Employing the architectural themes used by Aquinas in his discussion of Eternal Law, this article presents Natural Law as a frame for rational thought rooted in the foundation of the Eternal Law. The argument contrasts this theory of law, based on a close reading of Aquinas and Gratian, to both a Positivist theory of law as power as well as to other Natural Law theories not incorporating the foundation. Law is presented as a product of both reason and will. The genus of law is shown to involve both specific precepts as well as more general guiding principles. Law is not only the detailed specifications of a building but also the very idea of it as well as the overall blueprint for its construction. The relationships among end, being, goodness and justice are explored through an examination of how the Natural Law frame is connected to the Eternal Law foundation. Placing the Eternal Law in such a central location raises an important question. To what extent is belief in a personal God necessary to construct a Natural Law system? A nuanced answer to this question is taken up in the conclusion.