Abstract Quantum processes cannot be reduced, in a nontrivial way, to classical processes without specifying the context in the description of a measurement procedure. This requirement is implied by the Kochen–Specker theorem in the outcome-deterministic case and, more generally, by the Gleason theorem. The latter establishes that there is only one noncontextual classical model compatible with quantum theory, the one that trivially identifies the quantum state with the classical state.
