The historically earlier Helmholtz free energy is defined as . Its change is equal to the amount of reversible work done on, or obtainable from, a system at constant "T". Thus its appellation "work content", and the designation "A" from "Arbeit", the German word for work. Since it makes no reference to any quantities involved in work (such as "p" and "V"), the Helmholtz function is completely general: its decrease is the maximum amount of work which can be done "by" a system at constant temperature, and it can increase at most by the amount of work done "on" a system isothermally. The Helmholtz free energy has a special theoretical importance since it is proportional to the logarithm of the partition function for the canonical ensemble in statistical mechanics. (Hence its utility to physicists; and to gas-phase chemists and engineers, who do not want to ignore work.)