Objection 1: It would seem that the precepts of the decalogue should have included a precept of prudence. For the chief precepts should include a precept of the chief virtue. Now the chief precepts are those of the decalogue. Since then prudence is the chief of the moral virtues, it seems that the precepts of the decalogue should have included a precept of prudence.
Obj. 2: Further, the teaching of the Gospel contains the Law especially with regard to the precepts of the decalogue. Now the teaching of the Gospel contains a precept of prudence (Matt. 10:16): "Be ye . . . prudent [Douay: 'wise'] as serpents." Therefore the precepts of the decalogue should have included a precept of prudence.
Obj. 3: Further, the other lessons of the Old Testament are directed to the precepts of the decalogue: wherefore it is written (Malach. 4:4): "Remember the law of Moses My servant, which I commanded him in Horeb." Now the other lessons of the Old Testament include precepts of prudence; for instance (Prov. 3:5): "Lean not upon thy own prudence"; and further on (Prov. 4:25): "Let thine eyelids go before thy steps." Therefore the Law also should have contained a precept of prudence, especially among the precepts of the decalogue.
The contrary however appears to anyone who goes through the precepts of the decalogue.
I answer that, As stated above (I-II, Q. 100, A. 3; A. 5, ad 1) when we were treating of precepts, the commandments of the decalogue being given to the whole people, are a matter of common knowledge to all, as coming under the purview of natural reason. Now foremost among the things dictated by natural reason are the ends of human life, which are to the practical order what naturally known principles are to the speculative order, as shown above (Q. 47, A. 6). Now prudence is not about the end, but about the means, as stated above (Q. 47, A. 6). Hence it was not fitting that the precepts of the decalogue should include a precept relating directly to prudence. And yet all the precepts of the decalogue are related to prudence, in so far as it directs all virtuous acts.
Reply Obj. 1: Although prudence is simply foremost among all the moral virtues, yet justice, more than any other virtue, regards its object under the aspect of something due, which is a necessary condition for a precept, as stated above (Q. 44, A. 1; I-II, Q. 99, AA. 1, 5). Hence it behooved the chief precepts of the Law, which are those of the decalogue, to refer to justice rather than to prudence.
Reply Obj. 2: The teaching of the Gospel is the doctrine of perfection. Therefore it needed to instruct man perfectly in all matters relating to right conduct, whether ends or means: wherefore it behooved the Gospel teaching to contain precepts also of prudence.
Reply Obj. 3: Just as the rest of the teaching of the Old Testament is directed to the precepts of the decalogue as its end, so it behooved man to be instructed by the subsequent lessons of the Old Testament about the act of prudence which is directed to the means.