If our system is in state s 1 , then there would be a corresponding number of microstates available to the reservoir. Call this number Ω Ω R ( s 1 ) . By assumption, the combined system (of the system we are interested in and the reservoir) is isolated, so all microstates are equally probable. Therefore, for instance, if Ω Ω R ( s 1 ) = 2 Ω Ω R ( s 2 ) , we can conclude that our system is twice as likely to be in state s 1 than s 2 . In general, if P ( s i ) is the probability that our system is in state s i ,