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.