 Galen makes clear that he regards the commentator as a teacher and as an expositor of a text with an audience of students in mind. Moreover, as Galen presents the matter, the commentary was a teaching tool addressed to students with specific levels of knowledge. The students who were expected to read Galen on the Categories were not absolute beginners. They were students who either had previously benefited from the study of this treatise with a teacher or had become acquainted with the Categories through some more elementary commentaries. Galen refers to the commentaries written by Adrastus and Aspasius. By so doing, he gives us an idea of the profusion of commentaries that were written on the Categories. These commentaries were written one after the other as part of an already consolidated exegetical practice. Most of these commentaries have not reached us. We no longer have the commentaries that Adrastus and Aspasius wrote on the Categories, nor do we have the commentary that Galen wrote on the same treatise. This frustrating (at least for us) fact is ultimately due to the very nature of this literary production. Each generation of commentators read and interpreted Aristotle in the light of their own theoretical preoccupations and proclivities, only to be replaced by the next generation of commentators. One remarkable exception is Alexander of Aphrodisias, a younger contemporary of Galen. His commentaries survive because they were adopted as exemplary models by later commentators. But not all of them have come down to us. For example, Alexander wrote a commentary on the Categories (Simplicius, In Cat. 1.16). This commentary suffered the same fate as those written by Adrastus, Aspasius, Galen, and others. Today it survives only in fragmentary form, incorporated in the commentaries that a later generation of interpreters wrote on the Categories. 