Besides, all that approaches the heaven is either air or fire. What of it is air cannot affect the heaven. What of it is fire can neither influence the heaven, nor touch it, to act on it. Before acting on the heaven, it would have to assume its nature; besides, fire is less great or powerful than the heaven. Moreover, the action of fire consists in heating; whereas, 1, that which is to be heated cannot have been hot by itself; and as, 2, that which is to be dissolved by fire must first be heated, inasmuch as it is this heating which causes a change of nature. No other body is needed for either the subsistence of the heaven, or for the functioning of its natural revolutions.[220] Moreover, the heaven does not move in a straight line, because it is in the nature of celestial things to remain immovable, or to move in a circular orbit, and not to assume any other kind of movement without compulsion by some superior force. THE STARS ARE INEXHAUSTIBLE. AND NEED NO REFRESHMENT. Stars, therefore, stand in need of no feeding,[221] and we should not judge them according to our own circumstances. Indeed, our (human) soul, which contains our bodies, is not identical with the Soul that contains the heaven; our soul does not reside in the same place, while the world-Soul does not, like our composite bodies lose (excreta). Not as our bodies do the stars need continual metabolic replacing food. From our conception of celestial bodies we should remove all ideas of a change that could modify their constitution. Terrestrial bodies are animated by an entirely different nature[222]; which though because of its weakness is incapable of insuring them a durable existence, nevertheless imitates the superior nature (of the celestial Soul) by birth and generation. Elsewhere[223] we have shown that even this very celestial Soul cannot partake of the perfect immutability of intelligible things. FOURTH ENNEAD, BOOK SIX. Of Sensation and Memory. STOIC DOCTRINES OF SENSATIONS AND MEMORIES HANG TOGETHER.