Gary Rosenkrantz

A primary ontological presupposition of our ordinary conceptual scheme is the reality of enduring things, or individual substances, continuants such as people, horses, boulders, trees, and bridges.

This first goal of metaphysics is to provide a kind of inventory of the actual world.

Second, a complete ontological taxonomy would provisionally include every category whose instantiation we do not know to be impossible.

Hence, metaphysics also seeks to specify and organize all of the possible categories of being, that is, those categories whose instantiation is epistemically possible.

According to the system in question, the most general category is entity.

As the summum genus, every existing being is an instance of entity.

Intuitively, the next level below Level A should contain two mutually exclusive and exhaustive categories. These categories are concrete entity and abstract entity

Necessarily, anything that exists is either concrete or abstract, and it is impossible for anything to be both.

Apparent subdivisions of concrete entity include the categories of substance, event, place, time, boundary, absence, and (concrete) collection; and apparent divisions of abstract entity include the categories of property, relation, proposition, number, and (abstract) set.

Accordingly, these ontological categories appear to be at the next lower level of generality (at Level C).