I've always been really taken with Thomas Aquinas's account of the four cardinal virtues, which I think is actually quite brilliant, but the way he develops it, I think, obscures a bit what he is doing. In effect, Aquinas is having to deal with a peculiar set of problems. First, there are vast numbers of lists of virtues, constructed on all sorts of different principles, and, for that matter, there is necessarily a huge number of virtues to be expected. So we need some kind of way to keep track of them. We need reference-point virtues so that we can stay oriented in Virtue Country. Second, you might expect a very strong Aristotelian like Aquinas to favor Aristotle's own preferred set of reference-point virtues, but there's a practical problem. Aristotle's preferred set makes sense, but it's not particularly obvious or straightforward. Essentially, Aristotle picked out the virtues that were most important for life in a Greek city-state. This means that for a lot of other contexts, the selection has an odd flavor, including as it does things like magnanimity (originally linked to how one relates to social status in a Greek city-state), magnificence (originally linked to Greek taxation practices), eutrapelia (the virtue of playfulness and witty banter, originally linked to Greek customs of relaxation). The problem is not the virtues themselves -- they're all good candidates, they all have some importance somewhere, and St. Thomas certainly takes them all to be virtues, although occasionally with modification -- but while it's a great reference-point list for Greek city-states, picking out some of these virtues as the primary reference points is arguably something you'd only do for a Greek city-state. Third, the list of phronesis, dikaiosyne, andreia, and sophrosyne, or, to use the Latin, prudentia, iustitia, fortitudo, temperantia, is both much easier to use and far more widespread, due to the fact that it was favored by Platonists, Stoics, and (also important for St. Thomas) is specifically referred to in the Biblical book, the Wisdom of Solomon.
So what St. Thomas really needs is an Aristotelian reason to favor the more common list. If you are an Aristotelian and you had that, you wouldn't have to change much in Aristotle's ethics to adapt it to the common list. While Aristotle does not treat the four cardinal virtues as a list as particularly important, all four of the cardinal virtues are in Aristotle's list of virtues; what is more, all four of them are virtues which he discusses extensively. This isn't surprising; he was a student of Plato, who uses the list occasionally, and lived in Ancient Greek, where it was apparently a fairly common list, since Plato doesn't seem to have invented it. So this is what we want to find out: Is there a specifically Aristotelian reason why we might take the four cardinal virtues as our primary reference-point list for virtues? And St. Thomas solves this problem, conclusively, and indeed in such a way that it explains why the list of the four cardinal virtues is so extremely popular and practically useful. As I said, the way he sets it up in some ways obscures the brilliance of what he is doing, so what follows is a re-organizing of it to bring it out.
There is no more Aristotelian place to start than with Aristotle's definition of virtue. Aristotle tells us that a virtue is:
(1) a kind of second nature (hexis/habitus)
(2) concerned with choice
(3) consisting in a mean relative to us
(4) as determined by reason in the way a prudent person would determine it.
So, to fit this definition, a virtue is going to have to have certain qualities. You have something as a kind of second nature when you can easily and consistently perform the relevant actions in a wide variety of circumstances. So what is second nature is durable, enduring, consistent. From this we learn that a virtue needs to be STABLE. Virtues are concerned with choices, and choice is the structuring principle of deliberate human action, so from this we learn that virtues need to be concerned with action or ACTIVE. A mean is literally a middle between extremes, but Aristotle is clear that it's a matter of balancing requirements so as to avoid what is too much or too little, and thus not necessarily exactly in the middle. So from this we know that every virtue must in some way find a harmonious balance, that is, every virtue must be MODERATE. And, of course, the last portion of the definition tells us directly that every virtue must in some way be RATIONAL.
Thus every virtue, without exception, needs to be stable, active, moderate, and rational; moral life is to live according to a character that harmoniously blends these four qualities. But specific virtues concern different aspects of life and have different content. Honesty is very different from chastity or patience, despite the fact that each of these is in some way stable, in some way active, in some way moderate, and in some way rational. And when you look at specific virtues, it becomes clear that, while they all have these qualities, they do not all equally display these qualities. Compare modesty and patience. Both have all four qualities because both are virtues. But it's much more obvious that patience involves stability than that modesty does. Modesty, the virtue concerned with how you present yourself in public, is much more adaptable to different details in different situations; its stability is more abstract. If you are trying to be patient, you end up having to focus on remaining stable a lot; if you are trying to be modest, on the other hand, you will focus much more on finding a good balance. Patience involves balance or moderation, but it's not so obviously a balancing act as modesty is. This generally the case. All virtues have all four qualities, but different virtues will more clearly manifest one of the qualities than the others.
The implication is that virtues fall into four different families, depending on which of the four qualities they more obviously express. Some virtues more obviously involve rationality, others action, others stability, others moderation. Every virtue is related to every other virtue, but the natural grouping of virtues is by these families. So what we want for our reference-point list -- for the North, South, East, and West of our map of Virtue Country* -- is to pick out the virtue from each family that is most usable as a reference point. In practice, this means we need to pick out virtues that both (1) directly deal with very pervasive or common features of human life and (2) cover the most important cases in which a given quality of virtue needs to be manifested.
So let's start with the RATIONALITY FAMILY, the family of virtues that very obviously have to do with thinking and reasoning and being reasonable. It's the easiest, because the central virtue of the family falls directly into our lap from Aristotle's definition: PRUDENCE. And this makes sense. A lot of the virtues in this family obviously concern things that make it easier to make good decisions -- caution or carefulness, for instance, or reasonable estimation (good guessing), or teachableness (docility). But prudence is the virtue of actually making good decisions -- prudence pays attention to the circumstances and makes a decision appropriate to them -- and there's nothing else that is more properly characterized as pervasive and important for living as a human being than making decisions. All virtues in the Rationality family will be prudence-like in some ways, because prudence is, of all the virtues in the Rationality family, the one most commonly and completely expressive of the rationality of good character.
Now let's turn to the ACTIVITY FAMILY of virtue, the family that is most obviously expressive of the active nature of virtue. Every virtue is active, but the arena of human life in which we are obviously going to most clearly see moral action is the social arena -- an arena in which we face constant demands to act toward other people, with other people, and on behalf of other people. All of society is coordination of action. So while the Activity family of virtue is not exclusively social in its expression, every member of the family has a very obvious social side. So we need a virtue in this society-building family that concerns action-situations that are extremely common and also very important. There are several candidates (human social life is a vast field, so it covers a lot of very important things). But an extremely good candidate is the virtue that has as its particular province the preserving of goodness in human action. There are many ways in which I can act that would benefit me but only at a cost to you, and many ways in which you can act for your own benefit but only by leaving me worse off. If these actions are all we have, society collapses entirely into a struggle to exploit and coerce -- if you don't exploit others, you will lose everything for their benefit. In good social interactions, however, all parties should benefit, at least to the extent we can guarantee this by deliberate action. My benefit and your benefit do not have to be the same, but we need to come away with them being in some way even. The virtue that guarantees this is JUSTICE. Justice renders what is due so that we benefit evenly. There are lots of other virtues in the family, but we can easily understand them better by relating them to justice. For instance, some of the virtues of the family -- filial piety, religious devotion, gratitude -- handle situations where it's impossible to give an appropriate return to make things even (you will never be even with your parents or God, and those who give you gifts often do so in situations in which trying to make it even would be inappropriate); others, like everyday respect or honesty, deal with a different kind of equality or evennness, or else help make it easier to act justly.
The STABILITY FAMILY handles matters that most obviously require consistency, an ability not to be pushed around by circumstances. Thus, while they don't only deal with difficulties, they do obviously handle difficulties. So to find the best reference-point virtue, we need to consider a difficulty that is extremely common and has a huge amount of power to affect our choices. And there is one difficulty all human beings face, without exception, in one way or another, that affects our choices like nothing else: our own mortality. Death, and things that could kill us, and things that could people we care about -- these are universal and have an immense power to influence our choices. There are times when we face difficulties that we would regard as more difficult to face even than death -- but those are rare. There are difficulties that are as universal as death -- but none more devastating. So the central virtue of the Stability family, the virtue that most perfectly expresses moral strength and endurance, is FORTITUDE, endurance even in the face of death. To face even death and not be broken by it -- this is a test of the stability of your character, and one that we all at some point have to face. Other virtues in the Stability family, like patience, are obviously important, and they obviously have a lot in common with fortitude, revolving around it like planets around a sun, or like stars around an even bigger star. But there is no question which virtue has the greatest gravity.
That leaves the family of virtues that most obviously show that virtue involves a balance, the MODERATION FAMILY. All situations require balancing a lot of different things, but the situations where we most often have to focus specifically on finding a balance are situations that involve powerful motivators -- things associated with strong pleasures. Nothing is more likely to result in toppling over than reaching for something very pleasant. So all members of the Moderation family, while not exclusively concerned with pleasurable situations, nonetheless have something or other to do with pleasure and the strong motivations that are tied up with it. When somebody finds a balance in the face of very tempting pleasures -- that's the situation in which you most obviously see how virtue can be and must be moderate. So what we need to do to find our reference-point virtue is find a kind of pleasure that affects our choices to a large extent and is extremely common. It doesn't take much to find it. You'll have difficulty finding pleasures that are more common than those tied directly to the needs of our biology and the most basic of these -- our individual survival needs of food and drink, the need of our species to reproduce -- involve immensely powerful motivations. Thus the reference-point virtue for the Moderation family is TEMPERANCE, the virtue of restricting the pursuit of physical pleasures in light of what is more important (or more human, as St. Thomas likes to put it). Other virtues in the family -- eutrapelia, modesty, studiousness, and so forth -- also deal with pleasures, and so are temperance-like, but they are either pleasures that are more rare or pleasures that are easier to restrict.
Thus we have our North, South, East, and West of Virtue Country: Prudence, Justice, Fortitude, Temperance. Just like every direction is related to every other direction but they are not all equally related to each other, so also every virtue is related to every other virtue but not equally. Just like we pick as reference-points the directions that are easier to use for orienting ourselves in our environment (North from the north star or magnetic north, East and West from the rising and setting of the sun, and so forth), so we pick as our reference-points the virtues that are easiest for orienting ourselves (because they concern things that are very common and have a significant effect on our lives). And from Aristotle's definition of virtue, as Aquinas shows, we can prove why these four are such useful candidates for reference-points. We could pick any directions we wanted to be cardinal compass points, but we usually pick North, South, East, and West, for extremely good reasons. And likewise, since all virtues are related to all virtues in some way, you can start with any set of virtues and use them to help you understand every other virtue. But there are reasons why Prudence, Justice, Fortitude, and Temperance make a list that people keep coming back to, that people repeatedly find to be easy to use and valuable, and if you are an Aristotelian, Aristotle's definition of virtue explains that.
* The adjective 'cardinal' in the name 'cardinal virtues' is derived from the Latin word for a hinge, so it's common to say (and St. Thomas does) that they are in a sense the 'hinge virtues'. But St. Ambrose, who actually came up with the name 'cardinal virtues', almost certainly intended to draw a parallel with a figurative use of the term, to indicate the cardinal directions of the compass, and doesn't seem to have had the literal sense of the term in mind at all.