Heisenberg’s ontological interpretation of quantum physics has a number of current supporters. C. J. Isham writes: "The most common meaning attached to probability in classical physics is an epistemic one.. . . However, unless hidden variables are posited, the situation in quantum theory is very different. . . . In particular, there are no underlying microstates of whose precise values we are ignorant. If taken seriously, such a view of the probabilistic structure in quantum theory entails a radical departure from the philosophical position of classical physics. . . ." See Chris J. Isham, Lectures on Quantum Theory, 131-132. According to Paul Davies, "Prior to quantum theory, physics was ultimately deterministic.. . . The quantum factor . . . implies that we can never know in advance what is going to happen. . . . We shall see that this indeterminism is a universal feature of the micro-world." Davies, Quantum Mechanics, p. 4. For earlier sources and references on the ontological interpretation of indeterminacy, see H. Margenau, "Reality in quantum mechanics," Phil. Science 16 (1949), 287-302. See W. Heisenberg, Philosophic Problems of Nuclear Science, op. cit.; K. Popper, Quantum Theory and the Schism in Physics (London: Hutchinson, 1956).