In examining the truth of this question, where such diversity of
opinion exists, we shall do well to bear in mind that the union of
soul and body exists for the sake of the soul and not of the body;
for the form does not exist for the matter, but the matter for the
form. Now the nature and power of the soul are apprehended through
its operation, which is to a certain extent its end. Yet for some of
these operations, as sensation and nutrition, our body is a necessary
instrument. Hence it is clear that the sensitive and nutritive souls
must be united to a body in order to exercise their functions. There
are, however, operations of the soul, which are not exercised through
the medium of the body, though the body ministers, as it were, to
their production. The intellect, for example, makes use of the
phantasms derived from the bodily senses, and thus far is dependent
on the body, although capable of existing apart from it. It is not,
however, possible that the functions of nutrition, growth, and
generation, through which the nutritive soul operates, can be
exercised by the heavenly bodies, for such operations are
incompatible with a body naturally incorruptible. Equally impossible
is it that the functions of the sensitive soul can appertain to the
heavenly body, since all the senses depend on the sense of touch,
which perceives elemental qualities, and all the organs of the senses
require a certain proportion in the admixture of elements, whereas
the nature of the heavenly bodies is not elemental. It follows, then,
that of the operations of the soul the only ones left to be
attributed to the heavenly bodies are those of understanding and
moving; for appetite follows both sensitive and intellectual
perception, and is in proportion thereto. But the operations of the
intellect, which does not act through the body, do not need a body as
their instrument, except to supply phantasms through the senses.
Moreover, the operations of the sensitive soul, as we have seen,
cannot be attributed to the heavenly bodies. Accordingly, the union
of a soul to a heavenly body cannot be for the purpose of the
operations of the intellect. It remains, then, only to consider
whether the movement of the heavenly bodies demands a soul as the
motive power, not that the soul, in order to move the heavenly body,
need be united to the latter as its form; but by contact of power, as
a mover is united to that which he moves. Wherefore Aristotle (Phys.