Reason is a standard of natural law only to the degree that it discovers the standard of natura
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Of course, reason has an obvious role to play within descriptivism or naturalism. It discovers the good and evil of nature and presents it to the will. As such, it becomes a kind of conduit or messenger between nature and will. Nature is the true standard of human actions; reason is a standard only to the degree that it discovers the standard of nature. Aquinas himself says that reason is the rule of the will but reason itself is measured by a higher standard, namely, by the eternal law, Since the natural law is the rational participation in the eternal law, and since we have seen that the natural law is discovered through the inclinations in nature, might we also conclude that reason is measure by nature? These diverse layers by which human actions are measured do not detract from the role of reason. Reason is still the measure of human actions, even though it is measured by nature, which is in turn measured by the eternal law.
200 Reason is not simply a conduit; reason is the principle and source of human actions. When we want to know whether an action was human, we do not ask whether it arose from nature; we ask whether it arose from reason. We have noted that the will is the inclination of the whole person, so that nothing is done by the person except through his will. Likewise, nothing is done by the person except through his reason. Reason is not the inclination or motive force of the whole person. It is, however, the form of that inclination. We cannot move to anything without the force of will; at the same time, the will does not move to anything definite without the guidance of reason. None of this prevents the guidance of reason from finding its original source in an understanding of nature.
195 It is necessary that the operations of art imitate the operations of nature and that those things that are made by art imitate those things that are in nature. For if the teacher of an art makes a work of the art, then the student who learns the art from the teacher must examine his work, so that he himself can make similar work. Therefore, the human intellect, for whom the light of the intellect is derived from the divine intellect, must be informed in its actions by the examination of those things that are naturally made, and it must act in a similar way.1
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1 42. Sententia Politic, pr. I (LeoLeonine ed., v. 48, 69, 8-19) "Unde necesse est quod et operationes artis imitentur operationes nature, et ea que sunt secundum artem imitentur ea que sunt in natura. Si enim aliquis instructor alicuius artis opus artis efficeret, oporteret discipulum qui ab eo artem suscepisset, ad opus illius attendere ut ad eius similitudinem et ipse operaretur. Et ideo intellectus humanus, ad quem intelligibile lummen ab intellectu diuino deriuatur, necesse habet in hiis que facit informari ex inspectione eorum que sunt naturaliter facta, ut similiter operetur »