I answer that, As stated above (Question [123], Article [12]; Question [136], Article [1]), it belongs to moral virtue to safeguard the good of reason against the passions that rebel against reason. Now the movement of the soul's passions is twofold, as stated above (FS, Question [23], Article [2]), when we were treating of the passions: the one, whereby the sensitive appetite pursues sensible and bodily goods, the other whereby it flies from sensible and bodily evils.
The first of these movements of the sensitive appetite rebels against reason chiefly by lack of moderation. Because sensible and bodily goods, considered in their species, are not in opposition to reason, but are subject to it as instruments which reason employs in order to attain its proper end: and that they are opposed to reason is owing to the fact that the sensitive appetite fails to tend towards them in accord with the mode of reason. Hence it belongs properly to moral virtue to moderate those passions which denote a pursuit of the good.
On the other hand, the movement of the sensitive appetite in flying from sensible evil is mostly in opposition to reason, not through being immoderate, but chiefly in respect of its flight: because, when a man flies from sensible and bodily evils, which sometimes accompany the good of reason, the result is that he flies from the good of reason. Hence it belongs to moral virtue to make man while flying from evil to remain firm in the good of reason.
Accordingly, just as the virtue of fortitude, which by its very nature bestows firmness, is chiefly concerned with the passion, viz. fear, which regards flight from bodily evils, and consequently with daring, which attacks the objects of fear in the hope of attaining some good, so, too, temperance, which denotes a kind of moderation, is chiefly concerned with those passions that tend towards sensible goods, viz. desire and pleasure, and consequently with the sorrows that arise from the absence of those pleasures. For just as daring presupposes objects of fear, so too such like sorrow arises from the absence of the aforesaid pleasures.
