The Eternal Law, part 4
As for those areas where human law is not competent to direct human action, this does not mean, finally, that there is no law responsible for and capable of directing those actions. In such instances, rather, Aquinas says that they are ordered directly “by the Divine law… because more things are subject to a higher than to a lower cause.” In fact, he clarifies that “the very fact that human law does not meddle with matters it cannot direct comes under the ordination of the eternal law.” Eternal law, in other words, determines not only that area where human law is to operate, but also that (presumably much larger) area in which it is not to operate. Thus, not only in what it regulates, but also in what it doesn’t regulate, human law is to be subject to and derived from eternal law.
The Natural Law Libertarian 