Objection 1. It would seem possible for one man's will to be directed at the same time to several things, as last ends. For Augustine says (De Civ. Dei xix, 1) that some held man's last end to consist in four things, viz. "in pleasure, repose, the gifts of nature, and virtue." But these are clearly more than one thing. Therefore one man can place the last end of his will in many things.
Objection 2. Further, things not in opposition to one another do not exclude one another. Now there are many things which are not in opposition to one another. Therefore the supposition that one thing is the last end of the will does not exclude others.
Objection 3. Further, by the fact that it places its last end in one thing, the will does not lose its freedom. But before it placed its last end in that thing, e.g. pleasure, it could place it in something else, e.g. riches. Therefore even after having placed his last end in pleasure, a man can at the same time place his last end in riches. Therefore it is possible for one man's will to be directed at the same time to several things, as last ends.
On the contrary, That in which a man rests as in his last end, is master of his affections, since he takes therefrom his entire rule of life. Hence of gluttons it is written (Philippians 3:19): "Whose god is their belly": viz. because they place their last end in the pleasures of the belly. Now according to Matthew 6:24, "No man can serve two masters," such, namely, as are not ordained to one another. Therefore it is impossible for one man to have several last ends not ordained to one another.
I answer that, It is impossible for one man's will to be directed at the same time to diverse things, as last ends. Three reasons may be assigned for this. First, because, since everything desires its own perfection, a man desires for his ultimate end, that which he desires as his perfect and crowning good. Hence Augustine (De Civ. Dei xix, 1): "In speaking of the end of good we mean now, not that it passes away so as to be no more, but that it is perfected so as to be complete." It is therefore necessary for the last end so to fill man's appetite, that nothing is left besides it for man to desire. Which is not possible, if something else be required for his perfection. Consequently it is not possible for the appetite so to tend to two things, as though each were its perfect good.
The second reason is because, just as in the process of reasoning, the principle is that which is naturally known, so in the process of the rational appetite, i.e. the will, the principle needs to be that which is naturally desired. Now this must needs be one: since nature tends to one thing only. But the principle in the process of the rational appetite is the last end. Therefore that to which the will tends, as to its last end, is one.
The third reason is because, since voluntary actions receive their species from the end, as stated above (Article 3), they must needs receive their genus from the last end, which is common to them all: just as natural things are placed in a genus according to a common form. Since, then, all things that can be desired by the will, belong, as such, to one genus, the last end must needs be one. And all the more because in every genus there is one first principle; and the last end has the nature of a first principle, as stated above. Now as the last end of man, simply as man, is to the whole human race, so is the last end of any individual man to that individual. Therefore, just as of all men there is naturally one last end, so the will of an individual man must be fixed on one last end.
Reply to Objection 1. All these several objects were considered as one perfect good resulting therefrom, by those who placed in them the last end.
Reply to Objection 2. Although it is possible to find several things which are not in opposition to one another, yet it is contrary to a thing's perfect good, that anything besides be required for that thing's perfection.
Reply to Objection 3. The power of the will does not extend to making opposites exist at the same time. Which would be the case were it to tend to several diverse objects as last ends, as has been shown above (ad 2).
Source: Summa Theologica (New Advent)