I'm just reading "Equilibrium and Non-Equilibrium Statistical Thermodynamics" from Michel Le Bellac, Fabrice Mortessagne and G. George Batrouni. I encountered in this book a postulate I've heared many times before in various forms:
Postulate of maximum statistical entropy Among all the density operators consistent with the macroscopic constraints, we must choose the density operator D that gives the maximum statistical entropy S[D]. At equilibrium, a macrostate will therefore be represented by this density operator. In more intuitive terms, we choose the most disordered macrostate consistent with the available information. The density operator D is the one which contains no information beyond what is necessary to satisfy the macroscopic constraints.
Why is there a need for this postulate? I know it leads to all of the thermodynamics relations we want. But shouldn't the axioms of quantum mechanics be all we need to arrive at all results of quantum statistical physics/thermodynamics? Is it just that we don't know (yet) how to do it without this assumption or why is it that this postulate is necessary besides the QM axioms?