Of the reward of man according to his works, or of misery.
"It must be considered, however, that if there is a determined way of arriving at some end, those who proceed by a contrary way, or who turn aside from the right way, cannot attain that end. For a sick person is not cured if he uses the contrary things which the physician forbids, except perhaps by accident. Now there is a determined way of arriving at happiness, namely through virtue. For nothing attains its own end except by performing well what is proper to it: neither would a plant produce fruit if the mode of its natural operation were not preserved in it; nor would a runner reach the prize, or a soldier the palm, unless each operated according to his proper office. But for a man to operate rightly his proper operation is for him to operate according to virtue: for the virtue of each thing is that which makes its possessor good and renders his work good, as is said in Book II of the Ethics. Therefore, since the ultimate end of man is eternal life, of which it has been spoken, not all attain to it, but only those who operate according to virtue.Moreover, it has been shown above that under divine providence are contained not only natural things, but also human affairs, not only in the universal but also in the singular. But to him who has the care of individual men it belongs to render rewards to virtue and punishments to sin, because punishment is the medicine of fault and ordinative of it, as has been held above; but the reward of virtue is happiness, which is given to man from the divine goodness: therefore it belongs to God to render to those who act against virtue not happiness but the contrary as punishment, namely extreme misery."
— St. Thomas Aquinas, Comp. theol. I, C. 172
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