INTRODUCTION

In his programmatic opening to the Meteorologica (Meteor I.1, 338a20–339a10), Aristotle puts the issues discussed in De Caelo at the center of his science of nature. The study of “the stars ordered according to their upper motion” (Meteor I.1, 338a21–22), and of “the bodily elements, their number and nature, and their change into each other” (Meteor I.1, 338a22–24) is said to follow his examination of the first principles of nature and of natural change in general, but to precede his investigations of sublunary living nature. In accordance with this outline, Aristotle first – in De Caelo book I – argues for the existence of a fifth, heavenly element (i.e., aether) in addition to the familiar four sublunary, changeable ones (i.e., air, water, fire, and earth), and discusses the nature and characteristics of the universe in its entirety (i.e., its size, uniqueness, and eternity). Then, in book II, Aristotle turns to the motions and features of the heavens as a whole, of the individual planets and stars, and lastly of the Earth. Next, in book III, he focuses on the nature and motions of the four sublunary elements, saving his definitions of “weight” and “lightness” associated with these elements for book IV. By engaging with the heavenly phenomena in the first two books and only then turning to the sublunary elements, Aristotle mimics the order of exposition of Plato's Timaeus.