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.[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.