 The primary purpose of a commentary is to explain a text. Typically, the text is divided into lemmata (plural of lemma). A lemma is what is quoted from a text in order to explain and interpret it. Some times the text is quoted in its entirety. Other times only its beginning is recalled. The lemma is always followed by an analysis of the text. Commentaries were not written exclusively on philosophical works. Any poetical or scientific work that came to be regarded as authoritative could become the object of a commentary. For instance, the tradition of writing commentaries on the Hippocratic corpus started very early. Although most of this tradition is lost, the commentaries that Galen wrote on the Hippocratic corpus have come down to us. Galen was a doctor who lived and wrote in the second half of the 2nd century CE. Interestingly enough, his exegetical activity was not confined to Hippocrates. Galen wrote commentaries on several of Aristotle’s works, specifically on the De Interpretatione, the Prior and Posterior Analytics, and the Categories (Galen, On my Own Books, XIX 47). In all probability, these commentaries were not intended for publication but only for the use of a small group of friends and students. Here is what Galen says about his commentary on the Categories: