On the contrary, The Philosopher says (Topic. iii, 2) that "where one thing is by reason of another, there is but one." But there is no commanded act unless by reason of the command. Therefore they are one.
I answer that, Nothing prevents certain things being distinct in one respect, and one in another respect. Indeed, every multitude is one in some respect, as Dionysius says (Div. Nom. xiii). But a difference is to be observed in this, that some are simply many, 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. ________________________
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 (Confess. viii, 9): "The mind commands the mind to
will, and yet it does not." But to will is the act of the will.
Therefore the act of the will is not commanded.
Obj. 2: Further, to receive a command belongs to one who can understand the command. But the will cannot understand the command; for the will differs from the intellect, to which it belongs to understand. Therefore the act of the will is not commanded.