Commenting on the end of Aristotle's_ On the Heavens_ Book 3, Simplicius examines Aristotle's criticisms of Plato's theory of elemental chemistry in the _Timaeus_. Plato makes the characteristics of the four elements depend on the shapes of component corpuscles and ultimately on the arrangement of the triangles which compose them. Simplicius preserves and criticizes the contributions made to the debate in lost works by two other major commentators, Alexander the Aristotelian, and Proclus the Platonist. In Book 4, Simplicius identifies fifteen objections by Aristotle to Plato's views on weight in the four elements. He finishes Book 4 by elaborating Aristotle's criticisms of Democritus' theory of weight in the atoms, including Democritus' suggestions about the influence of atomic shape on certain atomic motions. This volume includes an English translation of Simplicius' commentary, a detailed introduction, extensive commentary notes and a bibliography.