Abstract

Karl Popper’s “Indeterminism in Quantum Physics and in Classical Physics” suffers unjust neglect. He judged determinism false: the future is open. In principle, replacing Laplace's variant of predetermination with predictable predetermination renders “scientific” determinism scientific and so refutable. Popper claimed that he had refuted it. Now a metaphysical system may have an extension—in the mathematical sense—that may render it explanatory and testable. If it exists, then it is not unique but has many alternative extensions. Popper’s proof is then inconclusive.