I present a very general and simple argument -- based on the no-signalling theorem -- showing that within the framework of the unitary Schroedinger equation it is impossible to reproduce the phenomenological description of quantum mechanical measurements (in particular the collapse of the state of the measured system) by assuming a suitable mixed initial state of the apparatus. The thrust of the argument is thus similar to that of the 'insolubility theorems' for the measurement problem of quantum mechanics (which, however, focus on the impossibility of reproducing the macroscopic measurement results). Although I believe this form of the argument is new, I argue it is essentially a variant of Einstein's reasoning in the context of the EPR paradox -- which is thereby illuminated from a new angle.
