and
one in a particular aspect: while with others it is the reverse. Now
"one" is predicated in the same way as "being." And substance is
being simply, whereas accident or being "of reason" is a being only
in a certain respect. Wherefore those things that are one in
substance are one simply, though many in a certain respect. Thus, in
the genus substance, the whole composed of its integral or essential
parts, is one simply: because the whole is being and substance
simply, and the parts are being and substances in the whole. But
those things which are distinct in substance, and one according to an
accident, are distinct simply, and one in a certain respect: thus
many men are one people, and many stones are one heap; which is unity
of composition or order. In like manner also many individuals that
are one in genus or species are many simply, and one in a certain
respect: since to be one in genus or species is to be one according
to the consideration of the reason.

Now just as in the genus of natural things, a whole is composed of
matter and form (e.g. man, who is one natural being, though he has
many parts, is composed of soul and body); so, in human acts, the act
of a lower power is in the position of matter in regard to the act of
a higher power, in so far as the lower power acts in virtue of the
higher power moving it: for thus also the act of the first mover is
as the form in regard to the act of its instrument. Hence it is
evident that command and the commanded act are one human act, just as
a whole is one, yet in its parts, many.

Reply Obj. 1: If the distinct powers are not ordained to one another,
their acts are diverse simply. But when one power is the mover of the
other, then their acts are, in a way, one: since "the act of the
mover and the act of the thing moved are one act" (Phys. iii, 3).

Reply Obj. 2: The fact that command and the commanded act can be
separated from one another shows that they are different parts.
Because the parts of a man can be separated from one another, and yet
they form one whole.

Reply Obj. 3: In those things that are many in parts, but one as a
whole, nothing hinders one part from preceding another. Thus the
soul, in a way, precedes the body; and the heart, the other members.
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FIFTH ARTICLE [I-II, Q. 17, Art. 5]

Whether the Act of the Will Is Commanded?

Objection 1: It would seem that the act of the will is not commanded.
For Augustine says (C