To the third one proceeds thus. It seems that habitude is not distinguished according to good and bad, for good and bad are contraries. But the same habitude is of contraries, as was said above. Therefore habitude is not distinguished according to good and bad.
Further, good is converted with being, and so, because it is common to all, it is not able to be taken as the differentia of some species, as is obvious from the Philosopher in Topic. IV. And likewise bad, because it is privation and not being, is not able to be the differentia of some being. Therefore habitude is not able to be distinguished in species according to good and bad.
Further, about the same there happen to be diverse bad habitudes, as intemperance and insensibility about concupiscence, and likewise for many good habitudes, such as human virtue and divine or heroic virtue, as is obvious from the Philosopher in Ethic. VII. Therefore habitude is not distinguished according to good and bad.
But contrariwise is that good habitude is contrary to bad habitude, as virtue to vice. But contraries are diverse according to species. Therefore habitude differs in species according to good and bad.
I reply that it must be said that, as was said, habitude is distinguished in species not only according to object and active principle, but also in ordering to nature. This can happen in two ways. In one way according to their fittingness to nature, or also to their unfittingness to it. And in this way good and bad habitudes are distinguished in species, since a habitude is called good that disposes to an act fitting to the nature of the agent, but a habitude is called bad that disposes to an act not fitting to the nature; as acts of virtues are fitting to human nature, in that they are according to reason, but acts of vices, because they are against reason, are discordant with human nature.
In another way that habitude is distinguished according to nature, because one habitude disposes to an act that is fitting to an inferior nature, but another habitude disposes to an act that is fitting to a superior nature. And so human virtue, which disposes to an act appropriate to human nature, is distinguished from divine or heroic virtue, which disposes to an act fitting to a superior nature.
To the first therefore it must be said that one habitude is able to be of contraries, inasmuch as the contraries converge into one notion. However, it never happens that contrary habitudes are of one species, for the contrariety of habitudes is according to the contrariety of notions. And thus habitudes are distinguished according to good and bad, namely, inasmuch as one habitude is good and another bad, but not from the fact that one is of good and the other of bad.
To the second it must be said that the good common to every human being is not the differentia constituting the species of some habitude, but rather a sort of determinate good, which is according to its fittingness to a determinate nature, namely, human. Likewise, the bad that is a differentia constitutive of habitude is not pure privation but is something repugnant to a determinate nature.
To the third it must be said that several good habitudes about the same species are distinguished according to fittingness to diverse natures, as was said. But several bad habitudes about the same action are distinguished according to diverse repugnances to that which is according to nature, as one virtue is contrary to diverse vices about the same matter.
[Thomas Aquinas, ST 2-1.54.3, my translation. The Latin is here, the Dominican Fathers translation is here, both of which, however, seem to have some minor textual problems.]
This, of course, is a key article, since it is (as is clear from the examples used) the foundation of the entire theory of virtues and vices.
