You see, she replied, there is a battle of the reason with them and a struggle to rid the soul of them; and there are men in whom this struggle has ended in success; it was so with Moses, as we know; he was superior both to anger and to desire; the history testifying of him in both respects, that he was meek beyond all men (and by meekness it indicates the absence of all anger and a mind quite devoid of resentment), and that he desired none of those things about which we see the desiring faculty in the generality so active. This could not have been so, if these faculties were nature, and were referable to the contents of man's essence. For it is impossible for one who has come quite outside of his nature to be in Existence at all. But if Moses was at one and the same time in Existence and not in these conditions, then it follows that these conditions are something other than nature and not nature itself. For if, on the one hand, that is truly nature in which the essence of the being is found, and, on the other, the removal of these conditions is in our power, so that their removal not only does no harm, but is even beneficial to the nature, it is clear that these conditions are to be numbered among externals, and are affections, rather than the essence, of the nature; for the essence is that thing only which it is. As for anger, most think it a fermenting of the blood round the heart; others an eagerness to inflict pain in return for a previous pain; we would take it to be the impulse to hurt one who has provoked us. But none of these accounts of it tally with the definition of the soul. Again, if we were to define what desire is in itself, we should call it a seeking for that which is wanting, or a longing for pleasurable enjoyment, or a pain at not possessing that upon which the heart is set, or a state with regard to some pleasure which there is no opportunity of enjoying. These and such-like descriptions all indicate desire, but they have no connection with the definition of the soul. But it is so with regard to all those other conditions also which we see to have some relation to the soul, those, I mean, which are mutually opposed to each other, such as cowardice and courage, pleasure and pain, fear and contempt, and so on; each of them seems akin to the principle of desire or to that of anger, while they have a separate definition to mark their own peculiar nature. Courage and contempt, for instance, exhibit a certain phase of the irascible impulse; the dispositions arising from cowardice and fear exhibit on the other hand a diminution and weakening of that same impulse. Pain, again, draws its material both from anger and desire. For the impotence of anger, which consists in not being able to punish one who has first given pain, becomes itself pain; and the despair of getting objects of desire and the absence of things upon which the heart is set create in the mind this same sullen state. Moreover, the opposite to pain, I mean the sensation of pleasure, like pain, divides itself between anger and desire; for pleasure is the leading motive of them both. All these conditions, I say, have some relation to the soul, and yet they are not the soul, but only like warts growing out of the soul's thinking part, which are reckoned as parts of it because they adhere to it, and yet are not that actual thing which the soul is in its essence.
And yet, I rejoined to the virgin, we see no slight help afforded for improvement to the virtuous from all these conditions. Daniel's desire was his glory; and Phineas' anger pleased the Deity. We have been told, too, that fear is the beginning of wisdom, and learned from Paul that salvation is the goal of the “sorrow after a godly sort.” The Gospel bids us have a contempt for danger; and the “not being afraid with any amazement” is nothing else but a describing of courage, and this last is numbered by Wisdom among the things that are good. In all this Scripture shows that such conditions are not to be considered weaknesses; weaknesses would not have been so employed for putting virtue into practice.
Source: On the Soul and the Resurrection (New Advent)