The central argument of this book is that the traditional notion of Natural Law has almost disappeared from the ethical and moral discourse of our time. For Thomas Aquinas, the author whose conception of Natural Law forms the foundation for the book, the ontological and ethical orders are not autonomous but inseparable-in effect, his ethical system is an ontological morality.For Thomas, the ethical (practical wisdom) must be understood as an extension of the metaphysical (speculative wisdom). Most modern philosophers, by contrast, consider these two orders to be entirely separate. Here Luis Cortest shows how traditional Natural Law (the form Thomas Aquinas developed from classical and medieval sources) was transformed by thinkers like John Locke and Kant into a doctrine compatible with early modern and modern notions of nature and morality. In early Modern Europe one of the first of the great debates about moral philosophy took place in sixteenth-century Spain, as a philosophical dispute concerning the humanity of the Native Americans. This foreshadowed debates in later centuries, which the author reevaluates in light of these earlier sources. The book also includes a close examination of the recent work of scholars like John Finnis and Brian Tierney, who argue that traditional Natural Law theorists were defenders of a doctrine of positive rights. Rather than attempt to make the traditional doctrine compatible with modern rights theory, however, the author argues that traditional Natural Law must be understood as a form of pre-Enlightenment ontological morality that has survived the onslaught of modernity.