Thomas Aquinas, Compendium of Theology

This leads to our next point. If there is a definite way of reaching a fixed end, they who travel along a road leading in the opposite direction or who turn aside from the right road, will never reach the goal. A sick man is not cured by using the wrong medicines, forbidden by the doctor, except, perhaps, quite by accident.

There is such a definite way of arriving at happiness, namely, the practice of virtue. Nothing will reach its end unless it performs well the operations proper to it. A plant will not bear fruit if the procedure natural to it is not followed. A runner will not win a trophy or a soldier a citation, unless each of them carries out his proper functions. To say that a man discharges his proper office is equivalent to saying that he acts virtuously; for the virtue of any being is that which makes its possessor good and also makes his work good, as is stated in the second book of the Ethics. Accordingly, since the ultimate end of man is eternal life, of which we spoke previously, not all attain it, but only those who act as virtue requires.

Besides, as we said above, not natural things alone, but also human affairs, are contained under divine providence, and this not only in general but in particular. But He who has care of individual men has disposal of the rewards to be assigned for virtue and of the punishments to be inflicted for sin. For punishment has a medicinal value with regard to sin and restores right order when violated by sin, as we stated above; and the reward of virtue is happiness, to be granted to man by God's goodness. Therefore God will not grant happiness to those who act against virtue, but will assign as punishment the opposite of happiness, namely, extreme wretchedness.