  A large proportion of the Byzantine philosophical texts consist of commentaries on ancient authorities—in fact, almost invariably Aristotle. The only Plato commentary from the period after c. 730 is George Pachymeres’ supplement to Proclus on the last 24 pages of the Parmenides. These commentaries are not, however, all made according to the same pattern. A few main categories can be discerned, and even within some of these categories, there is considerable variation. The most basic type of commentary is (a) simply a collection of scholia or notes, usually transmitted in the margin of the Aristotelian text. The individual scholia of such a collection may be the work of an indeterminate number of scholars. An example is Arethas’ scholia on the Isagoge, which draws on many earlier commentaries, some of which are now lost. It is Arethas’ commentary in the sense that he copied, and possibly collected and edited, the scholia in his Aristotle manuscript; in terms of authorship, however, it is a product of mass collaboration. Next, (b) the collection may be provided with an introduction dealing with preliminary questions. Rules for the composition of such introductions were established by the Alexandrian commentators on Porphyry and Aristotle in the 5th–7th centuries. The best examples in the period after c. 730 of collections of scholia with introductions are probably the commentaries on Porphyry’s and Aristotle’s logical works by Leo Magentenos (late 12th or early to mid-13th century). Further, (c) the notes can be integrated in a running commentary, so as to form a quasi-independent whole, each new section introduced by a lemma from the text. This is the standard type of so-called ‘exegetical commentary’ typified by Alexander of Aphrodisias and cultivated in Byzantine times by Michael of Ephesus and Eustratios of Nicaea in the early 12th century. (d) The Alexandrian 6th–7th-century commentaries are sometimes divided into more or less even chunks (praxeis), each starting with a lemma followed by a general explanation of its meaning (theōria), and then proceeding to comments on particular points (lexis). While there are no clear-cut examples of this type of commentary from Middle or Late Byzantium, its influence can be traced, for instance, in Eustratios’ commentary on Posterior Analytics 2. (e) An even more advanced form is represented by George (Gennadios) Scholarios’ commentaries, which, in the manner of contemporary Latin works, are divided into ‘lessons’ (anagnōseis = lectiones), each of which is subdivided into an introduction, a broad analysis of the text into sections, a detailed analysis with interpretations of particular points, and finally ‘investigations’ (zētēmata = quaestiones).