Thisness

Thisness, or haecceity, is a concept in metaphysics describing the quality that makes an individual thing to be itself, rather than any other thing. The term haecceity comes from the Latin haecceitas meaning "thisness" and was introduced by the medieval philosopher Duns Scotus to explain how a thing is individuated beyond its sensible properties and general nature.

For Scotus, haecceity is the non-qualitative aspect that grounds numerical identity. Two entities may share all qualitative properties and relations yet remain distinct objects because each possesses a unique haecceity that marks it as that particular thing. In this way haecceity functions as an individuating force beyond essence or predicates.

In modern metaphysics, haecceity is discussed as a potential primitive property or as a tool for accounting for individuation across possible worlds and modalities. Some theorists defend haecceities as ontologically real features, while others interpret them reductively in terms of essential properties, indices, or non-qualitative relations. The debate concerns whether haecceities are necessary or contingent, and how they relate to concepts of essence, quidditas, or the indiscernibility of identicals.

Usage notes: the English "thisness" and the Latin "haecceitas" are often invoked in discussions of what makes a thing the particular thing it is, beyond its properties. See also: quidditas, indiscernibility of identicals, identity across possible worlds.