law, as from general and indemonstrable principles, that the human reason needs to proceed to the more particular determination of certain matters.  These particular determinations, devised by human reason, are called human laws, provided the other essential conditions of law be observed, as stated above (Q. 90, AA. 2, 3, 4).  Wherefore Tully says in his Rhetoric (De Invent.  Rhet. ii) that “justice has its source in nature; thence certain things came into custom by reason of their utility; afterwards these things which emanated from nature and were approved by custom, were sanctioned by fear and reverence for the law.”

Reply Obj. 1:  The human reason cannot have a full participation of the dictate of the Divine Reason, but according to its own mode, and imperfectly.  Consequently, as on the part of the speculative reason, by a natural participation of Divine Wisdom, there is in us the knowledge of certain general principles, but not proper knowledge of each single truth, such as that contained in the Divine Wisdom; so too, on the part of the practical reason, man has a natural participation of the eternal law, according to certain general principles, but not as regards the particular determinations of individual cases, which are, however, contained in the eternal law.  Hence the need for human reason to proceed further to sanction them by law.

Reply Obj. 2:  Human reason is not, of itself, the rule of things:  but the principles impressed on it by nature, are general rules and measures of all things relating to human conduct, whereof the natural reason is the rule and measure, although it is not the measure of things that are from nature.

Reply Obj. 3:  The practical reason is concerned with practical matters, which are singular and contingent:  but not with necessary things, with which the speculative reason is concerned.  Wherefore human laws cannot have that inerrancy that belongs to the demonstrated conclusions of sciences.  Nor is it necessary for every measure to be altogether unerring and certain, but according as it is possible in its own particular genus. ________________________

FOURTH ARTICLE [I-II, Q. 91, Art. 4]

Whether There Was Any Need for a Divine Law?

Objection 1:  It would seem that there was no need for a Divine law.  Because, as stated above (A. 2), the natural law is a participation in us of the eternal law.  But the eternal law is a Divine law, as stated above (A. 1).  Therefore there was no need for a Divine law in addition to the natural law, and human laws derived therefrom.

Obj. 2:  Further, it is written (Ecclus. 15:14) that “God left man in the hand of his own counsel.”  Now counsel is an act of reason, as stated above (Q. 14, A. 1).  Therefore man was left to the direction of his reason.  But a dictate of human reason is a human law as stated above (A. 3).  Therefore there is no need for man to be governed also by a Divine law.