  There are many approaches to defining the division of the state space into distinct regions. The most common group together sets of microstates that match criteria such as being macroscopically or observationally indistinguishable or are accessible over time to the microstate evolution. For the systems considered here, these approaches generally define the same regions. We may conventionally refer to these regions as macrostates, while acknowledging this terminology is a little inappropriate when describing systems consisting only of a single molecule. In the case of the Szilard engine, for example, the macrostate of the system, when the partition is absent, consists of the set of all the microstates for which the molecule is in the box. When the partition is inserted in the box, the macrostate is the set of all the microstates for which the position of the molecule is on the same side of the partition as the actual location of the molecule. We will sometimes refer to the Boltzmann entropy of a macrostate: this is simply the Boltzmann entropy of the microstates within that macrostate.