i'm new to statistical mechanics and the more I read and view ,the more I get confused. So far I've got this. At equilibrium a system will have a fluctuating energy,and the specific energies follow Boltzmann's distribution. I also read that within one phase point the molecular velocities also follow a distribution.. . Question 1 is do all points in phase space that belong to a specific energy have a equal probability of existing? Question 2 ,involves Dr. Suskind's you tube lectures on statistical mechanics ,specifically the one deriving Boltzmann's distribution. He starts by considering an ensemble of identical systems and asked how these can be distributed over a set of energies. He comes up with the formula of N!/n1!n2!n3! and so forth. To my knowledge this is the formula for permutation of multisets. And implies that some members of the ensemble are distinguishable and some are not. The formula from combinatorics where say c indistinguishable items can be placed in b bags is ( c+b-1) ( b ) read this as number of combinations of c+b-1 taken b at a time. the assumption is that the number in each bag is unrestricted.. Why is he assuming that some members of the ensemble are distinguishable? Totally confused thanks for any help.