On the Heavens

On the Heavens (or "De Caelo") is Aristotle's chief cosmological treatise: it contains his astronomical theory. According to him, the heavenly bodies are the most perfect realities, (or "substances"), whose motions are ruled by principles other than those of bodies in the aether, so they are not subject to generation and corruption. Hence their motions are eternal and perfect, and the perfect motion is the circular one, which, unlike the earthly up- and down-ward locomotions, can last eternally selfsame. As substances, celestial bodies have matter (aether) and a form: it seems that Aristotle did regard them as living beings with a rational soul as their form (see also Metaphysics, bk. XII)