  Most of Buridan's works are in the form of commentaries on Aristotle. He wrote both expositiones (expositions), or literal commentaries consisting of detailed, line-by-line explanations of the meaning of Aristotle's words, and quaestiones (questions), or longer, critical studies of the philosophical issues raised by them, usually centered on a specific lemma from the text. Both genres originated in the classroom, a fact which becomes clear in occasional references to student queries and student concerns which survive in the written versions. Like teachers in our own day, Buridan lectured more than once on the same text over the course of his career, with the result that there are sometimes different versions of his commentary on the same work. For example, there are three versions of his Quaestiones on Aristotle's De anima, the last of which identifies itself as the “third or final lecture [tertia sive ultima lectura]”. Where there are multiple versions of the same commentary, their relationship is generally one of increasing length and sophistication over time.