Objection 1: It would seem that understanding is not a part of prudence. When two things are members of a division, one is not part of the other. But intellectual virtue is divided into understanding and prudence, according to Ethic. vi, 3. Therefore understanding should not be reckoned a part of prudence.
Obj. 2: Further, understanding is numbered among the gifts of the Holy Ghost, and corresponds to faith, as stated above (Q. 8, AA. 1, 8). But prudence is a virtue other than faith, as is clear from what has been said above (Q. 4, A. 8; I-II, Q. 62, A. 2). Therefore understanding does not pertain to prudence.
Obj. 3: Further, prudence is about singular matters of action (Ethic. vi, 7): whereas understanding takes cognizance of universal and immaterial objects (De Anima iii, 4). Therefore understanding is not a part of prudence.
On the contrary, Tully [*De Invent. Rhet. ii, 53] accounts "intelligence" a part of prudence, and Macrobius [*In Somn. Scip. i, 8] mentions "understanding," which comes to the same.
I answer that, Understanding denotes here, not the intellectual power, but the right estimate about some final principle, which is taken as self-evident: thus we are said to understand the first principles of demonstrations. Now every deduction of reason proceeds from certain statements which are taken as primary: wherefore every process of reasoning must needs proceed from some understanding. Therefore since prudence is right reason applied to action, the whole process of prudence must needs have its source in understanding. Hence it is that understanding is reckoned a part of prudence.
Reply Obj. 1: The reasoning of prudence terminates, as in a conclusion, in the particular matter of action, to which, as stated above (Q. 47, AA. 3, 6), it applies the knowledge of some universal principle. Now a singular conclusion is argued from a universal and a singular proposition. Wherefore the reasoning of prudence must proceed from a twofold understanding. The one is cognizant of universals, and this belongs to the understanding which is an intellectual virtue, whereby we know naturally not only speculative principles, but also practical universal principles, such as "One should do evil to no man," as shown above (Q. 47, A. 6). The other understanding, as stated in Ethic. vi, 11, is cognizant of an extreme, i.e. of some primary singular and contingent practical matter, viz. the minor premiss, which must needs be singular in the syllogism of prudence, as stated above (Q. 47, AA. 3, 6). Now this primary singular is some singular end, as stated in the same place. Wherefore the understanding which is a part of prudence is a right estimate of some particular end.
Reply Obj. 2: The understanding which is a gift of the Holy Ghost, is a quick insight into divine things, as shown above (Q. 8, AA. 1, 2). It is in another sense that it is accounted a part of prudence, as stated above.
Reply Obj. 3: The right estimate about a particular end is called both "understanding," in so far as its object is a principle, and "sense," in so far as its object is a particular. This is what the Philosopher means when he says (Ethic. v, 11): "Of such things we need to have the sense, and this is understanding." But this is to be understood as referring, not to the particular sense whereby we know proper sensibles, but to the interior sense, whereby we judge of a particular.