Determinism and the Paradox of Predictability

The inference from determinism to predictability, though intuitively plausible, needs to be qualified in an important respect. We need to distinguish between two different kinds of predictability. On the one hand, determinism implies external predictability, that is, the possibility for an external observer, not part of the universe, to predict, in principle, all future states of the universe. Yet, on the other hand, embedded predictability as the possibility for an embedded subsystem in the universe to make such predictions, does not obtain in a deterministic universe. By revitalizing an older result—the paradox of predictability—we demonstrate that, even in a deterministic universe, there are fundamental, non-epistemic limitations on the ability of one subsystem embedded in the universe to predict the future behaviour of other subsystems embedded in the same universe. As an explanation, we put forward the hypothesis that these limitations arise because the predictions themselves are physical events which are part of the law-like causal chain of events in the deterministic universe. While the limitations on embedded predictability cannot in any direct way show evidence of free human agency, we conjecture that, even in a deterministic universe, human agents have a take-it-or-leave-it control over revealed predictions of their future behaviour.