Finally, the third set or grade of natural law precepts includes more remote moral norms, derived from the precepts of the Decalogue as from their principles, and known only after much consideration by the “wise” — i.e., persons perfected in the virtue of prudence or, in Christian terms, saints.
This is clearly the structure of natural law according to St. Thomas.
EXCURSUS 1: St. Thomas and Ulpian’s Definition of Natural Law
Ulpian, a second-century Roman lawyer, defined natural law as “that which nature has taught all animals.” Several contemporary Catholic moral theologians — among them Charles E. Curran, Timothy E. O’Connell, and Richard M. Gula — claim that the teaching of St. Thomas on natural law is highly ambiguous. On the one hand, it stresses the role that reason has to play in constituting natural law. On the other hand, they say, his thought, because of the influence of Ulpian, definitely tends “to identify the demands of natural law with physical and biological processes”47 and leads to what they consider a “physicalistic” understanding of natural law.48 Because of this claim by some contemporary Catholic theologians, it will be worthwhile to examine the way in which Aquinas accommodated Ulpian’s definition of natural law into his own thought.
In one of his earliest writings, his Scriptum super IV Libros Sententiarum, Thomas takes up the question of natural law in connection with his discussion of a problem that had plagued his predecessors in their attempts to relate natural law to the teaching of Scripture. How could the polygyny of the Old Testament patriarchs be reconciled with natural law?49 My concern is not with this specific problem but rather with Thomas’s thought on natural law and his use of Ulpian’s definition of natural law.
In his analysis of this issue, Thomas first notes that all beings have within themselves, by their very nature, principles whereby they can not only bring about the actions proper to themselves but also direct them to their proper ends. Beings that lack knowledge act in a necessary way, but in beings that have knowledge, the principles of operation are knowledge and appetite. In beings capable of knowledge, consequently, there must be a natural concept (naturalis conceptio) in the cognitive faculty and a natural inclination (naturalis inclinatio) in the appetitive faculty, whereby these beings are ordered to the actions and ends appropriate to them. Since man differs radically in kind from other animals, however, in his ability to know the end as such and the relationship of means to ends, the “natural concept” in his case “is called natural law, whereas in other animals it is called a natural estimation (aestimatio).” Nonrational animals “are impelled by a force of nature to perform actions proper to them rather than acting by being regulated, as it were, by any kind of judgment properly so called.” Aquinas then continues: “Therefore natural law is nothing other than the concept naturally impressed upon man whereby he is directed to acting suitably in the actions that are proper to him, whether these actions pertain to his generic nature, such as generating life, eating, and so forth, or whether they pertain to his specific nature as man, such as thinking and things of this kind.”50
What is most important about the notion of natural law expressed in this passage from an early writing of St. Thomas is that natural law is explicitly related to human intelligence, to the fact that the human person, alone of all animals, is capable of knowing the end as such and of relating means to ends. The thought expressed here and elsewhere in St. Thomas’s Commentary on the Sentences is surely in accord with the thought set forth in the passages from the Summa theologiae that we have already considered: natural law, as law, is exclusively proper to human persons, and, precisely as law, natural law is related to human cognition.
As already noted, the discussion of natural law in this passage from the Commentary on the Sentences was prompted by the polygyny of the patriarchs. The question posed was “whether having many wives is contrary to natural law.” In the body of the article, from which the citations given above are taken, Thomas asserts that polygyny is indeed against the natural law in the sense that it impedes at least partially the marital good of fidelity and the peace and harmony that ought to reign in the family. He also notes that it completely destroys the good of the “sacrament” of marriage. He concedes, however, that it is not destructive of the principal end of the institution of marriage — namely, the procreation and education of children — and in this sense is not against natural law.51 One of the objections to the view (which Thomas held) that polygyny is against natural law had urged that a plurality of wives is in no way against natural law, inasmuch as the natural law is that which nature has taught all animals — i.e., Ulpian’s definition of natural law — and obviously in the animal kingdom it is by no means unnatural for one male to have more than one mate. In replying to this objection, Thomas distinguishes several ways in which natural law could be understood; and in commenting on these ways of understanding natural law, Thomas takes up the celebrated definition of Ulpian.
Thomas says that natural law can be understood, first, to refer to something that is natural by reason of its principle or source. It is in this way that Cicero understood natural law, for to him it was a kind of innate source or power, a principle intrinsic to things (ius naturae est quod non opinio genuit, sed quaedam innata vis inseruit). He goes on to note that the principle from which natural law springs can be extrinsic to the being regulated by natural law, and in this sense he makes room for the definition given by Gratian (incorrectly attributed, in Thomas’s text, to Isidore), that natural law is what is contained in law and gospel (ius naturale est quod in lege et in Evangelio continetur). Finally, he notes that natural law may be understood to refer to what is natural, not by reason of its source or principle but rather by reason of “nature,” i.e., by reason of the subject matter with which natural law is concerned (tertio dicitur ius naturale non solum a principio, sed a natura, quia de naturalibus est). If natural law is taken in this sense, nature is contrasted with or is opposed to reason. Consequently, “taking natural law in its most restricted or limited sense” (strictissimo modo accipiendi ius naturale),52 “those things that pertain only to men, although they follow from the dictate of reason, are not said to be of the natural law; but only those things that natural reason dictates about matters that are common to man and to other animals; and thus the aforesaid definition is given, namely, that ‘natural law is what nature has taught all animals.’ ”53
It is most important to be clear about Aquinas’s thought here. He is definitely not claiming that natural law, as law, is something infrarational, an instinct that human persons share with other animals. After all, in the body of the article he had stressed that natural law as such is unique to human beings and that it is a naturalis conceptio. In nonrational animals, he taught in the body of the article, there is no natural law; there is only a natural “estimation,” a power or force of nature impelling them to act in ways appropriate to achieve the ends for which they are made. Thus, in this celebrated passage, Thomas in no way repudiates what he has just said in the body of the article (or what he was to say subsequently in the texts from the Summa theologiae that we have already examined) about natural law as something brought into being or constituted by reason. He is making room for Ulpian’s understanding of natural law in only a limited way, namely, in the sense that it refers to the subject matter with which or about which natural law is concerned, for this is what he explicitly says. In other words, he is saying that the tendencies that human beings share with other animals are fitting objects of natural law taken in its proper sense as an achievement of reason — they are fit matter to be brought under the rule of law, i.e., under natural law understood as an achievement of human reason. This is evident from the text, for Thomas explicitly says that the natural law, understood in Ulpian’s sense, has to do only with those things that “natural reason dictates about matters that are common to man and to other animals” (illa tantum quae naturalis ratio dictat de his quae sunt homini aliisque communia).
Thomas also makes room for Ulpian’s definition in his later works, in passages in the Summa theologiae54 and in his commentary on Aristotle’s Nicomachean Ethics.55 But in all these places,