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 pseudos ...