Their designation of these as immaterial entities originally derives from the fact that a person abstracts universal intelligible notions from particular objects of sense perception. When he sees individual human beings, such as Zayd and ʿAmr, he apprehends an element common to human beings and to the universal, shared humanity conceived in his heart. When he sees horses, mules, donkeys, livestock, and other individual animals, he apprehends a universal element common to these individuals: universal animality as conceived by the intellect. When, alongside these, he considers animals, trees, and plants, he apprehends a common universal element: a body that grows and takes nourishment. They sometimes call this the vegetative soul.
When, alongside these, he considers all the other bodies—the celestial bodies above and the elemental bodies below—he apprehends a common universal element: body in its general, unrestricted sense. When he considers the other things that exist, he apprehends a common universal element: general, universal existence, which is divided into substance and accident. According to them, this existence is the subject matter of the “highest science,” which examines existence and its concomitants. This is what they call “first philosophy” and “supreme wisdom.”
They divide existence into substance and accident, and classify accidents into “nine kinds.” This is what Aristotle set forth. His followers include this within logic because it deals with the simple terms to which composite definitions ultimately reduce. The same is true of those who followed their approach in writing on this subject, such as Ibn Hazm and others.
Ibn Sīnā and his followers, however, said: “Discussion of this subject is not confined to logic.”