At the outset of the Physics Aristotle tells us that when the objects of any inquiry have ‘principles, causes, or elements’, knowledge and understanding are acquired by grasping these: ‘For we do not think we know a thing until we are familiar with its primary causes or first principles and have carried out our analysis as far as its elements. Clearly, then, also in the science of nature our first task will be to determine its principles’ (184a10–16). The aim of the next three chapters is to present Aristotle’s general model of substantial generation by examining those principles from which natural substances first come into being. Each of the three chapters focuses on a different set of texts. This chapter explores the basic hylomorphic model of coming-to-be from the perspective of Physics I, with an emphasis on the account in Physics I 7 (which is generally taken to be the locus classicus of that model).