I have studied statistical mechanics using F. Reif's book, and learned a lot, but there are still a couple very fundamental questions which still elude me. If anyone would be willing to share some insight, I would really appreciate it! So Reif early on discusses the fundamental postulate of statistical mechanics: "A system in equilibrium has equal probability of being found in any of it's accessible states." But here are my two questions (they sound kind of simple, but seem very tricky when I really try to think about them) 1. What exactly is equilibrium, in a rigorous sense? When the average energy [tex]\bar E[/tex] of the statistical system is time independent (constant)? When the "external parameters" such as volume are constant? I can't seem to come up with a totally inclusive definition of equilibrium. 2. What says that a system should relax to equilibrium to begin with? Is this a second postulate, or is there something simple I'm over looking? Reif never really touched on this point. Thanks for any help!