In Logic

In logic, the existential quantifier ョ is a notation for asserting that something exists. But logic itself has no vocabulary for describing the things that exist. Ontology fills that gap: it is the study of existence, of all the kinds of entities —- abstract and concrete —- that make up the world. It supplies the predicates of predicate calculus and the labels that fill the boxes and circles of conceptual graphs. The two sources of ontological categories are observation and reasoning. Observation profides knowledge of the physical world, and reasoning makes sense of observation by generating a framework of abstrractions called metaphysics.
In database theory the categories are usually called domains, in AI they are called types, in object-oriented systems they are called classes, and in logic they are called types or sorts.
Whatever they are called, the selection of categories determines everything that can be represented in a computer application or an entire family of applications. Any incompleteness, distortions, or restrictions in the framework of categories must inevitably limit the generality of every program and database that uses those categories.