes a theory true but what makes it scientific. Comparing Einstein's physics with the psychoanalysis and Marxist history popular in 1920s Vienna, he noticed an asymmetry that struck him as decisive.

Einstein's theory forbade things. It predicted that light would bend by a specific amount, and an observation of a different amount would have sunk it. The other theories explained every outcome after the fact and forbade nothing, which their supporters counted as strength.

Popper concluded that the mark of a scientific theory is falsifiability: it rules something out, and you can say in advance what observation would refute it.

Why this solves Hume's problem

Hume had shown that no number of confirming instances establishes a general law — the problem of induction.

Popper's response was to concede it entirely and deny that science needs induction. No observation confirms a universal claim, but one contrary observation refutes it. Science advances by bold conjecture and severe attempts at refutation, and a theory that survives is corroborated rather than proved. Nothing is ever established.

The open society

The same structure runs through his politics. The Open Society and Its Enemies (1945) attacks Plato, Hegel and Marx as historicists who claimed to know the direction of history, and argues for institutions that allow bad policies to be identified and removed without violence — the political form of falsification.

Criticism

Thomas Kuhn argued that working scientists do not abandon a theory at the first anomaly, and are right not to. Pierre Duhem and W. V. O. Quine had already noted that a failed prediction never identifies which assumption failed, since any hypothesis can be saved by adjusting something else. Falsification is less decisive in practice than it is on paper.