The Complete Aristotle (eng.)

we must
not ignore the question whether we have to suppose one such
substance or more than one, and if the latter, how many; we must
also mention, regarding the opinions expressed by others, that they
have said nothing about the number of the substances that can even
be clearly stated. For the theory of Ideas has no special
discussion of the subject; for those who speak of Ideas say the
Ideas are numbers, and they speak of numbers now as unlimited, now
as limited by the number 10; but as for the reason why there should
be just so many numbers, nothing is said with any demonstrative
exactness. We however must discuss the subject, starting from the
presuppositions and distinctions we have mentioned. The first
principle or primary being is not movable either in itself or
accidentally, but produces the primary eternal and single movement.
But since that which is moved must be moved by something, and the
first mover must be in itself unmovable, and eternal movement must
be produced by something eternal and a single movement by a single
thing, and since we see that besides the simple spatial movement of
the universe, which we say the first and unmovable substance
produces, there are other spatial movements-those of the
planets-which are eternal (for a body which moves in a circle is
eternal and unresting; we have proved these points in the physical
treatises), each of these movements also must be caused by a
substance both unmovable in itself and eternal. For the nature of
the stars is eternal just because it is a certain kind of
substance, and the mover is eternal and prior to the moved, and
that which is prior to a substance must be a substance. Evidently,
then, there must be substances which are of the same number as the
movements of the stars, and in their nature eternal, and in
themselves unmovable, and without magnitude, for the reason before
mentioned. That the movers are substances, then, and that one of
these is first and another second according to the same order as
the movements of the stars, is evident. But in the number of the
movements we reach a problem which must be treated from the
standpoint of that one of the mathematical sciences which is most
akin to philosophy-viz. of astronomy; for this science speculates
about substance which is perceptible but eternal, but the other
mathematical sciences, i.e. arithmetic and geometry, treat of no
substance. That the movements are more numerous than the bodies
that are moved is evident to those who have given even moderate
attention to the matter; for each of the planets has more than one
movement. But as to the actual number of these movements, we now-to
give some notion of the subject-quote what some of the
mathematicians say, that our thought may have some definite number
to grasp; but, for the rest, we must partly investigate for
ourselves, Partly learn from other investigators, and if those who
study this subject form an opinion contrary to what we have now
stated, we must esteem both parties indeed, but follow the more
accurate.
    Eudoxus supposed that the motion of the sun or of the moon
involves, in either case, three spheres, of which the first is the
sphere of the fixed stars, and the second moves in the circle which
runs along the middle of the zodiac, and the third in the circle
which is inclined across the breadth of the zodiac; but the circle
in which the moon moves is inclined at a greater angle than that in
which the sun moves. And the motion of the planets involves, in
each case, four spheres, and of these also the first and second are
the same as the first two mentioned above (for the sphere of the
fixed stars is that which moves all the other spheres, and that
which is placed beneath this and has its movement in the circle
which bisects the zodiac is common to all), but the poles of the
third sphere of each planet are in the circle which bisects the
zodiac, and the motion of the fourth sphere is in the circle which
is inclined at an angle to the equator of the third sphere; and the
poles of the third sphere are different for each of the other
planets, but those of Venus and Mercury are the same.
    Callippus made the position of the spheres the same as Eudoxus
did, but while he assigned the same number as Eudoxus did to
Jupiter and to Saturn, he thought two more spheres should be added
to the sun and two to the moon, if one is to explain the observed
facts; and one more to each of the other planets.
    But it is necessary, if all the spheres combined