Objection 1. It would seem that moral virtues should not be called cardinal or principal virtues. For "the opposite members of a division are by nature simultaneous" (Categor. x), so that one is not principal rather than another. Now all the virtues are opposite members of the division of the genus "virtue." Therefore none of them should be called principal.
Objection 2. Further, the end is principal as compared to the means. But the theological virtues are about the end; while the moral virtues are about the means. Therefore the theological virtues, rather than the moral virtues, should be called principal or cardinal.
Objection 3. Further, that which is essentially so is principal in comparison with that which is so by participation. But the intellectual virtues belong to that which is essentially rational: whereas the moral virtues belong to that which is rational by participation, as stated above (58, 3). Therefore the intellectual virtues are principal, rather than the moral virtues.
On the contrary, Ambrose in explaining the words, "Blessed are the poor in spirit" (Luke 6:20) says: "We know that there are four cardinal virtues, viz. temperance, justice, prudence, and fortitude." But these are moral virtues. Therefore the moral virtues are cardinal virtues.
I answer that, When we speak of virtue simply, we are understood to speak of human virtue. Now human virtue, as stated above (Question 56, Article 3), is one that answers to the perfect idea of virtue, which requires rectitude of the appetite: for such like virtue not only confers the faculty of doing well, but also causes the good deed done. On the other hand, the name virtue is applied to one that answers imperfectly to the idea of virtue, and does not require rectitude of the appetite: because it merely confers the faculty of doing well without causing the good deed to be done. Now it is evident that the perfect is principal as compared to the imperfect: and so those virtues which imply rectitude of the appetite are called principal virtues. Such are the moral virtues, and prudence alone, of the intellectual virtues, for it is also something of a moral virtue, as was clearly shown above (Question 57, Article 4). Consequently, those virtues which are called principal or cardinal are fittingly placed among the moral virtues.
Reply to Objection 1. When a univocal genus is divided into its species, the members of the division are on a par in the point of the generic idea; although considered in their nature as things, one species may surpass another in rank and perfection, as man in respect of other animals. But when we divide an analogous term, which is applied to several things, but to one before it is applied to another, nothing hinders one from ranking before another, even in the point of the generic idea; as the notion of being is applied to substance principally in relation to accident. Such is the division of virtue into various kinds of virtue: since the good defined by reason is not found in the same way in all things.
Reply to Objection 2. The theological virtues are above man, as stated above (58, 3, ad 3). Hence they should properly be called not human, but "super-human" or godlike virtues.
Reply to Objection 3. Although the intellectual virtues, except in prudence, rank before the moral virtues, in the point of their subject, they do not rank before them as virtues; for a virtue, as such, regards good, which is the object of the appetite.
Source: Summa Theologica (New Advent)