Austrian-born philosopher who escaped the Nazis to New Zealand, taught at the LSE from 1946, and produced work across two domains that he himself saw as unified. In philosophy of science, Popper argued against the logical positivists that what distinguishes scientific theories from non-scientific ones is not verifiability (you can never conclusively verify a universal claim) but falsifiability — a theory is scientific insofar as it forbids some possible observation and thus could in principle be shown wrong. Marxism and Freudianism, in his reading, failed this test: they could absorb any evidence, and that was precisely their weakness, not their strength.
The Open Society and Its Enemies (1945) extended the same epistemic humility to politics. Societies that treat their institutions as revisable responses to specific problems — "piecemeal social engineering" — are open; those that follow a grand blueprint derived from a theory of history's goal are closed. The enemies of the open society are the prophets of historical necessity: Plato, Hegel, Marx. Popper's polemic is uneven (his Hegel in particular is often a caricature), but his core point — that totalizing social theories tend to license totalizing regimes, and that a free society requires institutions robust enough to be criticized and changed — has proved durable.
He was a close intellectual ally of Friedrich A. Hayek, sharing his horror of comprehensive planning, though Popper was more comfortable with a substantial welfare state than Hayek.