Aquinas's theological approach to natural law primarily presents it as a participation in the eternal law. This fact has helped to mislead many into supposing that natural law must be understood as a divine imperative. Of course, Aquinas holds that God's will is prior to the natural law, since the natural law is an aspect of human existence and man is a free creation of God. But Aquinas does not describe natural law as eternal law passively received in man; he describes it rather as a participation in the eternal law. This participation is necessary precisely insofar as man shares the grand office of providence in directing his own life and that of his fellows. Every participation is really distinct from that in which it participates—a principle evidently applicable in this case, for the eternal law is God while the law of nature is a set of precepts.
(Germain G. Grisez, "The First Principle of Practical Reason: A Commentary on the Summa Theologiae, 1-2, Question 94, Article 2," Natural Law Forum 10 [1965]: 168-201, at 192 [italics in original; footnote omitted])
