A porphyrian tree is a diagrammatic representation of the relationship of genus, species, and individual in the category of substance. Based on a passage of the Isagoge of porphyry, it became common in logic texts of the Middle Ages and was called the scala predicamentalis (κλ[symbol omitted]μαξ). In its usual form, the predicamental line of the supreme genus, substance, is subdivided into its subjective parts by the specific differences given in the left-hand column. These divide the genus and constitute the species. In treating rational animal as a genus, Porphyry followed the Stoics, who posited the existence of another kind of rational being, eternal in time, with a body of a tenuous airlike matter. This type of division could be pursued in any of the categories of being.
See Also: predicables.