  The dynamics of irreversible relaxation of non-equilibrium macroscopic
systems is discussed. Arguments are developed showing that the general process
is supported by two independent successive mechanisms. One is mixing and it
follows pure conservative dynamics. It does not lead to thermodynamic
equilibrium. The second step involves dissipation to the outside world of the
information contained in the initial constraints. This step implies
interactions with the surroundings. Strict isolation is excluded. The first
step is only apparently irreversible. Strict irreversibility concerns only the
second one. The first step is isentropic. Entropy change implies the second
step. Strictly isolated non-equilibrium systems do not relax towards a new
thermodynamic equilibrium.
