Objection 1. It would seem that the moral virtues are not connected with one another. Because moral virtues are sometimes caused by the exercise of acts, as is proved in Ethic. ii, 1,2. But man can exercise himself in the acts of one virtue, without exercising himself in the acts of some other virtue. Therefore it is possible to have one moral virtue without another.
Objection 2. Further, magnificence and magnanimity are moral virtues. Now a man may have other moral virtues without having magnificence or magnanimity: for the Philosopher says (Ethic. iv, 2,3) that "a poor man cannot be magnificent," and yet he may have other virtues; and (Ethic. iv) that "he who is worthy of small things, and so accounts his worth, is modest, but not magnanimous." Therefore the moral virtues are not connected with one another.
Objection 3. Further, as the moral virtues perfect the appetitive part of the soul, so do the intellectual virtues perfect the intellective part. But the intellectual virtues are not mutually connected: since we may have one science, without having another. Neither, therefore, are the moral virtues connected with one another.
Objection 4. Further, if the moral virtues are mutually connected, this can only be because they are united together in prudence. But this does not suffice to connect the moral virtues together. For, seemingly, one may be prudent about things to be done in relation to one virtue, without being prudent in those that concern another virtue: even as one may have the art of making certain things, without the art of making certain others. Now prudence is right reason about things to be done. Therefore the moral virtues are not necessarily connected with one another.
On the contrary, Ambrose says on Luke 6:20: "The virtues are connected and linked together, so that whoever has one, is seen to have several": and Augustine says (De Trin. vi, 4) that "the virtues that reside in the human mind are quite inseparable from one another": and Gregory says (Moral. xxii, 1) that "one virtue without the other is either of no account whatever, or very imperfect": and Cicero says (Quaest. Tusc. ii): "If you confess to not having one particular virtue, it must needs be that you have none at all."
I answer that, Moral virtue may be considered either as perfect or as imperfect. An imperfect moral virtue, temperance for instance, or fortitude, is nothing but an inclination in us to do some kind of good deed, whether such inclination be in us by nature or by habituation. If we take the moral virtues in this way, they are not connected: since we find men who, by natural temperament or by being accustomed, are prompt in doing deeds of liberality, but are not prompt in doing deeds of chastity.
But the perfect moral virtue is a habit that inclines us to do a good deed well; and if we take moral virtues in this way, we must say that they are connected, as nearly as all are agreed in saying. For this two reasons are given, corresponding to the different ways of assigning the distinction of the cardinal virtues. For, as we stated above (61, A3,4), some distinguish them according to certain general properties of the virtues: for instance, by saying that discretion belongs to prudence, rectitude to justice, moderation to temperance, and strength of mind to fortitude, in whatever matter we consider these properties to be. In this way the reason for the connection is evident: for strength of mind is not commended as virtuous, if it be without moderation or rectitude or discretion: and so forth. This, too, is the reason assigned for the connection by Gregory, who says (Moral. xxii, 1) that "a virtue cannot be perfect" as a virtue, "if isolated from the others: for there can be no true prudence without temperance, justice and fortitude": and he continues to speak in like manner of the other virtues (cf. 61, 4, Objection 1). Augustine also gives the same reason (De Trin. vi, 4).
Others, however, differentiate these virtues in respect of their matters, and it is in this way that Aristotle assigns the reason for their connection (Ethic. vi, 13). Because, as stated above (Question 58, Article 4), no moral virtue can be without prudence; since it is proper to moral virtue to make a right choice, for it is an elective habit. Now right choice requires not only the inclination to a due end, which inclination is the direct outcome of moral virtue, but also correct choice of things conducive to the end, which choice is made by prudence, that counsels, judges, and commands in those things that are directed to the end. In like manner one cannot have prudence unless one has the moral virtues: since prudence is "right reason about things to be done," and the starting point of reason is the end of the thing to be done, to which end man is rightly disposed by moral virtue. Hence, just as we cannot have speculative science unless we have the understanding of the principles, so neither can we have prudence without the moral virtues: and from this it follows clearly that the moral virtues are connected with one another.
Reply to Objection 1. Some moral virtues perfect man as regards his general state, in other words, with regard to those things which have to be done in every kind of human life. Hence man needs to exercise himself at the same time in the matters of all moral virtues. And if he exercise himself, by good deeds, in all such matters, he will acquire the habits of all the moral virtues. But if he exercise himself by good deeds in regard to one matter, but not in regard to another, for instance, by behaving well in matters of anger, but not in matters of concupiscence; he will indeed acquire a certain habit of restraining his anger; but this habit will lack the nature of virtue, through the absence of prudence, which is wanting in matters of concupiscence. In the same way, natural inclinations fail to have the complete character of virtue, if prudence be lacking.
But there are some moral virtues which perfect man with regard to some eminent state, such as magnificence and magnanimity; and since it does not happen to all in common to be exercised in the matter of such virtues, it is possible for a man to have the other moral virtues, without actually having the habits of these virtues--provided we speak of acquired virtue. Nevertheless, when once a man has acquired those other virtues he possesses these in proximate potentiality. Because when, by practice, a man has acquired liberality in small gifts and expenditure, if he were to come in for a large sum of money, he would acquire the habit of magnificence with but little practice: even as a geometrician, by dint of little study, acquires scientific knowledge about some conclusion which had never been presented to his mind before. Now we speak of having a thing when we are on the point of having it, according to the saying of the Philosopher (Phys. ii, text. 56): "That which is scarcely lacking is not lacking at all."
This suffices for the Reply to the Second Objection.
Reply to Objection 3. The intellectual virtues are about divers matters having no relation to one another, as is clearly the case with the various sciences and arts. Hence we do not observe in them the connection that is to be found among the moral virtues, which are about passions and operations, that are clearly related to one another. For all the passions have their rise in certain initial passions, viz. love and hatred, and terminate in certain others, viz. pleasure and sorrow. In like manner all the operations that are the matter of moral virtue are related to one another, and to the passions. Hence the whole matter of moral virtues falls under the one rule of prudence.
Nevertheless, all intelligible things are related to first principles. And in this way, all the intellectual virtues depend on the understanding of principles; even as prudence depends on the moral virtues, as stated. On the other hand, the universal principles which are the object of the virtue of understanding of principles, do not depend on the conclusions, which are the objects of the other intellectual virtues, as do the moral virtues depend on prudence, because the appetite, in a fashion, moves the reason, and the reason the appetite, as stated above (9, 1; 58, 5, ad 1).
Reply to Objection 4. Those things to which the moral virtues incline, are as the principles of prudence: whereas the products of art are not the principles, but the matter of art. Now it is evident that, though reason may be right in one part of the matter, and not in another, yet in no way can it be called right reason, if it be deficient in any principle whatever. Thus, if a man be wrong about the principle, "A whole is greater than its part," he cannot acquire the science of geometry, because he must necessarily wander from the truth in his conclusion. Moreover, things "done" are related to one another, but not things "made," as stated above (ad 3). Consequently the lack of prudence in one department of things to be done, would result in a deficiency affecting other things to be done: whereas this does not occur in things to be made.
Source: Summa Theologica (New Advent)