  From the analysis of the measurement process we make the hypothesis that we
have to add to the quantum state psi a label z and a special function alpha in
order to describe completely the preparation of a (pure) quantum system . Given
such a terne every observable A receives a univocal value . Therefore all
physical quantities of the system are well defined independently from the
measuring process. Making the unknown parameter z to vary in its space we get
the probabilities prescribed by quantum mechanics.We show also that every
evolution of quantum states can be extended to an evolution of the ternes.
