Alexander of Aphrodisias, who was teaching at Athens in 200 CE, was recognized for centuries as the most authoritative exponent of Aristotle. His influence has probably been most far-reaching in the development of the theory of universals because he emphasized certain elements in Aristotle's not always unambiguous account. These were the unqualified priority of the particular substance and the existence of universals only as concepts, or "acts of intellect." The form was what made "this" matter (that is, an identifiable piece) what it was, but it was contingent whether the form was universal in the sense of generic. (Alexander does not notice that a class with only one member, like his case of the sun, is still a class.) What the form is as a subject remains unclear.

More famous is his doctrine about soul and intellect. A human being's intellectual faculty can exist in...