Natural law and moral disagreements
Other intellectual virtues, but not prudence, can exist without moral virtue. The reason for this is that prudence is right judgment about things to be done, and this not merely in general, but also in the particular instance, wherein action takes place. Prerequisite for right judgment are principles from which reason proceeds. Yet when reason is concerned with the particular, it needs not only universal principles, but also particular ones. So far as the general principles of practice are concerned, a man is rightly disposed by a natural understanding, by which he knows that he should do no evil, and by some normative science. Yet this is not enough in order that a man may reason rightly about particular cases. In fact, it happens sometimes that general principles and conclusions of understanding and science are swept away in the particular case by a passion. Thus to one who is overcome by lust, the object of his desire then seems good, although it is against his general convictions. Consequently, as by the habits of natural understanding and science, a man is rightly disposed with regard to general truths, so, in order that he be rightly disposed with regard to the particular principles of action, namely, their ends, he needs to be perfected by certain habits, whereby it becomes, as itwere, connatural to him to
judge rightly about an end. This is done by moral virtue, for the virtuous man judges
rightly of the end of virtue, because, as Aristotle says, “such as a man is, such does
the end seem to him.” Consequently right judgment about things to be done, namely
prudence, requires that a man has moral virtue. . . .