The subject of Aristotle's On the Heavens, Books 3-4, is the four elements of earth, air, fire and water, which exist below the heavens. Book 3, in chapters 1 to 7, frequently criticises the Presocratic philosophers. Because of this Simplicius' commentary is one of our main sources of quotations of the Presocratics. Mueller's translation gains added importance from its enabling us to see the context which guided Simplicius' selection of Presocratic texts to quote. Simplicius also criticises the lost commentary of the leading Aristotelian commentator, Alexander, and thereby gives us important information about that work.