Of Aristotle’s conception of a fifth element, the _quinta essentia_, or ether, superior to the others and permeating them, Isidore shows merely a trace. He says in one passage that “ether is the place where the stars are, and it signifies that fire which is separated on high from all the universe”.[73] He offers also another definition in which he confuses three of the elements of Aristotle: “Ether is the upper, fiery air”.[74] [73] _Etym._, 13, 5, 1. [74] _Diff._, 1, 82. The theory of the four elements, as has been already indicated, has a cosmological bearing. In the universe at large the elements were thought of as tending to arrange themselves in strata according to weight. Isidore says it is proved “that earth is the heaviest of all things created; and therefore, they say, it holds the lowest place in the creation, because by nature nothing but itself can support it. And we perceive that water is heavier than air in proportion as it is lighter than earth.... Fire, too, is apprehended to be in its nature above air, which is easily proved even in the case of fire that burns in earthy substance, since as soon as it is kindled, it directs its flame toward the upper spaces which are above the air, where there is an abundance of it, and where it has its place.”[75] [75] _De Ordine Creat. Liber_, 4, 5–6. _Cf. D. N. R._, 11. The problem of “the waters above the firmament,” which occupied the minds of the church fathers so much, and which is at variance with the cosmological side of the theory of the four elements, Isidore seems inclined to settle by regarding it as a miracle. _Cf. D. N. R._, 14. Thus the physical universe consists of the four kinds of matter, stratified according to the principle of weight. The notion was one in frequent use,[76] and it was brought into relation with animate existence by assigning to each of the four strata a peculiar population. Thus the fiery heavens were occupied by angels; the air, by birds and demons; the water, by fishes; the earth, by man and other animals.[77] [76] In the _De Natura Rerum_ and the _De Ordine Creaturarum_, as well as in Books XIII-XIV of the _Etymologies_, Isidore follows the order of the four elements in describing the universe. His fidelity to this order, as well as the variations of emphasis and of minor treatment which he introduced into it, are of interest. These may be exhibited in parallel form as follows: _Etymologies_ _De Natura _De Ordine _Books xiii and xiv_ Rerum_ Creaturarum_ xiii, chaps. 4–6 chaps. 9–27 4–6