Commentaries on Aristotle's writings have been produced since the 2nd century AD. This edition contains Greek commentaries on his work from the 3rd to the 8th centuries AD by, among others, Alexander of Aphrodiensias, Themistios, Joh. Philoponus, Simplicius in Greek.

Commentaries on Aristotle's writings have been produced since the 2nd century AD. This edition contains Greek commentaries on his work from the 3rd to the 8th centuries AD by, among others, Alexander of Aphrodiensias, Themistios, Joh. Philoponus, Simplicius in Greek.

Commentaries on Aristotle's writings have been produced since the 2nd century AD. This edition contains Greek commentaries on his work from the 3rd to the 8th centuries AD by, among others, Alexander of Aphrodiensias, Themistios, Joh. Philoponus, Simplicius in Greek.

Aristotle's "Physics "Book 3 covers two subjects: the definition of change and the finitude of the universe. Change enters into the very definition of nature as an internal source of change. Change receives two definitions in chapters 1 and 2, as involving the actualisation of the potential or of the changeable. Alexander of Aphrodisias is reported as thinking that the second version is designed to show that Book 3, like Book 5, means to disqualify change in relations from being genuine change. Aristotle's successor Theophrastus, we are told, and Simplicius himself, prefer to admit relational change. Chapter 3 introduces a general causal principle that the activity of the agent causing change is in the patient undergoing change, and that the causing and undergoing are to be counted as only one activity, however different in definition. Simplicius points out that this paves the way for Aristotle's God who moves the heavens, while admitting no motion in himself. It is also the basis of Aristotle's doctrine, central to Neoplatonism, that intellect is one with the objects it contemplates.In defending Aristotle's claim that the universe is spatially finite, Simplicius has to meet Archytas' question, "What happens at the edge?." He replies that, given Aristotle's definition of place, there is nothing, rather than an empty place, beyond the furthest stars, and one cannot stretch one's hand into nothing, nor be prevented by nothing. But why is Aristotle's beginningless universe not temporally infinite? Simplicius answers that the past years no longer exist, so one never has an infinite collection.

In chapter 1 of On the Heavens Aristotle defines body, and then notoriously ruptures dynamics by introducing a fifth element, beyond Plato's four, to explain the rotation of the heavens, which, like nearly all Greeks, Aristotle took to be real, not apparent. Even a member of his school, Xenarchus, we are told, rejected his fifth element. The Neoplatonist Simplicius seeks to harmonise Plato and Aristotle. Plato, he says, thought that the heavens were composed of all four elements but with the purest kind of fire, namely light, predominating. That Plato would not mind this being called a fifth element is shown by his associating with the heavens the fifth of the five convex regular solids recognised by geometry. Simplicius follows Aristotle's view that one of the lower elements, fire, also rotates, as shown by the behaviour of comets. But such motion, though natural for the fifth elements, is super-natural for fire. Simplicius reveals that the Aristotelian Alexander of Aphrodisias recognised the need to supplement Aristotle and account for the annual approach and retreat of planets by means of Ptolemy's epicycles or eccentrics. Aristotle's philosopher-god is turned by Simplicius, following his teacher Ammonius, into a creator-god, like Plato's. But the creation is beginningless, as shown by the argument that, if you try to imagine a time when it began, you cannot answer the question, 'Why not sooner?' In explaining the creation, Simplicius follows the Neoplatonist expansion of Aristotle's four 'causes' to six. The final result gives us a cosmology very considerably removed from Aristotle's.

Simplicius' commentary on Aristotle's Categories is the most comprehensive philosophical critique of the work ever written, representing 600 years of criticism. In his Categories, Aristotle divides what exists in the sensible world into ten categories of Substance, Quantity, Relative, Quality and so on. Simplicius starts with a survey of previous commentators, and an introductory set of questions about Aristotle's philosophy and about the Categories in particular. The commentator, he says, needs to present Plato and Aristotle as in harmony on most things. Why are precisely ten categories named, given that Plato did with fewer distinctions? We have a survey of views on this. And where in the scheme of categories would one fit a quality that defines a substance - under substance or under quality? In his own commentary, Porphyry suggested classifying a defining quality as something distinct, a substantial quality, but others objected that this would constitute an eleventh. The most persistent question dealt with here is whether the categories classify words, concepts, or things.

Aristotle believed that the outermost stars are carried round us on a transparent sphere. There are directions in the universe and a preferred direction of rotation. The sun moon and planets are carried on different revolving spheres. The spheres and celestial bodies are composed of an everlasting fifth element, which has none of the ordinary contrary properties like heat and cold which could destroy it, but only the facility for uniform rotation. But this creates problems as to how the heavenly bodies create light, and, in the case of the sun, heat. The value of Simplicius' commentary on On the Heavens 2,1-9 lies both in its preservation of the lost comments of Alexander and in Simplicius' controversy with him. The two of them discuss not only the problem mentioned, but also whether soul and nature move the spheres as two distinct forces or as one. Alexander appears to have simplified Aristotle's system of 55 spheres down to seven, and some hints may be gleaned as to whether, simplifying further, he thinks there are seven ultimate movers, or only one.