  Both the ‘Introduction to Philosophy’ and the commentary on Porphyry arose out of the author’s teaching activity, presumably at the school of Alexandria. The text is divided throughout into more or less self-contained ‘lectures,’ called ‘praxeis,’ a formal structure that was first used by Olympiodorus. The ‘Introduction to Philosophy’ consists of 24 such lectures. In fact, the texts we have state in their title that they were written down “from the voice” of David; this means that while the teacher lectured, a student would take copious notes that were then copied out and circulated, a practice that was not uncommon in the Alexandrian school.