thatness

Thatness, often discussed in philosophy under the closer term haecceity, refers to the individuating aspect of a thing that makes it this particular object rather than any other. It is the “thisness” or “thatness” that differentiates one entity from another when their qualitative properties might otherwise appear indistinguishable. In this sense, whatness or quiddity describes the general nature or essence of a kind, while thatness designates the particular instance of a thing.

Etymology and origin: the term haecceity derives from Latin haecceitas, from haec meaning “this.” It was developed in medieval scholasticism, notably by Duns Scotus, to account for how individuals are singled out beyond universal natures. In English, haecceity is commonly rendered as “thisness” or “thatness,” with the latter sometimes used to emphasize the singular, individual aspect.

Philosophical role and debates: thatness is often treated as an individuating primitive or non-qualitative property—one that cannot be reduced to color, shape, or other attributes. The central question is whether haecceities are real properties of objects or convenient theoretical tools for explaining identity conditions across possible worlds and counterfactual scenarios. Some contemporary discussions frame haecceities in terms of primitive identity conditions, while others integrate them with modal or possible-world analyses. Critics argue that haecceities can seem obscure or lead to excessive ontological commitment, whereas defenders view them as a necessary conceit for explaining how two qualitatively identical items can nonetheless be distinct.

Relation to other concepts: related terms include thisness, this-ness, and quiddity (whatness). While quiddity concerns the general essence of a kind, thatness concerns the particularity of an individual instance. The term remains a topic in metaphysics, philosophy of language, and discussions of object identity.