Standard accounts of the micro-reduction of phenomenological to kinetic thermostatics, based on the postulate relating empirical absolute temperature to mean kinetic energy (ū = (3/2)kT), face two problems. The standard postulate also allows ‘reduction’ in the other direction and it can be criticized from the point of view that reduction postulates need to be ontological identities.

This paper presents a detailed account of the reduction, based on the postulate that thermal equilibrium is ontologically identical to having equal mean kinetic energy. In particular, it is shown that this postulate enables reduction only in the appropriate direction, but leaves room for ‘evidence transport’ in the other. Moreover, it makes possible the derivation (explanation) of the standard postulate, using the existential kinetic hypothesis and phenomenological laws with which it turns out to be laden.