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