The topic “the categories of being” is intermediate between the topic “the nature of being” and the topics that fall under the post-Medieval conception of metaphysics for a reason that can be illustrated by considering the problem of universals.  Universals, if they indeed exist, are, in the first instance, properties or qualities or attributes (i.e., “ductility” or “whiteness”) that are supposedly universally “present in” the members of classes of things and relations (i.e., “being to the north of”) that are supposedly universally present in the members of classes of sequences of things. “In the first instance”: it may be that things other than qualities and relations are universals, although qualities and relations are the items most commonly put forward as examples of universals. It may be that the novel War and Peace is a universal, a thing that is in some mode present in each of the many tangible copies of the novel. It may be that the word “horse” is a universal, a thing that is present in each of the many audible utterances of the word. And it may be that natural classes or kinds are themselves universals—it may be that there is such a thing as “the horse” or the species Equus caballus, distinct from its defining attribute “being a horse” or “equinity”, and in some sense “present in” each horse. (Perhaps some difference between the attribute “being a horse” and the attribute “being either a horse or a kitten” explains why the former is the defining attribute of a kind and the latter is not.  Perhaps the former attribute exists and the latter does not; perhaps the former has the second-order attribute “naturalness” and the latter does not; perhaps the former is more easily apprehended by the intellect than the latter.)