Consider a system S embedded into a heat bath B. Let the total energy of both systems be E. Let p i denote the probability that the system S is in a particular microstate, i, with energy E i. According to the fundamental postulate of statistical mechanics (which states that all attainable microstates of a system are equally probable), the probability p i will be inversely proportional to the number of microstates of the total closed system (S, B) in which S is in microstate i with energy E i. Equivalently, p i will be proportional to the number of microstates of the heat bath B with energy E − E i : p i = Ω Ω B ( E − − E i ) Ω Ω ( S , B ) ( E ) .