Happiness is the attainment of the Perfect Good. Whoever, therefore, is capable of the Perfect Good can attain Happiness. Now, that man is capable of the Perfect Good, is proved both because his intellect can apprehend the universal and perfect good, and because his will can desire it. And therefore man can attain Happiness. This can be proved again from the fact that man is capable of seeing God, as stated in I, 12, 1: in which vision, as we stated above (Question 3, Article 8) man's perfect Happiness consists.
A certain participation of Happiness can be had in this life: but perfect and true Happiness cannot be had in this life. This may be seen from a twofold consideration.
First, from the general notion of happiness. For since happiness is a "perfect and sufficientgood," it excludes every evil, and fulfils every desire. But in this life every evil cannot be excluded. For this present life is subject to many unavoidable evils; to ignorance on the part of the intellect; to inordinate affection on the part of the appetite, and to many penalties on the part of the body; as Augustine sets forth in De Civ. Dei xix, 4. Likewise neither can the desire for good be satiated in this life. For man naturally desires the good, which he has, to be abiding. Now the goods of the present life pass away; since life itself passes away, which we naturally desire to have, and would wish to hold abidingly, for man naturally shrinks from death. Wherefore it is impossible to have true Happiness in this life.
Secondly, from a consideration of the specific nature of Happiness, viz. the vision of the Divine Essence, which man cannot obtain in this life, as was shown in the I, 12, 11. Hence it is evident that none can attain true and perfect Happiness in this life.
If we speak of imperfect happiness, such as can be had in this life, in this sense it can be lost. This is clear of contemplative happiness, which is lost either by forgetfulness, for instance, when knowledge is lost through sickness; or again by certainoccupations, whereby a man is altogether withdrawn from contemplation.
This is also clear of active happiness: since man's will can be changed so as to fall to vicefrom the virtue, in whose act that happiness principally consists. If, however, the virtueremain unimpaired, outward changes can indeed disturb such like happiness, in so far as they hinder many acts of virtue; but they cannot take it away altogether because there still remains an act of virtue, whereby man bears these trials in a praiseworthy manner. And since the happiness of this life can be lost, a circumstance that appears to be contrary to thenature of happiness, therefore did the Philosopher state (Ethic. i, 10) that some are happy in this life, not simply, but "as men," whose nature is subject to change.
But if we speak of that perfect Happiness which we await after this life, it must be observed that Origen (Peri Archon. ii, 3), following the error of certain Platonists, held that man can become unhappy after the final Happiness.
ST I-II. Q5. A 2,3,4.