Metaphysics is the branch of philosophy that studies the nature of reality. It asks what exists, what it is like, and what categories we use to describe it. The word comes from the Greek ta meta ta physika, the books after the Physics, referring to the order of Aristotle's works. Aristotle himself called the subject first philosophy. It includes ontology, the study of being; cosmology, the study of the universe; and philosophical theology, the study of God and ultimate reality.

Metaphysical questions include: What is a substance? What is a property? What is a cause? What is time? Is the world deterministic? Is there free will? Are there abstract objects? Metaphysics is often contrasted with epistemology, which studies knowledge, and with science, which studies particular features of the world. But the boundaries are porous. Physics, for example, raises metaphysical questions about the nature of space, time, and matter. Metaphysics has been criticized as speculative and unverifiable. Defenders argue that it is unavoidable. Any scientific theory presupposes metaphysical assumptions about what exists and how it can be known. Metaphysics is the attempt to make those assumptions explicit and to evaluate them.

Metaphysical topics

• Ontology. What exists?
• Cosmology. What is the structure of the universe?
• Mind. What is the relation between mind and body?
• Free will. Are our choices determined?
• Time. Is time real? Does it flow?

Metaphysics is not a science. It is a conversation about the assumptions that science and everyday life presuppose.