  We introduce a model for a two configurations system, and we study the
transition from quantum to classical behaviour. We first consider the effect of
the interaction with the environment as an external noise and we show that it
produces decoherence and suppression of tunnelling. These features are widely
accepted as definition of classicality, while we believe that classicality
implies that quantum delocalized states spontaneously evolve into localized
ones. We than show that this evolution take place only when both noise and non
linearity in the equations are present.
