"The usual conclusion... in quantum mechanics is that when the momentum of a particle is known, its coordinate has no physical reality. More generally, it is shown in quantum mechanics that, if the operators corresponding to two physical quantities... do not commute... then the precise knowledge of one of them precludes such knowledge of the other. Furthermore, any attempt to determine the latter experimentally will alter the state of the system in such a way as to destroy the knowledge of the first. From this follows that either (1) the quantum mechanical description of reality given by the wave function is not complete or (2) when the operators corresponding to the two physical quantities do not commute the two quantities cannot have simultaneous reality."