formula and the demon’s objective description of our sequence of measurements is the replacement of the commuting unitary $\Phi$’s in the complete system by the noncommuting idempotent Hermitian $\Pi$’s in the measured system alone.

Thus is valid precisely to the extent that the time-dependent operators (in the Heisenberg picture), as calculated by the Heisenberg equation for the [*complete*]{} experimental setup at the time the next measurement is taken, can be approximated in the assumed (time-independent) state of the investigated [*subsystem*]{} by a two-sided multiplication with these projectors.

This shows clearly [*the aim of a correct measurement theory:*]{} It must exhibit broad sufficient conditions for a system that guarantee, for all observables to be measured, the validity of this approximation, and hence a complete reduction of the wave packet.

I’d like to challenge the adherents of to devise and perform experiments testing validity, accuracy and limits of when more than two (noncommuting) projectors are involved and the measurements are not ideal. The findings should coincide with the interpretation in the Heisenberg picture just given. (For the realization of arbitrary discrete projectors, note that these can always be brought into the form $UDU^*$ with diagonal projectors $D$ and unitary $U$; hence [Reck]{} 