Consider first the relatively simple case of gas temperature, which has often been supposed reducible to the average kinetic energy of the gas’s molecules (cf. Nagel 1961, p. 343). A particular average kinetic energy of the molecules is only a necessary condition for having a given temperature, however. Only gases at equilibrium have a definite temperature, when all the spatial parts have the same temperature as the whole (reflecting the fact that temperature is an intensive property). A suffici