Differentia

In scholastic logic, differentia (also called differentia specifica) is one of the predicables; it is that part of a definition which is predicable in a given genus only of the definiendum; or the corresponding "metaphysical part" of the object. Plato implicitly employed the concept of differentia when he conceived his method of diairesis. Aristotle was the first to use the term diaphora (διαφορά) in a systematic fashion; but he had no explicit theory about it, and his understanding of the term is controversial. A theory was only provided by Porphyry's explicit treatment of the predicables presented in his Isagoge. The elaborate scholastic theory of the predicables evolved οn the basis of Boethius' translation of the Isagoge, where the Greek term diaphora was rendered in Latin as "differentia". In ancient Greek adiaphora - is the negation of diaphora - is an important term in Hellenistic philosophy. However, only in Pyrrhonism does it appear to be a denial of Aristotle's notion of diaphora. In the original, logical sense, a differentia is a concept — the notion of "differentia" is a second-order concept, or a "second intention", in the scholastic nomenclature. In the scholastic theory it is a kind of essential predicate — a predicate that belongs to its subjects de re necessarily. It is distinguished against the species by expressing the (specific) essence of the object only partially and against the genus by expressing the determining rather than the determined part of the essence. Although the primary meaning of "differentia" is logical or second-order, it may under certain assumptions have an ontological, first-order application. If it is assumed that the structuring of an essence into "determining" and "determinable" metaphysical parts (which corresponding to a differentia and a genus respectively) exists in reality independently of its being conceived, one can apply the notion "differentia" also to the determining metaphysical part itself, and not just to the concept that expresses it.