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The 1964 theorem of John Bell shows that no model that reproduces thepredictions of quantum mechanics can simultaneously satisfy the assumptions oflocality and determinism. On the other hand, the assumptions of \emph{signallocality} plus \emph{predictability} are also sufficient to derive Bellinequalities. This simple theorem, previously noted but published onlyrelatively recently by Masanes, Acin and Gisin, has fundamental implicationsnot entirely appreciated. Firstly, nothing can be concluded about theontological assumptions of locality or determinism independently of each other-- it is possible to reproduce quantum mechanics with deterministic models thatviolate locality as well as indeterministic models that satisfy locality. Onthe other hand, the operational assumption of signal locality is an empiricallytestable (and well-tested) consequence of relativity. Thus Bell inequalityviolations imply that we can trust that some events are fundamentally\emph{unpredictable}, even if we cannot trust that they are indeterministic.This result grounds the quantum-mechanical prohibition of arbitrarily accuratepredictions on the assumption of no superluminal signalling, regardless of anypostulates of quantum mechanics. It also sheds a new light on an early stage ofthe historical debate between Einstein and Bohr.

Bell nonlocality, signal locality and unpredictability (or What Bohr could have told Einstein at Solvay had he known about Bell experiments)