Book excerpt: Supporting the twelve volumes of translation of Simplicius' great commentary on Aristotle's Physics , published between 1992 and 2021, this volume presents a general introduction to the commentary. It covers the philosophical aims of Simplicius' commentaries on the Physics and the related text On the Heaven ; Simplicius' methods and his use of earlier sources; key themes and comparison with Philoponus' commentary on the same text. In the first chapters of his work, Aristotle raises the question of the number and character of the first principles of nature and feels the need to oppose the challenge of the paradoxical Eleatic philosophers who had denied that there could be more than one unchanging thing. By 1.7, Aristotle reaches the conclusion that we must distinguish one substratum and two contrary states that it may possess: a form and a privation of that form. But this only foreshadows what is to follow. In book 2, Aristotle introduces four kinds of explanatory factor: besides the material substratum of a thing and its form, there is its function or purpose, and the efficient cause of its taking on new forms. He goes on in Books 3 to 8 to discuss causation, chance and necessity, motion, infinity, vacuum, spatial relations and the continuum and he postulates the need for a divine first mover as the source of purposive motion in celestial bodies.