Objection 1. It would seem that choice is not only of the means. For the Philosopher says (Ethic. vi, 12) that "virtue makes us choose aright; but it is not the part of virtue, but of some other power to direct aright those things which are to be done for its sake." But that for the sake of which something is done is the end. Therefore choice is of the end.
Objection 2. Further, choice implies preference of one thing to another. But just as there can be preference of means, so can there be preference of ends. Therefore choice can be of ends, just as it can be of means.
On the contrary, The Philosopher says (Ethic. iii, 2) that "volition is of the end, but choice of the means."
I answer that, As already stated (01, ad 2), choice results from the decision or judgment which is, as it were, the conclusion of a practical syllogism. Hence that which is the conclusion of a practical syllogism, is the matter of choice. Now in practical things the end stands in the position of a principle, not of a conclusion, as the Philosopher says (Phys. ii, 9). Wherefore the end, as such, is not a matter of choice.
But just as in speculative knowledge nothing hinders the principle of one demonstration or of one science, from being the conclusion of another demonstration or science; while the first indemonstrable principle cannot be the conclusion of any demonstration or science; so too that which is the end in one operation, may be ordained to something as an end. And in this way it is a matter of choice. Thus in the work of a physician health is the end: wherefore it is not a matter of choice for a physician, but a matter of principle. Now the health of the body is ordained to the good of the soul, consequently with one who has charge of the soul's health, health or sickness may be a matter of choice; for the Apostle says (2 Corinthians 12:10): "For when I am weak, then am I powerful." But the last end is nowise a matter of choice.
Reply to Objection 1. The proper ends of virtues are ordained to Happiness as to their last end. And thus it is that they can be a matter of choice.
Reply to Objection 2. As stated above (Question 1, Article 5), there is but one last end. Accordingly wherever there are several ends, they can be the subject of choice, in so far as they are ordained to a further end.
Source: Summa Theologica (New Advent)