Index (also: observer index) is information that disambiguates an observer from other observers which is not contained in its state, and is therefore unknowable to the observer. Index is the unknown variable where there is indexical uncertainty. Observers with different indices but indistinguishable observations may be generated by different mechanisms (e.g. a human in base reality vs a simulation vs a Boltzmann brain), or the same (indeterministic) mechanism but have different futures (e.g. the versions of you who will observe the photon going through the right vs left slit).