Think carefully about this chapter, and how it fits in with our understanding of science. Wills can’t be changed in dead bodies.
1 From these points this is clear: souls immediately after their separation from the body become unchangeable in will, with the result that the will of man cannot further be changed, neither from good to evil, nor from evil to good.
2 As long as the soul can be changed from good to evil or evil to good, it is in a state of struggle and warfare, for it must with solicitude resist evil lest it be conquered by evil, or it must try to be freed from it. Immediately after the soul is separated from the body it will not be in a state of warfare or struggle, but in a state of receiving reward or punishment, because it “has lawfully or unlawfully striven” (2 Tim. 2:5). For it was shown that reward or punishment follows immediately. No longer, then, is the soul able to be changed in its willing, whether from good to evil, or from evil to good.
3 Then, too, in Book III it was shown that beatitude which consists in the vision of God is everlasting; and in like fashion we showed in the same Book that mortal sin deserves eternal punishment. But a soul cannot be blessed if its will is not going to be right and it ceases to be right by being tamed away from the end: but it cannot simultaneously be turned away from the end and enjoying the end. Necessarily, then, the rectitude of the will in the blessed soul is everlasting; as a result, it cannot be changed from good to evil.
4 The rational creature, furthermore, naturally desires to be happy; hence, it cannot wish not to be happy. For all that, its will can be deflected from Him in whom its true beatitude consists; this is the perversity of will. And this takes place because that in which there is the true beatitude is not grasped essentially as beatitude, but something else is, and toward this the disordered will is deflected as though to an end.
For example, take the man who puts his end in bodily pleasures; he thinks they are the greatest good, and this is essential to his beatitude. But those who are already happy grasp that in which there truly is beatitude essentially as beatitude and as ultimate end; otherwise, there would be therein no quiet of the appetite and, in consequence, they would not be happy.
Therefore, all those who are happy cannot turn their wills away from Him in whom the true happiness is. Therefore, they can have no perversity of will.
Notes Being forever hungry is not to be happy.
5 Then, too, when what one has suffices him, he seeks nothing beyond it. But whoever is happy has what suffices him in the true beatitude; otherwise, his desire would not be fulfilled. Therefore, whoever is happy seeks nothing which does not belong to that in which true beatitude consists. But no one has a perverse will unless he wills something repugnant to Him in whom true beatitude consists. Therefore, there is no one of the blessed whose will can be changed to evil.
6 There is more. Sin cannot take place in the will, without some sort of ignorance in the intellect, for we will nothing but the good whether true or apparent. For this reason Proverbs (14:27) says: “They err who work evil”; and in the Ethics the Philosopher says “every evil man is ignorant.” But the soul which is truly happy cannot be in ignorance at all, since in God it sees everything which belongs to its. perfection. Therefore, there is no way for it to have a bad will, especially since that vision of God is always actual, as was shown in Book III.
Notes You see, I trust, the argument against relativism embedded here.
7 Our intellect, again, can be in error about some conclusion before a resolution into the first principles is made; once the resolution into the principles is made, one has knowledge of the conclusions in which there can be no falsity. “But what the principle of demonstration is in speculative matters, so the end is in matters of appetite.” Therefore, as long as we do not achieve the ultimate end our will can be perverted, but not after it arrives at the enjoyment of the ultimate end which is desirable in itself, just as the first principles of demonstration are known in themselves.
8 The good, furthermore, is precisely as good the lovable. Therefore, that which is grasped as the best is the most lovable. But a happy rational substance that sees God grasps Him as the best. Therefore, it loves Him the most. But this is an essential of love: the wills of those who love each other are in conformity. Therefore, the wills of the blessed are most in conformity with God, and this makes rightness of will, since the divine will is the first rule of all wills. Therefore, the wills of those who see God cannot be rendered perverse.
9 Once more: So long as a thing is by nature changeable to another it does not have its ultimate end. Therefore, if the blessed soul can still be changed from good to evil, it is not yet in its ultimate end. And this is against the essentials of beatitude. It is clear, then, that the souls which immediately after death are beatified become immutable in their wills.