Objection 1. It would seem that volition is not of the means, but of the end only. For the Philosopher says (Ethic. iii, 2) that "volition is of the end, while choice is of the means."
Objection 2. Further, "For objects differing in genus there are corresponding different powers of the soul" (Ethic. vi, 1). Now, the end and the means are in different genera of good: because the end, which is a good either of rectitude or of pleasure, is in the genus "quality," or "action," or "passion"; whereas the good which is useful, and is directed to and end, is in the genus "relation" (Ethic. i, 6). Therefore, if volition is of the end, it is not of the means.
Objection 3. Further, habits are proportionate to powers, since they are perfections thereof. But in those habits which are called practical arts, the end belongs to one, and the means to another art; thus the use of a ship, which is its end, belongs to the (art of the) helmsman; whereas the building of the ship, which is directed to the end, belongs to the art of the shipwright. Therefore, since volition is of the end, it is not of the means.
On the contrary, In natural things, it is by the same power that a thing passes through the middle space, and arrives at the terminus. But the means are a kind of middle space, through which one arrives at the end or terminus. Therefore, if volition is of the end, it is also of the means.
I answer that, The word "voluntas" sometimes designates the power of the will, sometimes its act [See note: above 01, Reply to Obj. 1. Accordingly, if we speak of the will as a power, thus it extends both to the end and to the means. For every power extends to those things in which may be considered the aspect of the object of that power in any way whatever: thus the sight extends to all things whatsoever that are in any way colored. Now the aspect of good, which is the object of the power of the will, may be found not only in the end, but also in the means.
If, however, we speak of the will in regard to its act, then, properly speaking, volition is of the end only. Because every act denominated from a power, designates the simple act of that power: thus "to understand" designates the simple act of the understanding. Now the simple act of a power is referred to that which is in itself the object of that power. But that which is good and willed in itself is the end. Wherefore volition, properly speaking, is of the end itself. On the other hand, the means are good and willed, not in themselves, but as referred to the end. Wherefore the will is directed to them, only in so far as it is directed to the end: so that what it wills in them, is the end. Thus, to understand, is properly directed to things that are known in themselves, i.e. first principles: but we do not speak of understanding with regard to things known through first principles, except in so far as we see the principles in those things. For in morals the end is what principles are in speculative science (Ethic. viii, 8).
Reply to Objection 1. The Philosopher is speaking of the will in reference to the simple act of the will; not in reference to the power of the will.
Reply to Objection 2. There are different powers for objects that differ in genus and are on an equality; for instance, sound and color are different genera of sensibles, to which are referred hearing and sight. But the useful and the righteous are not on an equality, but are as that which is of itself, and that which is in relation to another. Now such like objects are always referred to the same power; for instance, the power of sight perceives both color and light by which color is seen.
Reply to Objection 3. Not everything that diversifies habits, diversifies the powers: since habits are certain determinations of powers to certain special acts. Moreover, every practical art considers both the end and the means. For the art of the helmsman does indeed consider the end, as that which it effects; and the means, as that which it commands. On the other hand, the ship-building art considers the means as that which it effects; but it considers that which is the end, as that to which it refers what it effects. And again, in every practical art there is an end proper to it and means that belong properly to that art.
Source: Summa Theologica (New Advent)