Lewis principle revisited

In 1967, R.M. Lewis formulated a principle by means of which simplified descriptions of complex systems may be obtained from Liouvilleʹs equation. This description is given in terms of the pertinent equations of motion and an entropy functional. The principle applies both to equilibrium as well as to non-equilibrium situations. In this paper we show how this principle may be understood within the context of the general theory of non-equilibrium statistical mechanics. This analysis clarifies the physical content of the premises under which the principle is based, as well as the scope of its possible applications