An unambiguous treatment of the rate of chemical transformation for homogeneous reactions is sought by pointing out the key elements of any mathematical theory of physicochemical phenomena and relating them to some of the concepts of reaction engineering. Within this framework, the rate of generation per unit volume of a species through a reaction step emerges as a primitive. The status of the species balance equation as a law (rather than a definition) and that of commonly used rate expressions as constitutive relations are brought forth. These concepts are illuminated using analogies from classical mechanics and thermodynamics.