Objection 1. It would seem that reason should not be reckoned a part of prudence. For the subject of an accident is not a part thereof. But prudence is in the reason as its subject (Ethic. vi, 5). Therefore reason should not be reckoned a part of prudence.
Objection 2. Further, that which is common to many, should not be reckoned a part of any one of them; or if it be so reckoned, it should be reckoned a part of that one to which it chiefly belongs. Now reason is necessary in all the intellectual virtues, and chiefly in wisdom and science, which employ a demonstrative reason. Therefore reason should not be reckoned a part of prudence
Objection 3. Further, reason as a power does not differ essentially from the intelligence, as stated above (I, 79, 8). If therefore intelligence be reckoned a part of prudence, it is superfluous to add reason.
On the contrary, Macrobius [In Somn. Scip. i], following the opinion of Plotinus, numbers reason among the parts of prudence.
I answer that, The work of prudence is to take good counsel, as stated in Ethic. vi, 7. Now counsel is a research proceeding from certain things to others. But this is the work of reason. Wherefore it is requisite for prudence that man should be an apt reasoner. And since the things required for the perfection of prudence are called requisite or quasi-integral parts of prudence, it follows that reason should be numbered among these parts.
Reply to Objection 1. Reason denotes here, not the power of reason, but its good use.
Reply to Objection 2. The certitude of reason comes from the intellect. Yet the need of reason is from a defect in the intellect, since those things in which the intellective power is in full vigor, have no need for reason, for they comprehend the truth by their simple insight, as do God and the angels. On the other hand particular matters of action, wherein prudence guides, are very far from the condition of things intelligible, and so much the farther, as they are less certain and fixed. Thus matters of art, though they are singular, are nevertheless more fixed and certain, wherefore in many of them there is no room for counsel on account of their certitude, as stated in Ethic. iii, 3. Hence, although in certain other intellectual virtues reason is more certain than in prudence, yet prudence above all requires that man be an apt reasoner, so that he may rightly apply universals to particulars, which latter are various and uncertain.
Reply to Objection 3. Although intelligence and reason are not different powers, yet they are named after different acts. For intelligence takes its name from being an intimate penetration of the truth [Cf. II-II, 08, 1, while reason is so called from being inquisitive and discursive. Hence each is accounted a part of reason as explained above (2; 47, 2,3).
Source: Summa Theologica (New Advent)