Ontology

What it asks

Metaphysics asks what exists. Ontology asks the question one level up: what KINDS of thing exist, and what would settle a dispute about the list.

Aristotle's answer was a table of ten categories - substance, quantity, quality, relation and so on - into which everything sortable falls. The modern answer is different in method rather than ambition. Quine argued that you find out what a theory says exists by writing it out formally and seeing what its variables have to range over. That turned a question about the world into a question about the commitments of a theory, which is answerable, and it is why the branch became tractable again after Kant.

The recurring problems

Abstract objects. Numbers, sets, propositions and possibilities do nothing and are nowhere, yet mathematics and logic quantify over them constantly. Admitting them makes existence a much stranger property than it looked; refusing them means explaining how arithmetic can be true anyway.

Parts and wholes. Is there an object composed of your left shoe and the Eiffel Tower? Mereology says yes, on the grounds that any principled reason to refuse also refuses ordinary objects. Most people find that intolerable and have trouble saying why.

Dependence. A hole depends on what it is in; a smile on a face; a nation on its people. Existence looks less like a yes-or-no property and more like a structure once dependence is taken seriously.

Whether the disputes are real. If the realist and the nominalist mean different things by exists, they are not contradicting each other. That objection is the most serious one the branch faces.

How it developed

Aristotle's Categories is the founding text and was the framework for two thousand years. The scholastics extended it enormously, and coined the name in the early seventeenth century as the study of being in general, separated from theology.

Kant's attack on metaphysics took ontology with it, and the logical positivists finished the job by declaring the questions meaningless.

Quine's On What There Is in 1948 rebuilt the subject on a single test: to be is to be the value of a bound variable. Ask what your best theory quantifies over and that is what you are committed to. The move was deliberately deflationary and had the opposite effect, giving the field a method precise enough to argue with.

Since then the work has been technical - mereology, dependence, grounding, the theory of categories - and it has acquired an unexpected second life outside philosophy, in computer science, where an ontology is a formal specification of the entities in a domain.

The positions within it

Realism and nominalism about abstract objects; Platonism in mathematics; trope theory as a middle way; universalism and nihilism about composition. All are covered in the Schools section.

Ontological deflationism holds that the disputes are verbal rather than substantive, and it is a position within the branch, not a rejection of it.

Where it stands now

Grounding has become the central notion: not what exists, but what holds in virtue of what. It gives the field a way to say that both the table and the particles exist without treating them as two competing inventories.

The deflationary challenge remains live and unresolved. And the computer-science usage now vastly outweighs the philosophical one in volume, which has made the word ambiguous in a way it was not twenty years ago.