The art of thinking · Philosophy and logic

Ontology (philosophy)

Also known as: Philosophical ontology, Ontologia, Theory of being

German: Ontologie (Philosophie)

In philosophy, ontology is the part of metaphysics that asks what there is: which things exist, what the most general kinds or categories of entity are — objects, properties, relations, events, states of affairs — and how entities depend on one another. It also includes the question of how such claims can be settled at all, which is why the Stanford Encyclopedia of Philosophy treats ontology as several related projects rather than one. The word is modern: ontologia appears in Jacob Lorhard's Ogdoas Scholastica of 1606, while the questions go back at least to Aristotle's study of "being qua being". The information-science sense of "ontology" — a machine-readable model of concepts and relations — borrows the name from this tradition but is a different thing.

Philosophy
Thinking models

In one sentence

Ontology in philosophy: the study of what exists, of the most general categories of entity, and of how entities depend on each other.

Example

A team arguing whether a "release" is a thing in its own right or only a property of a document version is doing ontology in the philosophical sense — asking what kinds of entity their domain really contains — before anyone writes a single class in a model.

How it applies

Two questions, not one: The Stanford Encyclopedia of Philosophy describes ontology as a first approximation as the study of what there is, and splits the project in two: saying what exists, and saying what the most general features and relations of those things are. Both halves reappear whenever a domain model is built — an inventory of entity kinds, plus the relations allowed between them.
Origin of the questions: Aristotle described his subject as "first philosophy" or the study of being qua being; the Stanford Encyclopedia notes that this is not a special subject matter but a manner of studying being, and that it leads to the study of substance. His Categories supplies the oldest surviving list of kinds of being.
Origin of the word: The Latin ontologia is credited to Jacob Lorhard (1561–1609), who used it in Ogdoas Scholastica (1606), at first close in meaning to metaphysica; it became the name of a distinct discipline over the following century, notably through later German school philosophy. Accounts of exactly who used it first and in which sense differ, so treat the 1606 date as the earliest known printed use rather than a settled point of origin.
Categories and dependence: Ontology asks not only which entities there are but which depend on which — properties on their bearers, parts on wholes, events on participants. The Stanford Encyclopedia treats ontological dependence as a family of relations in which a being depends on others for its existence or for its identity. In documentation models the same question is asked as: can this object exist without its parent?
Ontological commitment: Quine's meta-ontological move was to read off what a theory says exists from the quantifiers of its formalized version — the ontological commitments of a theory are, roughly, what the theory says exists. The practical echo: every schema, data model or controlled vocabulary commits its users to a set of things, and that commitment is worth making explicit rather than leaving in a field name.
Why a metadata layer touches it: Deciding whether "product", "variant", "configuration" and "model year" name four kinds of thing or four views of one thing is a question about categories, not about tooling. Answering it first tends to keep taxonomies, faceted classification and publication metadata stable when the tool is replaced.
Limits: Ontological clarity is an aid to thinking and modeling. It does not by itself make a model correct, a publication conformant or a product safe, and it never replaces a risk assessment or the safety information a product must carry.

Ontology (philosophy) vs. ontology (information science)

Philosophical ontology asks what exists and in which categories; it is argued about, not published in a file format. The information-science sense — widely traced to Thomas Gruber's 1993 formulation of an ontology as an explicit specification of a conceptualization — names an artifact: a formal, machine-readable model of classes, properties and axioms in a bounded domain, as built with RDF Schema, OWL or SKOS. One is a field of inquiry whose results are contested; the other is an engineering deliverable that can be versioned, validated and reused. The glossary entry Ontology covers the artifact sense in the Information architecture and ontology glossary. Closely related for documentation work is the distinction between a hierarchy of kinds and a hierarchy of parts: the first is classification (taxonomy), the second is mereology.

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