The reference is to the Pythagorean and Aristotelian ideas of the heavens as being like the body of Deity, something uncorrupt, different from the four elements, and therefore called a fifth body, or element (στοιχεῖον). In his Meteor. i. 3, De Cœlo i. 3, &c., Aristotle speaks of the Ether as extending from the heaven of the fixed stars down to the moon, as of a nature specially adapted for circular motion, as the first element in rank, but as the fifth, “if we enumerate beginning with the elements directly known by the senses.…the subsequently so-called πέμπτον στοιχεῖον, quinta essentia.” The other elements, he taught, had the upward motion, or the downward: the earth having the attribute of heaviness, and its natural place in the world being the lowest; fire being the light element, and “its place the sphere next adjoining the sphere of the ether.” See Ueberweg’s History of Philosophy, Vol. I. p. 167, Morris’s translation, and the chapter on the De Cœlo in Grote’s Aristotle, Vol. II. pp. 389, &c.