  Related to the eternity of the world is the question of the nature of the celestial bodies. Aristotle argues in De Caelo I 2 that the celestial bodies, which move with a natural circular motion, must be made of a simple substance different from the four sublunary simple bodies (whose natural movements are in a straight line: up or down). This ‘fifth element,’ which is by nature imperishable, is the ether. With this explanation Aristotle seems to oppose the view Plato defends in Timaeus where it is said that the Demiurge made the divine celestial bodies “mostly out of fire” (40a2–4). Proclus admits that the heaven is composed out of the four elements with a preponderance of fire, but he insists that the elements are not present in the celestial bodies in the same mode as they exist in the sublunary bodies. Therefore Aristotle is right when he considers the heavens to constitute a fifth nature besides the four elements. “For in the heavens the elements are not the same as they are here, but are rather the summits of them” (In Tim. II 49.27–29). If one counts the whole heaven composed out of the best of the elements as one nature and adds to it the four sublunary elements, we may speak of five natures altogether.