Karl Raimund Popper, the philosopher and methodologist of science, died on 17 September 1994 at the age of 92 years. Popper's intellectual life spanned more than 75 years, with little interruption. He worked, and thought, and wrote, and rethought, and rewrote, almost unceasingly. Popper's interest in the logic and methodology of science had started long before his attendance at Buhler's lectures. A theory, Popper's criterion of demarcation says, must be falsifiable in principle if it is to belong to empirical science, if it is to say anything about the world that we experience. It is a natural consequence of Popper's insistence on the objectivity of scientific knowledge that he should have sought to purge quantum theory of the subjectivism implanted into it by Heisenberg, preeminently the subjectivist interpretation of probabilities as measures of ignorance, rather than as statistical frequencies.