Ontology is the study of being and existence. It asks what exists, what categories of things there are, and what it means for something to exist. The word comes from the Greek ontos, meaning being, and logos, meaning study. Ontology is a branch of metaphysics, and it is one of the oldest areas of philosophy. Parmenides asked what is. Aristotle studied being in its many senses. Medieval philosophers distinguished essence from existence. Modern ontologists debate whether abstract objects like numbers and propositions exist, whether possible worlds are real, and whether there are fundamental particles or only fields.

Ontology is not just a philosophical exercise. It matters for science, where theories presuppose commitments about what exists. It matters for computer science, where ontologies are formal representations of knowledge. It matters for law, where courts must decide what counts as a person, a property, or an action. Ontological questions are often implicit. When a physicist says that an electron exists, she is making an ontological claim. When a lawyer says that a corporation is a person, he is making an ontological claim. Ontology makes those claims explicit and asks whether they are justified.

Ontological questions

• What exists? Physical objects, minds, numbers, universals?
• What is a substance? What are its properties?
• What is a category? How do we classify things?
• What is existence? Is it a property, a predicate, or something else?

Ontology is not the same as epistemology. Epistemology studies knowledge. Ontology studies what is known.