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 Obj. 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 Obj. 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 Obj. 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.
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EIGHTH ARTICLE [I-II, Q. 2, Art. 8]