An Introduction To Moral Theology, 2nd Edition. William May

reason, that Thomas always considers to be the essential element of law and of natural law in the formal, proper sense.⁵⁶ Ulpian’s definition can never be used in reference to natural law as that which is in something as ruling and measuring. It uniformly refers to natural law only in an accommodated sense, as found in something only “participatively, as it were.”⁵⁷ In short, Ulpian’s definition of natural law, as St. Thomas appropriates it, refers to the tendencies or inclinations that human beings possess by virtue of being, in truth, animals, albeit animals of a special and unique kind — tendencies that can be grasped by practical reason along with the real goods toward which they incline the human person, and thereby formally brought under the dominion of reason and capable of being expressed in principles or propositions that serve as starting points for thinking about what is to be done in and through human action — that serve, in other words, as the principles that go to make up the content of natural law as law.⁵⁸

      There was good reason, I believe, why Thomas took pains to make room for Ulpian’s definition of natural law within his thought. Aquinas was no dualist. For him, a human being is not an incarnate spirit, a “separated substance” in some way accidentally united to a material body. For him, a human being is first and foremost an animal, a very special kind of animal to be sure, different in kind from all other animals by reason of his intelligence and power to determine his life by his own free choices, but an animal nonetheless. Thus, the tendencies that humans possess in virtue of their animality are basic human tendencies, fundamental inclinationes naturales, and the goods correlative to these tendencies, goods such as the procreation and education of children, are basic human goods meriting the respect of human intelligence.

      This analysis of the way in which Thomas incorporated Ulpian’s definition of natural law into his own thought on the subject shows that he never accepted Ulpian’s understanding of natural law as a nonrational kind of instinct. Rather, he consistently held that natural law, formally and properly as law, is the work of practical reason. He accepted Ulpian’s definition only as a very restricted or limited way of understanding natural law, as referring to those tendencies that human beings share with other animals and which, in the human animal, must be brought under the rule of reason, under the tutelage of natural law.

EXCURSUS 2: St. Thomas’s Teaching on Natural Law in the Summa Contra Gentes