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 differentia of a species (eg the rationality of humans) fits into the scheme of categories. Another is why Aristotle elevates the category of Quantity to second place, above the category of Quality. Further, de Haas shows how Simplicius distinguishes different kinds of universal order to solve some of the problems.
