Ontology is the study of what exists and of the categories into which existing things fall. It is the core of metaphysics.

The two questions

The first question is what kinds of thing there are. Physical objects are uncontroversial. Numbers, properties, events, fictional characters, holes and possible worlds are all disputed, and the arguments usually turn on whether talk that appears to refer to them can be paraphrased away.

The second is what it means to say something exists. W. V. O. Quine proposed a sharp test in 1948: the ontological commitments of a theory are whatever must exist for the theory's statements to be true. If your best physics quantifies over fields, you are committed to fields. The method converts a metaphysical question into a question about the logical form of your best theories.

Realism and nominalism

The oldest dispute concerns universals. Realists hold that properties exist over and above the particulars that have them — redness itself, not just red things. Plato gave the strongest version with the theory of forms, placing forms outside space and time.

Nominalists hold that only particulars exist and that general terms are devices for grouping them. William of Ockham argued in the fourteenth century that entities should not be multiplied beyond necessity, and that universals are names rather than things.

Aristotle took a middle route: forms are real and exist only in the particulars that instantiate them, which is why his substance is a composite of matter and form.

Modern use

Computer science borrowed the word for a specified vocabulary of entities and relations in a domain. The senses are related — both concern what a description commits you to — and the philosophical question of whether those entities exist is left open by the engineering one.