(6) Why is it said, “But he did not divide the birds?” (#Ge 15:10). He is shadowing forth a fifth and periodical nature, from which the ancients say that the heaven was made; for the four elements are mixtures rather than elements: by which he subdivides those things which are already divided into those materials of which they were originally composed, as the earth includes within itself a portion of the elements of water, and also of air, and also of fire, which however obtains the appellation not so much in accordance with our apprehension of it, as with our sight; and again the water is not so clear or pure, as not to have some participation in wind and earth; and so also in each of the other elements there is a certain tempering and combination; but the fifth substance is the only one which has been made unmixed and pure, on which account it was not accustomed to be mentioned at all. Therefore it is well said, he did not divide the birds; since the heavenly nature, both of the planets and also of the fixed stars, is raised on high like that of birds, in the similitude of both kinds, that is to say, of clean birds, the turtle dove and the pigeon, which scarcely admit of being divided or cut up; for the indivisible nature is of a fifth essence, more unmixed and pure than the others, and therefore it more closely resembles unity.