Aquinas on True Happiness
Now, one should note that many things are set down here about the beatitudes. . . Yet one should know that all complete happiness is included in these words: for all men desire happiness, but they differ in their judgments about happiness, and for this reason some men desire this, others desire that.
But we find four opinions about happiness. For some believe that it consists only in exterior things: namely, in an abundance of temporal things; “they have called the people happy, that have these things” (Ps 144:15). Others, that perfect beatitude consists in the fact that a man fulfills his own will; hence we say, blessed is he who lives as he wants. “And I have known that there was no better thing than to rejoice, and to do well in this life” (Eccl 3:12). Others say that perfect beatitude consists in the virtues of the active life. Others, that it consists in the virtues of the contemplative life, namely of divine and intelligible things, as Aristotle says.
But all these opinions are false, although not in the same way. Hence the Lord condemns all these.
He condemns the opinion of those who say that it consists in the abundance of exterior things: hence, he says, “blessed are the poor” (Matt 5:3) as though to say, the wealthy are not happy.
And he condemns the opinion of those who place beatitude in the satisfaction of appetite when he says, “blessed are the merciful” (Matt 5:7).
But one should know that there are three appetites in man. The irascible, which desires vengeance on enemies, and he condemns this when he says, “blessed are the meek” (Matt 5:5). The concupiscible, whose good is to rejoice and be delighted; he condemns this when he says, “blessed are they who mourn” (Matt 5:4). The will, which is twofold, according as it seeks two things: first, that the will be forced by no higher law; second, that it be able to bind others as subjects: hence it desires to command, and not to be under another. But the Lord shows the contrary regarding both. And regarding the first, he says, “blessed are they who hunger and thirst after justice” (Matt 5:6). Regarding the second, he says, “blessed are the merciful” (Matt 5:7). Therefore both those who place beatitude in the abundance of exterior things and those who place it in the satisfaction of appetite are wrong.
Those, however, who place beatitude in the actions of the active life, namely, in moral acts, are wrong, but less so, because that is the way to beatitude. Hence the Lord does not condemn them as though evil, but shows the ordering to beatitude. Because the virtues of the active life are either ordered to oneself, such as temperance and others of this kind, and their end is cleanness of heart, because they enable one to conquer the passions; or they are ordered to another, and so their end is peace, and of this kind: “the work of justice will be peace” (Isa 32:17). And therefore these virtues are ways to beatitude, and not beatitude itself; and this is “blessed are the clean of heart: for they will see God” (Matt 5:8). He does not say, “they see,” because this would be beatitude itself. And similarly, “blessed are the peacemakers” (Matt 5:9) not because they make peace, but because they tend toward something else: “for they will be called children of God.”
Now, the opinion of those who say that beatitude consists in the contemplation of divine things is condemned by the Lord only as to the timing, for otherwise it is true; because the last happiness does consist in the vision of the best intelligible, namely God; hence he says, “they will see.”
And one should note that, according to the Philosopher, two things are required for the contemplative act to make one happy: one has to do with the substance, namely, that it be the act of the highest intelligible, which is God; the other has to do with form, namely love and delight, for delight perfects happiness just as beauty perfects youth. And therefore the Lord sets down two things, “they will see God” (Matt 5:8) and “they will be called children of God” (Matt 5:9) for this pertains to the union of love. Behold what manner of charity the Father has bestowed upon us, that we should be called, and should be the sons of God (1 John 3:1).
Commentary on Matthew, 5.2.404–408. Cited from https://aquinas.cc/la/en/~Matt.C5.L2.n404