  The relevance that the property of complete positivity has had in the
determination of quantum structures is briefly reviewed, together with recent
applications to neutron optics and quantum Brownian motion. A possible useful
application and generalization of this property to the description of
macroscopic systems inside quantum mechanics is discussed on the basis of
recent work on the derivation of subdynamics in Heisenberg picture of slowly
varying degrees of freedom inside nonrelativistic quantum field theory.
