Objection 1: It would seem that the prohibitive precepts relating to the vices opposed to prudence are unfittingly propounded in the Old Law. For such vices as imprudence and its parts which are directly opposed to prudence are not less opposed thereto, than those which bear a certain resemblance to prudence, such as craftiness and vices connected with it. Now the latter vices are forbidden in the Law: for it is written (Lev.19:13): |Thou shalt not calumniate thy neighbor,| and (Dt.25:13): |Thou shalt not have divers weights in thy bag, a greater and a less.| Therefore there should have also been prohibitive precepts about the vices directly opposed to prudence.
Objection 2: Further, there is room for fraud in other things than in buying and selling. Therefore the Law unfittingly forbade fraud solely in buying and selling.
Objection 3: Further, there is the same reason for prescribing an act of virtue as for prohibiting the act of a contrary vice. But acts of prudence are not prescribed in the Law. Therefore neither should any contrary vices have been forbidden in the Law.
The contrary, however, appears from the precepts of the Law which are quoted in the first objection.
I answer that, As stated above (A), justice, above all, regards the aspect of something due, which is a necessary condition for a precept, because justice tends to render that which is due to another, as we shall state further on (Q, A). Now craftiness, as to its execution, is committed chiefly in matters of justice, as stated above (Q, A): and so it was fitting that the Law should contain precepts forbidding the execution of craftiness, in so far as this pertains to injustice, as when a man uses guile and fraud in calumniating another or in stealing his goods.
Reply to Objection 1: Those vices that are manifestly opposed to prudence, do not pertain to injustice in the same way as the execution of craftiness, and so they are not forbidden in the Law, as fraud and guile are, which latter pertain to injustice
Reply to Objection 2: All guile and fraud committed in matters of injustice, can be understood to be forbidden in the prohibition of calumny (Lev.19:13). Yet fraud and guile are wont to be practiced chiefly in buying and selling, according to Ecclus.26:28, |A huckster shall not be justified from the sins of the lips|: and it is for this reason that the Law contained a special precept forbidding fraudulent buying and selling.
Reply to Objection 3: All the precepts of the Law that relate to acts of justice pertain to the execution of prudence, even as the precepts prohibitive of stealing, calumny and fraudulent selling pertain to the execution of craftiness.