Another reply is that the interaction taking place in the human body may not be described by "billiard ball" classical mechanics. If a nondeterministic interpretation of quantum mechanics is correct then microscopic events are indeterminate, where the degree of determinism increases with the scale of the system. Philosophers Karl Popper and John Eccles and physicist Henry Stapp have theorized that such indeterminacy may apply at the macroscopic scale. However, Max Tegmark has argued that classical and quantum calculations show that quantum decoherence effects do not play a role in brain activity. Indeed, macroscopic quantum states have only ever been observed in superconductors near absolute zero.