4 OF THE DIFFERENCE OF THE PARTS. AND OF THE FACULTIES OF THE SOUL.
We shall now explain the difference obtaining between a part and a faculty of the soul. One part differs from another by the characteristics of its genus (or, kind); while different faculties may relate to a common genus. That is why Aristotle did not allow that the soul contained parts, though granting that it contained faculties. Indeed, the introduction of a new part changes the nature of the subject, while the diversity of faculties does not alter its unity. Longinus did not allow in the animal (or, living being) for several parts, but only for several faculties.