Objection 1. It would seem that shrewdness is not a part of prudence. For shrewdness consists in easily finding the middle term for demonstrations, as stated in Poster. i, 34. Now the reasoning of prudence is not a demonstration since it deals with contingencies. Therefore shrewdness does not pertain to prudence.
Objection 2. Further, good counsel pertains to prudence according to Ethic. vi, 5,7,9. Now there is no place in good counsel for shrewdness [Ethic. vi, 9; Poster. i, 34 which is a kind of eustochia, i.e. "a happy conjecture": for the latter is "unreasoning and rapid," whereas counsel needs to be slow, as stated in Ethic. vi, 9. Therefore shrewdness should not be accounted a part of prudence.
Objection 3. Further, shrewdness as stated above (Article 48) is a "happy conjecture." Now it belongs to rhetoricians to make use of conjectures. Therefore shrewdness belongs to rhetoric rather than to prudence.
On the contrary, Isidore says (Etym. x): "A solicitous man is one who is shrewd and alert [solers citus]." But solicitude belongs to prudence, as stated above (Question 47, Article 09). Therefore shrewdness does also.
I answer that, Prudence consists in a right estimate about matters of action. Now a right estimate or opinion is acquired in two ways, both in practical and in speculative matters, first by discovering it oneself, secondly by learning it from others. Now just as docility consists in a man being well disposed to acquire a right opinion from another man, so shrewdness is an apt disposition to acquire a right estimate by oneself, yet so that shrewdness be taken for eustochia, of which it is a part. For eustochia is a happy conjecture about any matter, while shrewdness is "an easy and rapid conjecture in finding the middle term" (Poster. i, 34). Nevertheless the philosopher [Andronicus; Cf. 48, Objection 1] who calls shrewdness a part of prudence, takes it for eustochia, in general, hence he says: "Shrewdness is a habit whereby congruities are discovered rapidly."
Reply to Objection 1. Shrewdness is concerned with the discovery of the middle term not only in demonstrative, but also in practical syllogisms, as, for instance, when two men are seen to be friends they are reckoned to be enemies of a third one, as the Philosopher says (Poster. i, 34). On this way shrewdness belongs to prudence.
Reply to Objection 2. The Philosopher adduces the true reason (Ethic. vi, 9) to prove that euboulia, i.e. good counsel, is not eustochia, which is commended for grasping quickly what should be done. Now a man may take good counsel, though he be long and slow in so doing, and yet this does not discount the utility of a happy conjecture in taking good counsel: indeed it is sometimes a necessity, when, for instance, something has to be done without warning. It is for this reason that shrewdness is fittingly reckoned a part of prudence.
Reply to Objection 3. Rhetoric also reasons about practical matters, wherefore nothing hinders the same thing belonging both to rhetoric and prudence. Nevertheless, conjecture is taken here not only in the sense in which it is employed by rhetoricians, but also as applicable to all matters whatsoever wherein man is said to conjecture the truth.
Source: Summa Theologica (New Advent)