Objection 1. It would seem that there is only one moral virtue. Because just as the direction of moral actions belongs to reason which is the subject of the intellectual virtues; so does their inclination belong to the appetite which is the subject of moral virtues. But there is only one intellectual virtue to direct all moral acts, viz. prudence. Therefore there is also but one moral virtue to give all moral acts their respective inclinations.
Objection 2. Further, habits differ, not in respect of their material objects, but according to the formal aspect of their objects. Now the formal aspect of the good to which moral virtue is directed, is one thing, viz. the mean defined by reason. Therefore, seemingly, there is but one moral virtue.
Objection 3. Further, things pertaining to morals are specified by their end, as stated above (Question 1, Article 3). Now there is but one common end of all moral virtues, viz. happiness, while the proper and proximate ends are infinite in number. But the moral virtues themselves are not infinite in number. Therefore it seems that there is but one.
On the contrary, One habit cannot be in several powers, as stated above (Question 56, Article 2). But the subject of the moral virtues is the appetitive part of the soul, which is divided into several powers, as stated in the I, 80, 2; I, 81, 2. Therefore there cannot be only one moral virtue.
I answer that, As stated above (58, A1,2,3), the moral virtues are habits of the appetitive faculty. Now habits differ specifically according to the specific differences of their objects, as stated above (Question 54, Article 2). Again, the species of the object of appetite, as of any thing, depends on its specific form which it receives from the agent. But we must observe that the matter of the passive subject bears a twofold relation to the agent. For sometimes it receives the form of the agent, in the same kind specifically as the agent has that form, as happens with all univocal agents, so that if the agent be one specifically, the matter must of necessity receive a form specifically one: thus the univocal effect of fire is of necessity something in the species of fire. Sometimes, however, the matter receives the form from the agent, but not in the same kind specifically as the agent, as is the case with non-univocal causes of generation: thus an animal is generated by the sun. In this case the forms received into matter are not of one species, but vary according to the adaptability of the matter to receive the influx of the agent: for instance, we see that owing to the one action of the sun, animals of various species are produced by putrefaction according to the various adaptability of matter.
Now it is evident that in moral matters the reason holds the place of commander and mover, while the appetitive power is commanded and moved. But the appetite does not receive the direction of reason univocally so to say; because it is rational, not essentially, but by participation (Ethic. i, 13). Consequently objects made appetible by the direction of reason belong to various species, according to their various relations to reason: so that it follows that moral virtues are of various species and are not one only.
Reply to Objection 1. The object of the reason is truth. Now in all moral matters, which are contingent matters of action, there is but one kind of truth. Consequently, there is but one virtue to direct all such matters, viz. prudence. On the other hand, the object of the appetitive power is the appetible good, which varies in kind according to its various relations to reason, the directing power.
Reply to Objection 2. This formal element is one generically, on account of the unity of the agent: but it varies in species, on account of the various relations of the receiving matter, as explained above.
Reply to Objection 3. Moral matters do not receive their species from the last end, but from their proximate ends: and these, although they be infinite in number, are not infinite in species.
Source: Summa Theologica (New Advent)