Book 2 of the Physics is arguably the best introduction to Aristotle's ideas, as well as being the most interesting and representative book in the whole of his corpus. It defines nature and distinguishes natural science from mathematics. It introduces the seminal idea of four causes, or four modes of explanation. It defines chance, but rejects a theory of chance and natural selection in favour of purpose in nature.
Simplicius, writing in the sixth century AD, adds his own considerable contribution to this work. Seeing Aristotle's God as a Creator, he discusses how nature relates...

In Categories chapters 7 and 8 Aristotle considers his third and fourth categories - those of Relative and Quality. Critics of Aristotle had suggested for each of the non-substance categories that they could really be reduced to relatives, so it is important how the category of Relative is defined. Aristotle offers two definitions, and the second, stricter, one is often cited by his defenders in order to rule out objections. The second definition of relative involves the idea of something changing its relationship through a change undergone by its correlate, not by itself. There were...

Chapters 5 and 6 of Aristotle's "Categories" describe his first two categories, Substance and Quantity. It is usually taken that Plotinus attacked Aristotle's "Categories", but that Porphyry and Iamblichus restored it to the curriculum once and for all. Nonetheless, the introduction to this text stresses how much of the defence of Aristotle Porphyry was able to draw out of Plotinus' critical discussion. Simplicius' commentary is our most comprehensive account of the debate on the validity of Aristotle's "Categories". One subject discussed by Simplicius in these chapters is where the...