parthood

Parthood is a basic relation in ontology that holds between two entities when one is included as a component or constituent of the other. If A is a part of B, then A is contained within B’s material or structural make-up. A proper part is a part that is not identical to the whole (A ≠ B) and is included in B. The standard view of parthood is transitive: if A is a part of B and B is a part of C, then A is a part of C. Parthood can apply to spatial objects, temporal stages, or even abstract entities, depending on the framework.

Mereology is the systematic study of parthood relations. In extensional mereology, the identity of a whole is determined by the identity of its parts; two wholes with exactly the same parts are identical. A central operation is the mereological sum or fusion, the object formed by a given collection of parts. The notions of overlap (two objects sharing a common part) and disjointness (no common parts) clarify how parts and wholes relate.

Philosophical positions about parthood vary. Mereological nihilism denies ordinary composite objects and holds that only simples exist; all complex objects are reducible to their simplest parts. Mereological universalism maintains that any collection has a fusion, even when it seems intuitively problematic; some moderate or restricted theories constrain what counts as a legitimate fusion. Other accounts balance these extremes, allowing composition under certain conditions.

Ongoing issues include boundary problems, the nature of composition, and how parthood works across domains such as time (temporal parts) and function (functional parts). Parthood thus informs debates in metaphysics, science, and language about identity, causation, and categorization.