A long standing conundrum for a quantum system interacting with a detector is how the quantum system evolves from a pure state to a mixture. The claim [5] that the nature (pure, mixture) of the quantum states are determined by the observables (instruments) actually available is exploited to show how the so-called wave function collapse actually consists in the collapse of the wave function of the detector. Indeed, both the quantum system and the detector should obey the laws of quantum mechanics and be described by a total wave function. What distinguishes a detector from a genuine quantum system is the nonexistence of observables that connect the different pointer states of the detector which, therefore, turns out to be the only "legal" states of the detector. The problem of the time evolution of the system interacting with the detector is discussed with a simple model.
