  The derivation of the ten categories is most originally presented in the Commentary on the “Metaphysics.” Aquinas begins by dividing being into three modes: outside the mind (extra animam), in the mind (in mente), and as divided into act and potency (per potentiam et actum). Being, considered in the first way (which is the one pertinent to the discussion here), is not divided as a genus (e.g., animal) is into species (e.g., human) by means of differences (differentiae, e.g., rational), because such differences need to be outside the genus's essence (e.g., rationality is not included in animality), and nothing lies outside being. Being, as found outside the mind, is instead divided into categories (praedicamenta) on the basis of how it is predicated (modi praedicandi). Predication occurs in three fundamental ways to indicate: (1) what a subject is (id quod est subiectum); (2) that something inheres in a subject (inest subiecto); and (3) that something does not inhere in the subject and is beyond it but nonetheless affects it (sumatur ab eo quod est extra subiectum). These three modes of predication are in turn divided and subdivided to account for the ten categories.