How did John Duns Scotus explain universals and common natures?

John Duns Scotus, often called the "Doctor Subtilis," developed a nuanced theory of universals that expanded upon traditional medieval realism.

He argued that while universals exist only as abstract concepts in the mind, they are grounded in the reality of "common natures." These common natures, such as humanity, are real and possess a unity that is distinct from the specific unity of the individuals in which they reside.

To explain how a common nature becomes a specific individual, Duns Scotus introduced the concept of an "individual difference" or "thisness," known as haecceitas. This haecceitas is an added positive reality that determines a common nature to be a particular individual, such as Socrates. By distinguishing between the common nature shared by many and the unique "thisness" that characterizes an individual, he provided a sophisticated metaphysical framework for understanding existence and identity.12