Generally, categories are understood to express the most general features of reality. Yet, since categories have this special status, obtaining a correct list of them is difficult. This question is addressed by examining how Thomas Aquinas establishes the list of categories through a technique of identifying diversity in how predicates are per se related to their subjects. A sophisticated critique by Duns Scotus of this position is also examined, a rejection which is fundamentally grounded in the idea that no real distinction can be made from a logical one. It is argued Aquinas's approach can be rehabilitated in that real distinctions are possible when specifically considering per se modes of predication. This discussion between Aquinas and Scotus bears fruit in a contemporary context insofar as it bears upon, strengthens, and seeks to correct E. J. Lowe's four-category ontology view regarding the identity and relation of the categories.