 We first introduce the main elements of the Boltzmannian framework and then turn to the use of ergodicity in it. Every system can posses various macrostates \(M_1 ,\ldots ,M_k\). These macrostates are characterised by the values of macroscopic variables, in the case of a gas pressure, temperature, and  volume.[14]  In the introductory example one macro-state corresponds to the gas being confined to the left half, another one to it being spread out. In fact, these two states have special status: the former is the gas’ initial state (also referred to as the ‘past state’); the latter is the gas’ equilibrium state. We label the states \(M_p\) and \(M_{eq}\) respectively.