  It is argued that in the description of macroscopic systems inside quantum
mechanics the study of the dynamics of selected degrees of freedom slowly
varying on a suitable time scale, corresponding to relevant observables for the
given reduced description, is particularly meaningful. A formalism developing
these ideas in the more simple case of a microsystem interacting with a
macroscopic system is briefly outlined, together with an application to the
field of neutron optics. The obtained reduced description relies on a T-matrix
formalism and has the property of complete positivity.
