From the statistical point of view, β is a numerical quantity relating two macroscopic systems in equilibrium. The exact formulation is as follows. Consider two systems, 1 and 2, in thermal contact, with respective energies E 1 and E 2. We assume E 1 + E 2 = some constant E. The number of microstates of each system will be denoted by Ω 1 and Ω 2. Under our assumptions Ω i depends only on E i. We also assume that any microstate of system 1 consistent with E 1 can coexist with any microstate of system 2 consistent with E 2. Thus, the number of microstates for the combined system is