This book develops the investigation I began in Corpi e movimenti: il De caelo di Aristotele e la sua fortuna nel mondo antico (Naples, 2001). There I discussed Aristotle's reasons for the view that the celestial bodies are made of a special body which naturally performs circular motion and is different from, and not reducible to, earth, water, air, and fire. I have also shown that very few in antiquity, even within the school of Aristotle, were prepared to accept this doctrine, though many, if not most of them, shared Aristotle's view that the celestial world is a special and somehow distinct region of the natural world. This book incorporates material from the Italian one but presents it in the light of a new project. By studying the reception of the view that the heavens are made of a special body, I have come to appreciate not only how unusual Aristotle's conception of the natural world is; I have also come to understand how this conception may have affected the way Aristotle conceives of the science of nature. This book is an attempt to explore the significance of the study of the celestial bodies for Aristotle's project of investigation of the natural world.

While Aristotle argues, against his predecessors, that the celestial world is radically different from the sublunary world, he is not envisioning two disconnected, or only loosely connected, worlds.