In the summer of 1919, a 16-year-old Karl Popper watched police fire on unarmed socialist youths in Vienna, killing people he had marched beside. Instead of radicalizing him, the event and the Marxist leadership’s dismissal of the deaths as historically necessary triggered a lifelong suspicion of ideologies that can never be proven wrong. This episode follows Popper from a childhood home holding 12,000 to 14,000 books, through road construction and an apprenticeship as a cabinet maker, to volunteer work in Alfred Adler’s clinics, where he noticed that psychoanalysis could explain any behavior and therefore predicted nothing.

The contrast with Einstein’s risky prediction that starlight bends around the sun, confirmed in Eddington’s 1919 eclipse expedition, led Popper to his demarcation criterion: falsifiability separates science from pseudoscience. The conversation explains how he answered Hume’s problem of induction with bold conjectures and ruthless refutations, illustrated by the search for the black swan, and how The Logic of Scientific Discovery in 1934 became a life raft that got him to New Zealand before the Nazi annexation of Austria. There he wrote The Open Society and Its Enemies, attacking historicism in both fascism and communism, and set out the paradox of tolerance, including its frequently lost nuance about where the threshold actually lies. The episode ends with his years at the London School of Economics, his knighthood, his student George Soros, and a challenge to name the evidence that would change your own mind.

• The 1919 Vienna shooting and why the Marxist response turned Popper away from closed ideological loops
• Adler, Freud, and the trap of theories that explain everything, versus Einstein’s testable prediction
• Falsifiability as the demarcation between science and pseudoscience, and Popper’s answer to Hume’s problem of induction
• Historicism, the high-speed train analogy, and the open society’s trial-and-error approach to policy
• The paradox of tolerance and the specific threshold Popper set: refusing debate and resorting to violence