Objection 1. It would seem that the commanded act is not one with the command itself. For the acts of different powers are themselves distinct. But the commanded act belongs to one power, and the command to another; since one is the power that commands, and the other is the power that receives the command. Therefore the commanded act is not one with the command.
Objection 2. Further, whatever things can be separate from one another, are distinct: for nothing is severed from itself. But sometimes the commanded act is separate from the command: for sometimes the command is given, and the commanded act follows not. Therefore command is a distinct act from the act commanded.
Objection 3. Further, whatever things are related to one another as precedent and consequent, are distinct. But command naturally precedes the commanded act. Therefore they are distinct.
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 to Objection 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 to Objection 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 to Objection 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.
Source: Summa Theologica (New Advent)