Objection 1. It would seem that the will is moved by the same act, to the end and to the means. Because according to the Philosopher (Topic. iii, 2) "where one thing is on account of another there is only one." But the will does not will the means save on account of the end. Therefore it is moved to both by the same act.
Objection 2. Further, the end is the reason for willing the means, just as light is the reason of seeing colors. But light and colors are seen by the same act. Therefore it is the same movement of the will, whereby it wills the end and the means.
Objection 3. Further, it is one and the same natural movement which tends through the middle space to the terminus. But the means are in comparison to the end, as the middle space is to the terminus. Therefore it is the same movement of the will whereby it is directed to the end and to the means.
On the contrary, Acts are diversified according to their objects. But the end is a different species of good from the means, which are a useful good. Therefore the will is not moved to both by the same act.
I answer that, Since the end is willed in itself, whereas the means, as such, are only willed for the end, it is evident that the will can be moved to the end, without being moved to the means; whereas it cannot be moved to the means, as such, unless it is moved to the end. Accordingly the will is moved to the end in two ways: first, to the end absolutely and in itself; secondly, as the reason for willing the means. Hence it is evident that the will is moved by one and the same movement, to the end, as the reason for willing the means; and to the means themselves. But it is another act whereby the will is moved to the end absolutely. And sometimes this act precedes the other in time; for example when a man first wills to have health, and afterwards deliberating by what means to be healed, wills to send for the doctor to heal him. The same happens in regard to the intellect: for at first a man understands the principles in themselves; but afterwards he understands them in the conclusions, inasmuch as he assents to the conclusions on account of the principles.
Reply to Objection 1. This argument holds in respect of the will being moved to the end as the reason for willing the means.
Reply to Objection 2. Whenever color is seen, by the same act the light is seen; but the light can be seen without the color being seen. In like manner whenever a man wills the means, by the same act he wills the end; but not the conversely.
Reply to Objection 3. In the execution of a work, the means are as the middle space, and the end, as the terminus. Wherefore just as natural movement sometimes stops in the middle and does not reach the terminus; so sometimes one is busy with the means, without gaining the end. But in willing it is the reverse: the will through (willing) the end comes to will the means; just as the intellect arrives at the conclusions through the principles which are called "means." Hence it is that sometimes the intellect understands a mean, and does not proceed thence to the conclusion. And in like manner the will sometimes wills the end, and yet does not proceed to will the means.
The solution to the argument in the contrary sense is clear from what has been said above (02, ad 2). For the useful and the righteous are not species of good in an equal degree, but are as that which is for its own sake and that which is for the sake of something else: wherefore the act of the will can be directed to one and not to the other; but not conversely.
Source: Summa Theologica (New Advent)