Aristotle's On Interpretation, the centrepiece of his logic, examines the relationship between conflicting pairs of statements. The first eight chapters, analysed in this volume, explain what statements are, starting from their basic components - the words - and working up to the character of opposed affirmations and negations. Ammonius, who in his capacity as Professor at Alexandria from around A.D. 470 taught almost all the great sixth-century commentators, left just this one commentary in his own name, although his lectures on other works of Aristotle have been written up by his pupils, who included Philoponus and Asclepius. His ideas on Aristotle's On Interpretation were derived partly from his own teacher, Proclus, and partly from the great lost commentary of Porphyry. The two most important extant commentaries on On Interpretation, of which this is one (the other being by Boethius), both draw on Porphyry's work, which can be to some extent reconstructed from them.