"A quantum environment that emerges as a classical system is a measuring apparatus"

We consider a quantum principal system ?, with an environment ? made of N elementary quantum components. By using properties of generalized coherent states for the environment we demonstrate that whenever ? behaves according to an effective classical theory such theory emerges from the quantum description of a measuring apparatus in the large-N limit, regardless of the actual interaction between ? and ?. In fact, we show how to construct a hamiltonian model for the dynamical process that rules the coexistence of any two quantum systems whenever one of them is macroscopic and behaves classically. This result makes it clear, and formally rigorous, the qualitative argument that the reason why we do not observe state superposition is the continual measurement acted upon by whatever macroscopic environment.