I’ve just now taken notice of these discussions by Stephen Grabill on natural law and Protestant catholicism. These are excellent surveys in which the catholic nature of the Reformation is thoroughly demonstrated. Grabill quotes Zanchi’s complex definition of natural law:
Natural law is the will of God, and consequently, the divine rule and principle for knowing what to do and what not to do. It is, the knowledge of what is good or bad, fair or unfair, upright or shameful, that was inscribed upon the hearts of all people by God himself also after the fall. For this reason, we are all universally taught what activities should be pursued and what should be avoided; that is, to do one thing and to avoid another, and we know that we are obligated and pushed to act for the glory of God, our own good, and the welfare of our neighbor both in private and in public. In addition, we know that if we do what should be avoided or avoid what we should do, we are condemned; but if we do the opposite, we are defended and absolved. (Girolamo Zanchi, quoted by Stephen Grabill, “Part VI: Recovering and Reviving the Catholicity of Protestant Ethics” at percaritatem.com)
One interesting thing to note in this definition, as Grabill points out, is the fact that God inscribes the natural law upon the hearts of man again after the fall. Thus, Zanchi does not use the doctrine of a natural law merely for the explanation of the acts of certain noble pagans and civilizations but this “divine principle for knowing what to do” also sparks a passion that pushes men toward God.