St. Thomas asks in this Article whether human acts “receive their species” from their ends – whether their ends are what make them the species, or kinds, of acts that they are. If so, then these ends are the proper basis for defining and classifying them. This part of the Treatise on Happiness and Ultimate Purpose is more elliptical than most, for it assumes that the reader knows certain things about the structure of human action, which St. Thomas explains only elsewhere in the Summa. As the Thomist philosopher Christopher Kaczor points out, St. Thomas analyzes every complete human action by using at least the following elements, though the agent is not necessarily thinking of each of them explicitly.