Decoherence versus the idealization of microsystems as correlation carriers between macrosystems

It is argued that the appropriate framework to describe a microsystem as a correlation carrier between a source and a detector is non-equilibrium statistical mechanics for the compound source-detector system. An attempt is given to elucidate how this idealized notion of microsystem might arise inside a field theoretical description of isolated macrosystems: then decoherence appears as the natural limit of this idealization.