haecceitas

Haecceitas, typically rendered as haecceity or “thisness,” is a term in scholastic metaphysics used to designate the intrinsic principle by which a particular thing is individuated. The concept was introduced by the 13th‑century philosopher Duns Scotus to explain how individuals of the same kind can be numerically distinct. In Scotus’s framework, beings have both a quiddity (their whatness or essence) and a haecceitas (their thisness).

Haecceitas is understood as an intrinsic, non‑qualitative feature that cannot be captured by a positive predicate applied to many objects. It is the factor that makes this particular thing the one it is, rather than another, beyond its observable properties. Thus two objects could share all qualitative properties (shape, color, size) yet remain numerically different because they possess different haecceitæ.

In medieval debates, haecceitas served to account for individuation of substances, including angels and other immaterial beings, without reducing numerical identity to qualitative similarity alone. Critics, such as William of Ockham, questioned the necessity or definability of haecceities, arguing that individuation might be explained without positing further non‑qualitative properties.

In modern philosophy, haecceitas is discussed less as a positive property and more as a historical precursor to discussions about primitive identity conditions, indexical properties, or the limits of predication. It is frequently contrasted with quiddity (whatness) to illustrate how an entity’s identity can be secured independently of its, or its properties’, qualitative aspects.