Nicomachean Ethics of Aristotle (Bk. V, chap. vii), where he in like manner divides law into natural and conventional.
In what senses the term ‘natural law’ is employed by the different writers. However, with respect to ‘natural law’, it should be noted that this term is variously understood by the philosophers, the jurisconsults and the theologians. For Plato, in the works above cited, apparently understands ‘natural law’ as referring to every natural inclination implanted in things by their Creator, whereby they severally tend towards the acts and ends proper to them. For just as he has said that the divine law is the eternal rational principle dwelling in God, whereby all things are governed, even so has he given the name of natural law to the participation in this rational principle that has been instilled into all creatures in order that they may tend toward their appointed ends. St. Thomas (I.–II, qu. 91, art. 2) has even said that all things ruled by divine providence partake in some fashion of the eternal law, to the degree that they derive from its efficacy, propensities toward their proper acts and ends. But the jurisconsults, while they hold that the natural law is common to other living beings as well as to men, apparently exclude inanimate things from participation in this law, a fact which is evidenced by the Institutes (I. ii, § 1) and by the Digest (I. i. 1).
8. ‘Law’, in its strict meaning, is not to be attributed to insensate things. However, as I stated in the First Chapter, ‘law’ is to be attributed to
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insensate things, not in its strict sense, but metaphorically. Accordingly, of natural law in that first and most general acceptation, we need say nothing more at present than what we have already remarked in Chapter One, and what we shall remark below,5 in connexion with the eternal law. Not even brute animals are capable of [participating in] law in a strict sense, since they have the use neither of reason nor of liberty; so that it is only by a like metaphor that natural law may be ascribed to them. For, even though they differ from insensate things in this respect, namely, that they are guided not merely by the force of nature, but also by knowledge and natural instinct, an instinct which is for them a form of law; and although the second interpretation of the jurisconsults can therefore be sustained, after a fashion; nevertheless, absolutely speaking, that interpretation is metaphorical and to a great extent dependent upon analogy. Accordingly, we shall pass over it also, for the present; for later (in Bk. II, chap. viii [chap. xvii]), in our exposition of ius gentium,6 we shall attempt to explore the true meaning of the [divisions of law] above mentioned.
9. What constitutes natural law, strictly speaking. Natural law, then, in the proper sense of the term—the natural law which pertains to moral doctrine and to theology—is that form of law which dwells within the human mind, in order that the righteous may be distinguished from the evil, in accordance with the passage in the Psalms (iv[, vv. 6, 7]): ‘Who sheweth us good things? The light of Thy countenance, O Lord, is signed upon us.’ Such is the explanation of St. Thomas in the passage (I.–II, qu. 91, art. 2) wherein he concludes that the natural law is, ‘a participation in the eternal law on the part of the rational creature.’
Moreover, in another passage (on the Sentences, Bk. IV, dist. xxxiii, art. 1), St. Thomas says: ‘Because man [alone] among living beings is cognizant of the essential nature of his end and of the comparative relationship between the work and the end, the natural power of comprehension implanted in
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him, which is directed toward befitting action, is therefore spoken of as the lex naturalis, or ius naturale (natural law), while in the case of the other animals, it is called naturalis aestimatio (instinct). This is plainly Cicero’s opinion, also (On Laws, Bk. II [, chap. iv, no. 8]). For, after writing the words above quoted, on the eternal law, he adds: ‘Wherefore that law which the gods have given to the human race has been justly praised; since it is the reason and mind of a wise being, suited to commanding and to restraining.’
So it is, then, that the law in question is called natural, not only in so far as the natural is distinguished from the supernatural, but also in that [what is natural] is distinguished from what is a matter of free choice.7 This is the case, not because the execution of that law is natural, or the result of necessity, as is the execution of the natural inclination of the brutes or of inanimate objects; but because the law in question is (so to speak) a kind of characteristic of nature, and because God Himself has annexed that law to nature. Moreover, in this respect the natural law is also divine, being decreed, as it were, directly by God Himself. Such was the opinion of St. Thomas, as expressed in the above-mentioned passage (Qu. 91 and qu. 94, art. 6), where he cites the words of St. Augustine (Confessions, Bk. II, chap. iv), spoken to God, ‘Thy law is written in the hearts of men’, words which had reference to natural law; wherefore Augustine has said, in another work (On the Sermon of Our Lord on the Mount, Bk. II, chap. ix), that there is no soul, ‘in whose conscience God does not speak. For who save God writes the natural law in the hearts of men?’ Isidore (Etymologies, Bk. V, chap. ii) more explicitly calls this law divine. And finally, it is evident from the foregoing how necessary and useful the said law is; since on it rests the capacity of discriminating between the righteous and the evil8 in the rational nature. All this, however, requires a lengthy explanation; but let us reserve that explanation for the following Book, lest we invert the proper order and cause complete confusion.
10. A certain subdivision of natural law remains for discussion. But first, we must say a word concerning Plato’s fourth division of law, called by him the human, and relating to the law designated by Aristotle [Nicomachean
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Ethics, Bk. V, chap. vii] as ‘conventional law’, which he describes thus: ‘It is that form of law which is a matter of indifference, originally, but of great moment, once it has been established [as a law].’9 This comment is to be understood as referring to the subject-matter of the said law, since the latter relates to actions which apart from that law would not be a matter of obligation, but which are rendered obligatory by it. Finally, to this same division belong those laws which Cicero (On Laws, Bk. II [, chap. v, no. 11]) distinguishes from eternal and natural law, calling them popular. We, however, divide created or temporal law into natural and positive, after the manner of the theologians; since the term ‘positive’ covers a wider field than does ‘human’. For it is to be noted that the philosophers have not recognized man’s supernatural end but have dealt only with a certain felicity in this life, or rather, with a certain state conducive to passing it in peace and in justice, and have considered the subject of laws, from the standpoint of this temporal end; so that they have merely distinguished natural law from human law, which we may call ‘civil’, and to which we shall presently devote some words.10 However, since it is a doctrine of the faith that men are ordained to the supernatural end of the future life by fitting means which are to be sought after in this life, sacred theology rightly infers that this natural law is necessary for a reason vastly different [from the reason recognized by the philosophers], and that men need more laws of a positive nature than were discerned by those same philosophers.
11. In what ways human nature may be considered, with respect to the laws which it needs. With respect to the natural law, then, it is the teaching of theology that man may be considered from the standpoints of a twofold nature and dual light of reason. The first standpoint deals with pure nature, or the substance of the rational soul, and consequently with the light of reason that is connatural to man. The second deals with the nature of grace infused into man from above, and with the divine and supernatural light of faith which rules and guides him in this life. Moreover, in accordance with these two principles, [theology] distinguishes two aspects of natural
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law. The one is absolutely natural, [even] with respect to man. The other, although it is supernatural in its relation to man (since the whole order of grace is supernatural in that respect), may nevertheless be called natural in relation to grace. For grace also has an essence and a nature of its own, to which the infused light is connatural, and to which it is connatural not only to direct men toward righteous, good, and fitting behaviour in supernatural matters, but also to dispel darkness and errors relating to the purely natural law itself and to enjoin on the basis of a higher reason the observance of that same natural law. Two aspects of natural law, then, may be