Haecceity

Haecceity, from the Latin haecceitas meaning thisness, is a philosophical concept that denotes the non-qualitative aspect of a thing that individuates it as this particular object rather than any other. It is most closely associated with the medieval scholastic tradition, especially the 13th‑century philosopher Duns Scotus, who introduced haecceity as the principle of individuation that goes beyond genus, species, and accidental properties.

For Scotus, every concrete thing possesses a haecceity that cannot be captured by its essential or qualitative properties. Thus two entities with exactly the same size, shape, color, and other properties can still be numerically distinct because they have different haecceities. Haecceity is often contrasted with quiddity, or whatness, which denotes the general nature of a thing; haecceity supplies the singular, individuating mark.

In modern metaphysics, haecceity is discussed as a potential primitive or irreducible aspect of individuals, sometimes framed as a non-qualitative attribute that grounds identity across possible worlds. The idea interacts with debates over Leibniz’s law of indiscernibility, which claims that if two entities share all properties, they are identical. Haecceities are invoked to explain how distinct individuals can share all qualitative properties yet remain different.

Critics contend that haecceities are obscure, metaphysically costly, or empirically inaccessible, and they challenge their coherence with established principles of identity. Some philosophers view haecceity as a historically useful but now obsolete notion, while others incorporate it into broader theories of primitive thisness, tropes, or alternative accounts of individuation that do not rely on non-qualitative properties alone.