Objection 1. It would seem that the intention of the end and the volition of the means are not one and the same movement. For Augustine says (De Trin. xi, 6) that "the will to see the window, has for its end the seeing of the window; and is another act from the will to see, through the window, the passersby." But that I should will to see the passersby, through the window, belongs to intention; whereas that I will to see the window, belongs to the volition of the means. Therefore intention of the end and the willing of the means are distinct movements of the will.
Objection 2. Further, acts are distinct according to their objects. But the end and the means are distinct objects. Therefore the intention of the end and the willing of the means are distinct movements of the will.
Objection 3. Further, the willing of the means is called choice. But choice and intention are not the same. Therefore intention of the end and the willing of the means are not the same movement of the will.
On the contrary, The means in relation to the end, are as the mid-space to the terminus. Now it is all the same movement that passes through the mid-space to the terminus, in natural things. Therefore in things pertaining to the will, the intention of the end is the same movement as the willing of the means.
I answer that, The movement of the will to the end and to the means can be considered in two ways. First, according as the will is moved to each of the aforesaid absolutely and in itself. And thus there are really two movements of the will to them. Secondly, it may be considered accordingly as the will is moved to the means for the sake of the end: and thus the movement of the will to the end and its movement to the means are one and the same thing. For when I say: "I wish to take medicine for the sake of health," I signify no more than one movement of my will. And this is because the end is the reason for willing the means. Now the object, and that by reason of which it is an object, come under the same act; thus it is the same act of sight that perceives color and light, as stated above (8, 3, ad 2). And the same applies to the intellect; for if it consider principle and conclusion absolutely, it considers each by a distinct act; but when it assents to the conclusion on account of the principles, there is but one act of the intellect.
Reply to Objection 1. Augustine is speaking of seeing the window and of seeing, through the window, the passersby, according as the will is moved to either absolutely.
Reply to Objection 2. The end, considered as a thing, and the means to that end, are distinct objects of the will. But in so far as the end is the formal object in willing the means, they are one and the same object.
Reply to Objection 3. A movement which is one as to the subject, may differ, according to our way of looking at it, as to its beginning and end, as in the case of ascent and descent (Phys. iii, 3). Accordingly, in so far as the movement of the will is to the means, as ordained to the end, it is called "choice": but the movement of the will to the end as acquired by the means, it is called "intention." A sign of this is that we can have intention of the end without having determined the means which are the object of choice.
Source: Summa Theologica (New Advent)