  The surviving, often voluminous, Greek commentaries on Aristotle’s logical works by Alexander of Aphrodisias (fl. c. 200 CE), Porphyry (234–c. 305), Ammonius Hermeiou (5th century), Philoponus (c. 500) and Simplicius (6th century) and the Latin ones by Boethius (c. 480–524) are mainly important for preserving alternative interpretations of Aristotle’s logic and as sources for lost Peripatetic and Stoic works. They also allow us to trace the gradual development from a Peripatetic exegesis of Aristotle’s Organon to a more eclectic logic that resulted from the absorption and inclusion of elements not just from Stoic and Platonist theories but also from mathematics and rhetoric. Two of the commentators in particular deserve special mention in their own right: Porphyry, for writing the Isagoge or Introduction (i.e. to Aristotle’s Categories), in which he discusses the five notions of genus, species, differentia, property and accident as basic notions one needs to know to understand the Categories. For centuries, the Isagoge was the first logic text a student would tackle, and Porphyry’s five predicables (which differ from Aristotle’s four) formed the basis for the medieval doctrine of the quinque voces. The second is Boethius. In addition to commentaries, he wrote a number of logical treatises, mostly simple explications of Aristotelian logic, but also two very interesting ones: (i) His On Topical Differentiae bears witness to the elaborated system of topical arguments that logicians of later antiquity had developed from Aristotle’s Topics under the influence of the needs of Roman lawyers. (ii) His On Hypothetical Syllogisms systematically presents wholly hypothetical and mixed hypothetical syllogisms as they are known from the early Peripatetics; it may be derived from Porphyry. Boethius’ insistence that the negation of ‘If it is A, it is B’ is ‘If it is A, it is not B’ suggests a suppositional understanding of the conditional, a view for which there is also some evidence in Ammonius, but that is not attested for earlier logicians. Historically, Boethius is most important because he translated all of Aristotle’s Organon into Latin, making these texts (except the Posterior Analytics) available to philosophers of the medieval period.