Enneads II.1: Of the Heaven
Celestial Light Is Not Exposed to Any Wastage
8. Since this light subsists in elevated regions, because the purity of its nature forces it to remain in pure regions, it cannot be subject to any wastage (or, leakage). Such a nature could not allow any escape either downwards or upwards, nor could it meet anything that would force it to descend. Moreover, it will be remembered that there is a great difference of condition in a body united to, or separated from a soul; and in this case the body of the heaven is everywhere united to the (universal) Soul.
THE HEAVEN DOES NOT NEED THE ACTION OF EITHER AIR OR FIRE.
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. 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.