Objection 1. It would seem that prudence is not a special virtue. For no special virtue is included in the definition of virtue in general, since virtue is defined (Ethic. ii, 6) "an elective habit that follows a mean appointed by reason in relation to ourselves, even as a wise man decides." Now right reason is reason in accordance with prudence, as stated in Ethic. vi, 13. Therefore prudence is not a special virtue.
Objection 2. Further, the Philosopher says (Ethic. vi, 13) that "the effect of moral virtue is right action as regards the end, and that of prudence, right action as regards the means." Now in every virtue certain things have to be done as means to the end. Therefore prudence is in every virtue, and consequently is not a special virtue.
Objection 3. Further, a special virtue has a special object. But prudence has not a special object, for it is right reason "applied to action" (Ethic. vi, 5); and all works of virtue are actions. Therefore prudence is not a special virtue.
On the contrary, It is distinct from and numbered among the other virtues, for it is written (Wisdom 8:7): "She teacheth temperance and prudence, justice and fortitude."
I answer that, Since acts and habits take their species from their objects, as shown above (I-II, 01, 3; I-II, 18, 2; I-II, 54, 2), any habit that has a corresponding special object, distinct from other objects, must needs be a special habit, and if it be a good habit, it must be a special virtue. Now an object is called special, not merely according to the consideration of its matter, but rather according to its formal aspect, as explained above (I-II, 54, 2, ad 1). Because one and the same thing is the subject matter of the acts of different habits, and also of different powers, according to its different formal aspects. Now a yet greater difference of object is requisite for a difference of powers than for a difference of habits, since several habits are found in the same power, as stated above (I-II, 54, 1). Consequently any difference in the aspect of an object, that requires a difference of powers, will "a fortiori" require a difference of habits.
Accordingly we must say that since prudence is in the reason, as stated above (Article 2), it is differentiated from the other intellectual virtues by a material difference of objects. "Wisdom," "knowledge" and "understanding" are about necessary things, whereas "art" and "prudence" are about contingent things, art being concerned with "things made," that is, with things produced in external matter, such as a house, a knife and so forth; and prudence, being concerned with "things done," that is, with things that have their being in the doer himself, as stated above (I-II, 57, 4). On the other hand prudence is differentiated from the moral virtues according to a formal aspect distinctive of powers, i.e. the intellective power, wherein is prudence, and the appetitive power, wherein is moral virtue. Hence it is evident that prudence is a special virtue, distinct from all other virtues.
Reply to Objection 1. This is not a definition of virtue in general, but of moral virtue, the definition of which fittingly includes an intellectual virtue, viz., prudence, which has the same matter in common with moral virtue; because, just as the subject of moral virtue is something that partakes of reason, so moral virtue has the aspect of virtue, in so far as it partakes of intellectual virtue.
Reply to Objection 2. This argument proves that prudence helps all the virtues, and works in all of them; but this does not suffice to prove that it is not a special virtue; for nothing prevents a certain genus from containing a species which is operative in every other species of that same genus, even as the sun has an influence over all bodies.
Reply to Objection 3. Things done are indeed the matter of prudence, in so far as they are the object of reason, that is, considered as true: but they are the matter of the moral virtues, in so far as they are the object of the appetitive power, that is, considered as good.
Source: Summa Theologica (New Advent)