is proper to the heavenly bodies, is not
by contraries, whereas the movements of the elements are mutually
opposite, one tending upwards, another downwards: so, therefore, the
heavenly body is without contrariety, whereas the elemental bodies
have contrariety in their nature. And as generation and corruption are
from contraries, it follows that, whereas the elements are
corruptible, the heavenly bodies are incorruptible. But in spite of
this difference of natural corruption and incorruption, Avicebron
taught unity of matter in all bodies, arguing from their unity of
form. And, indeed, if corporeity were one form in itself, on which the
other forms that distinguish bodies from each other supervene, this
argument would necessarily be true; for this form of corporeity would
inhere in matter immutably and so far all bodies would be
incorruptible. But corruption would then be merely accidental through
the disappearance of successive forms--that is to say, it would be
corruption, not pure and simple, but partial, since a being in act
would subsist under the transient form. Thus the ancient natural
philosophers taught that the substratum of bodies was some actual
being, such as air or fire. But supposing that no form exists in
corruptible bodies which remains subsisting beneath generation and
corruption, it follows necessarily that the matter of corruptible and
incorruptible bodies is not the same. For matter, as it is in itself,
is in potentiality to form.

Considered in itself, then, it is in potentiality in respect to all
those forms to which it is common, and in receiving any one form it is
in act only as regards that form. Hence it remains in potentiality to
all other forms. And this is the case even where some forms are more
perfect than others, and contain these others virtually in themselves.
For potentiality in itself is indifferent with respect to perfection
and imperfection, so that under an imperfect form it is in
potentiality to a perfect form, and _vice versa._ Matter, therefore,
whilst existing under the form of an incorruptible body, would be in
potentiality to the form of a corruptible body; and as it does not
actually possess the latter, it has both form and the privation of
form; for want of a form in that which is in potentiality thereto is
privation. But this condition implies corruptibility. It is therefore
impossible that bodies by nature corruptible, and those by nature
incorruptible, should possess t