then (and with air Nature, and with Nature inconstancy) must end before Sky began. Above the air, in true Sky, was a different substance, which he called aether. Thus ‘the aether encompasses the divine bodies, but immediately below the aethereal and divine nature comes that which is passible, mutable, perishable, and subject to death’.7 By the word divine Aristotle introduces a religious element; and the placing of the important frontier (between Sky and Nature, Aether and Air) at the Moon’s orbit