Metaphysics is the branch of philosophy that studies, at the most general level, what exists and what the things that exist are like, including being, identity, change, causation, space, time, and the relation between mind and matter.
Metaphysics is the part of philosophy that asks what reality contains and how its contents are structured. Where a particular science studies one region of the world, say living cells or distant galaxies, metaphysics steps back to ask the questions that no single science is set up to answer: what it is for anything to exist at all, what kinds of things there are, and what holds across every region of reality rather than within one of them. The word itself dates from the editing of Aristotle's works, where the treatises placed after the Physics came to be called the metaphysics, and Aristotle himself described their subject as first philosophy.[1]
Two questions organize most of the field. The first is the question of existence: what is there? The second is the question of nature: given that something exists, what is it, and what features does it have necessarily rather than by accident? These look simple, but they reach quickly into territory that resists easy answers. NoteA familiar gloss separates metaphysics from science by saying the sciences study the specific how of reality, while metaphysics studies the general what and why.
Ontology: what exists
The study of what there is goes by the name of ontology, and it forms the core of metaphysics. Ontology does not merely list the contents of the world; it asks what categories those contents fall into and whether some of them are more fundamental than others. Are there only concrete physical objects, or also abstract ones such as numbers, sets, and propositions? Do properties like redness exist over and above the particular red things that have them? Such questions divide those who admit a wide range of entities from those who try to account for the same facts with a sparser inventory.[2]
A long-running version of this dispute concerns universals and particulars. A particular is a single thing, this cup on this desk. A universal is something that can be shared, the property of being a cup, present wherever there is a cup. Realists hold that universals exist in their own right; nominalists deny this and try to explain shared features without them. The debate matters because so much else, including how we understand resemblance, classification, and the laws of nature, depends on what answer is given.
The central questions
Beyond the inventory of what exists, metaphysics examines the most general features that things display: that they are, that they persist or alter, and that they could have been otherwise.
Being and identity
To ask after being is to ask what it is for something to exist, and whether existence is itself a property a thing can have or lack. Closely linked is the problem of identity: what makes a thing the very thing it is, and what allows it to remain that thing over time. A person, a river, or a ship may exchange every part and still be called the same; explaining how that is possible, or arguing that it is not, is among the oldest tasks in the subject.[3]
Change and causation
Things alter, and one event brings about another. Accounting for change requires saying how a thing can gain and lose properties while remaining one thing, and accounting for causation requires saying what the link is between a cause and its effect, beyond the bare fact that the second follows the first. NoteDavid Hume argued that we never observe a necessary connection between cause and effect, only constant conjunction, a challenge later theories of causation still answer.
Possibility and necessity
Some truths could have been false, while others seem to hold no matter what. The study of modality, of possibility and necessity, asks what grounds this difference. One influential approach analyzes such claims in terms of possible worlds, treating a thing as possible if it holds in some world and necessary if it holds in every world. Whether possible worlds are to be taken as real or only as a useful manner of speaking is itself a metaphysical question.[4]
Metaphysics and science
Metaphysics is sometimes contrasted with the empirical sciences and sometimes treated as continuous with them. On the first view, its questions are too general to be settled by observation: no experiment tells us whether numbers exist or whether causation involves more than regular succession. On the second view, the best science already carries metaphysical commitments, and the philosopher's task is to make those commitments explicit and coherent. Much contemporary work occupies the middle ground, taking the findings of physics and biology seriously while pressing the questions those findings leave open.
A current angle
Artificial intelligence has given several old metaphysical questions a new edge. When a system processes language and reports on its own states, debates about mind and matter, and about what would have to be true for something to count as a thinking thing, become more than academic. Questions of identity return as well: if a model is copied, retrained, or run in many instances at once, which of them, if any, is the same entity, and what settles that? These are not engineering problems, and they are not answered by better hardware. They are the existence and identity questions of classical metaphysics, met again in an unfamiliar form, and they show why the discipline that asks what exists and why remains in use.