 It is one of the fundamental posits of the Boltzmann approach that macrostates supervene on microstates, meaning that a change in a system’s macrostate must be accompanied by a change in its microstate (for a discussion of supervenience see McLaughlin and Bennett 2005, and references therein). For instance, it is not possible to change the pressure of a system and at the same time keep its micro-state constant. Hence, to every given microstate \(x\) there corresponds exactly one macrostate. Let us refer to this macrostate as \(M(x)\). This determination relation is not one-to-one; in fact many different \(x\) can correspond to the same macrostate. We now group together all microstates \(x\) that correspond to the same macro-state, which yields a partitioning of the phase space in non-overlapping regions, each corresponding to a macro-state. For this reason we also use the same letters, \(M_1 ,\ldots ,M_k\), to refer to macro-states and the corresponding regions in phase space. This is illustrated in Figure 6a.