In philosophy, ontology (from the Greek ὄν, genitive ὄντος: of being (part. of εἶναι: to be) and -λογία: science, study, theory) is the most fundamental branch of metaphysics. It studies being or existence and its basic categories and relationships, to determine what entities and what types of entities exist. Ontology thus has strong implications for conceptions of reality.
Some philosophers, notably of the Platonic school, contend that all nouns refer to entities. Other philosophers contend that some nouns do not name entities but provide a kind of shorthand way of referring to a collection (of either objects or events). In this latter view, mind, instead of referring to an entity, refers to a collection of mental events experienced by a person; society refers to a collection of persons with some shared characteristics, and geometry refers to a collection of a specific kind of intellectual activity. Any ontology must give an account of which words refer to entities, which do not, why, and what categories result. When one applies this process to nouns such as electrons, energy, contract, happiness, time, truth, causality, and god, ontology becomes fundamental to many branches of philosophy.
Ontology has one basic question: "What actually exists?" Different philosophers provide different answers to this question.
One common approach is to divide the extant entities into groups called "categories". However, these lists of categories are also quite different from one another. It is in this latter sense that ontology is applied to such fields as theology, library science and artificial intelligence.
Further examples of ontological questions include:
What is existence?
Is existence a property?
Why does something exist rather than nothing?
What constitutes the identity of an object?
What is a physical object?
What features are the essential, as opposed to merely accidental, attributes of a given object?
Can one give an account of what it means to say that a physical object exists?
What are an object's properties or relations and how are they related to the object itself?
When does an object go out of existence, as opposed to merely changing?