In the first two books of the Physics Aristotle discusses philosophical issues involved in the investigation of the physical universe. He introduces his distinction between form and matter and his fourfold classification of causes or explanatory factors, and defends teleological explanation. These books therefore form a natural entry into Aristotle's system as a whole, and also occupy an important place in the history of scientific thought. The present volume provides a close literal translation, which can be used by serious students without Greek. The introduction and commentary deal with the interpretation and assessment, from a philosophical standpoint, of what Aristotle says. This translation was first published in 1970.