Thomas Aquinas, Compendium of Theology

Since the wretchedness to which vice leads is opposed to the happiness to which virtue leads, whatever pertains to wretchedness must be understood as being the opposite of all we have said about happiness. We pointed out above that man's ultimate happiness, as regards his intellect, consists in the unobstructed vision of God. And as regards man's affective life, happiness consists in the immovable repose of his will in the first Good. Therefore man's extreme unhappiness will consist in the fact that his intellect is completely shut off from the divine light, and that his affections are stubbornly turned against God's goodness. And this is the chief suffering of the damned. It is known as the punishment of loss.

However, as should be clear from what we said on a previous occasion, evil cannot wholly exclude good, since every evil has its basis in some good. Consequently, although suffering is opposed to happiness, which will be free from all evil, it must be rooted in a good of nature. The good of an intellectual nature consists in the contemplation of truth by the intellect, and in the inclination to good on the part of the will. But all truth and all goodness are derived from the first and supreme good, which is God. Therefore the intellect of a man situated in the extreme misery of hell must have some knowledge of God and some love of God, but only so far as He is the principle of natural perfections. This is natural love. But the soul in hell cannot know and love God as He is in Himself, nor so far as He is the principle of virtue or of grace and the other goods through which intellectual nature is brought to perfection by Him; for this is the perfection of virtue and glory.

Nevertheless men buried in the misery of hell are not deprived of free choice, even though their will is immovably attached to evil. In the same way the blessed retain the power of free choice, although their will is fixed on the Good. Freedom of choice, properly speaking, has to do with election. But election is concerned with the means leading to an end. The last end is naturally desired by every being. Hence all men, by the very fact that they are intellectual, naturally desire happiness as their last end, and they do so with such immovable fixity of purpose that no one can wish to be unhappy. But this is not incompatible with free will, which extends only to means leading to the end. The fact that one man places his happiness in this particular good while another places it in that good, is not characteristic of either of these men so far as he is a man, since in such estimates and desires men exhibit great differences. This variety is explained by each man's condition. By this I mean each man's acquired passions and habits; and so if a man's condition were to undergo change, some other good would appeal to him as most desirable.

This appears most clearly in men who are led by passion to crave some good as the best. When the passion, whether of anger or lust, dies down, they no longer have the same estimate of that good as they had before.

Habits are more permanent, and so men persevere more obstinately in seeking goods to which habit impels them. Yet, so long as habit is capable of change, man's desire and his judgment as to what constitutes the last end are subject to change. This possibility is open to men only during the present life, in which their state is changeable. After this life the soul is not subject to alteration. No change can affect it except indirectly, in consequence of some change undergone by the body.

However, when the body is resumed, the soul will not be governed by changes occurring in the body. Rather, the contrary will take place. During our present life the soul is infused into a body that has been generated of seed, and therefore, as we should expect, is affected by changes experienced in the body. But in the next world the body will be united to a pre-existing soul, and so will be completely governed by the latter's conditions. Accordingly the soul will remain perpetually in whatever last end it is found to have set for itself at the time of death, desiring that state as the most suitable, whether it is good or evil. This is the meaning of Ecclesiastes 11:3: "If the tree fall to the south or to the north, in what place soever it shall fall, there shall it be." After this life, therefore, those who are found good at the instant of death will have their wills forever fixed in good. But those who are found evil at that moment will be forever obstinate in evil.