What role does haecceitas play in John Duns Scotus’s philosophy?

In the philosophy of Blessed John Duns Scotus, haecceitas—often translated as "thisness" or "individual difference"—serves as the metaphysical principle that accounts for the individuality of a specific being.12

Duns Scotus argued that while common natures (such as "humanity") are real and possess their own unity, they exist in many individuals. To explain how a common nature becomes a unique, specific individual like Socrates, he proposed that an added positive reality must be present. This reality is haecceitas. It is not a universal quality but an ultimate, singular determination that distinguishes one entity from all others of the same species.123

By developing this doctrine, Duns Scotus provided an original contribution to the medieval realism of universals, earning him the title "Doctor Subtilis" for the subtlety of his metaphysical distinctions.12