  In this paper we address the problem to give a concrete support to the idea,
originally stemming from Niels Bohr, that quantum mechanics must be rooted
inside the physics of macroscopic systems. It is shown that, starting from the
formalism of the non-equilibrium statistical operator, which is now a
consolidated part of quantum statistical mechanics, particular correlations
between two isolated systems can be singled out and interpreted as
microsystems. In this way also a new framework is established in which
questions of decoherence can be naturally addressed.
