Objection 1. It would seem that some good of the soul constitutes man's happiness. For happiness is man's good. Now this is threefold: external goods, goods of the body, and goods of the soul. But happiness does not consist in external goods, nor in goods of the body, as shown above (4,5). Therefore it consists in goods of the soul.
Objection 2. Further, we love that for which we desire good, more than the good that we desire for it: thus we love a friend for whom we desire money, more than we love money. But whatever good a man desires, he desires it for himself. Therefore he loves himself more than all other goods. Now happiness is what is loved above all: which is evident from the fact that for its sake all else is loved and desired. Therefore happiness consists in some good of man himself: not, however, in goods of the body; therefore, in goods of the soul.
Objection 3. Further, perfection is something belonging to that which is perfected. But happiness is a perfection of man. Therefore happiness is something belonging to man. But it is not something belonging to the body, as shown above (Article 5). Therefore it is something belonging to the soul; and thus it consists in goods of the soul.
On the contrary, As Augustine says (De Doctr. Christ. i, 22), "that which constitutes the life of happiness is to be loved for its own sake." But man is not to be loved for his own sake, but whatever is in man is to be loved for God's sake. Therefore happiness consists in no good of the soul.
I answer that, As stated above (Question 1, Article 8), the end is twofold: namely, the thing itself, which we desire to attain, and the use, namely, the attainment or possession of that thing. If, then, we speak of man's last end, it is impossible for man's last end to be the soul itself or something belonging to it. Because the soul, considered in itself, is as something existing in potentiality: for it becomes knowing actually, from being potentially knowing; and actually virtuous, from being potentially virtuous. Now since potentiality is for the sake of act as for its fulfilment, that which in itself is in potentiality cannot be the last end. Therefore the soul itself cannot be its own last end.
In like manner neither can anything belonging to it, whether power, habit, or act. For that good which is the last end, is the perfect good fulfilling the desire. Now man's appetite, otherwise the will, is for the universal good. And any good inherent to the soul is a participated good, and consequently a portioned good. Therefore none of them can be man's last end.
But if we speak of man's last end, as to the attainment or possession thereof, or as to any use whatever of the thing itself desired as an end, thus does something of man, in respect of his soul, belong to his last end: since man attains happiness through his soul. Therefore the thing itself which is desired as end, is that which constitutes happiness, and makes man happy; but the attainment of this thing is called happiness. Consequently we must say that happiness is something belonging to the soul; but that which constitutes happiness is something outside the soul.
Reply to Objection 1. Inasmuch as this division includes all goods that man can desire, thus the good of the soul is not only power, habit, or act, but also the object of these, which is something outside. And in this way nothing hinders us from saying that what constitutes happiness is a good of the soul.
Reply to Objection 2. As far as the proposed objection is concerned, happiness is loved above all, as the good desired; whereas a friend is loved as that for which good is desired; and thus, too, man loves himself. Consequently it is not the same kind of love in both cases. As to whether man loves anything more than himself with the love of friendship there will be occasion to inquire when we treat of Charity.
Reply to Objection 3. Happiness, itself, since it is a perfection of the soul, is an inherent good of the soul; but that which constitutes happiness, viz. which makes man happy, is something outside his soul, as stated above.
Source: Summa Theologica (New Advent)