  The heat theorem (i.e. the second law of thermodynamics or the existence of
entropy) is a manifestation of a general property of hamiltonian mechanics and
of the ergodic Hypothesis. In nonequilibrium thermodynamics of stationary
states the chaotic hypothesis plays a similar role: it allows a unique
determination of the probability distribution (called {\rm SRB} distribution on
phase space providing the time averages of the observables. It also implies an
expression for a few averages concrete enough to derive consequences of
symmetry properties like the fluctuation theorem or to formulate a theory of
coarse graining unifying the foundations of equilibrium and of nonequilibrium.
