Objection 1. It would seem that there can be intellectual without moral virtue. Because perfection of what precedes does not depend on the perfection of what follows. Now reason precedes and moves the sensitive appetite. Therefore intellectual virtue, which is a perfection of the reason, does not depend on moral virtue, which is a perfection of the appetitive faculty; and can be without it.
Objection 2. Further, morals are the matter of prudence, even as things makeable are the matter of art. Now art can be without its proper matter, as a smith without iron. Therefore prudence can be without the moral virtue, although of all the intellectual virtues, it seems most akin to the moral virtues.
Objection 3. Further, prudence is "a virtue whereby we are of good counsel" (Ethic. vi, 9). Now many are of good counsel without having the moral virtues. Therefore prudence can be without a moral virtue.
On the contrary, To wish to do evil is directly opposed to moral virtue; and yet it is not opposed to anything that can be without moral virtue. Now it is contrary to prudence "to sin willingly" (Ethic. vi, 5). Therefore prudence cannot be without moral virtue.
I answer that, Other intellectual virtues can, but prudence cannot, be without moral virtue. The reason for this is that prudence is the right reason about things to be done (and this, not merely in general, but also in particular); about which things actions are. Now right reason demands principles from which reason proceeds to argue. And when reason argues about particular cases, it needs not only universal but also particular principles. As to universal principles of action, man is rightly disposed by the natural understanding of principles, whereby he understands that he should do no evil; or again by some practical science. But this is not enough in order that man may reason aright about particular cases. For it happens sometimes that the aforesaid universal principle, known by means of understanding or science, is destroyed in a particular case by a passion: thus to one who is swayed by concupiscence, when he is overcome thereby, the object of his desire seems good, although it is opposed to the universal judgment of his reason. Consequently, as by the habit of natural understanding or of science, man is made to be rightly disposed in regard to the universal principles of action; so, in order that he be rightly disposed with regard to the particular principles of action, viz. the ends, he needs to be perfected by certain habits, whereby it becomes connatural, as it were, to man to judge aright to the end. This is done by moral virtue: for the virtuous man judges aright of the end of virtue, because "such a man is, such does the end seem to him" (Ethic. iii, 5). Consequently the right reason about things to be done, viz. prudence, requires man to have moral virtue.
Reply to Objection 1. Reason, as apprehending the end, precedes the appetite for the end: but appetite for the end precedes the reason, as arguing about the choice of the means, which is the concern of prudence. Even so, in speculative matters the understanding of principles is the foundation on which the syllogism of the reason is based.
Reply to Objection 2. It does not depend on the disposition of our appetite whether we judge well or ill of the principles of art, as it does, when we judge of the end which is the principle in moral matters: in the former case our judgment depends on reason alone. Hence art does not require a virtue perfecting the appetite, as prudence does.
Reply to Objection 3. Prudence not only helps us to be of good counsel, but also to judge and command well. This is not possible unless the impediment of the passions, destroying the judgment and command of prudence, be removed; and this is done by moral virtue.
Source: Summa Theologica (New Advent)