Quantum probabilities from combination of Zurek's envariance and Gleason's theorem

The quantum-mechanical rule for probabilities, in its most general form of positive-operator valued measure (POVM), is shown to be a consequence of the environment-assisted invariance (envariance) idea suggested by Zurek [Phys. Rev. Lett. 90, 120404 (2003)], being completed by Gleason's theorem. This provides also a method for derivation of the Born rule.