By Ryan J. Brady, Ph.D.

In this essay, my endeavor is to establish some of the indispensable principles for understanding St. Thomas’ notion of the Moral Precepts of the Old Law. In keeping with Aquinas’ own modus operandi, I will give some consideration to the wider context of law in general and the Old Law’s relationship to the New Law of grace; nevertheless, the goal is a simple and straightforward presentation of the nature of these precepts of the Old Law. In the future, we expect to have some essays that engage modern scholarship more directly, but we trust that this small contribution to the advancement of Thomistic scholarship will prove worthwhile.

The Moral Precepts of the Old Law in the Thought of Thomas Aquinas

In accordance with St. Thomas’ custom of establishing principles, we will begin by placing the moral precepts of the Old Law in their proper context. Accordingly, we will first consider the notion of ‘law’ in general and then its general divisions followed by a consideration of how the moral precepts of the Old Law presuppose, reinforce, and build upon those of the natural law. Finally, we will see how the moral precepts of the Old Law relate to the ceremonial and judicial precepts of the Old Law and the New Law in general.

The Nature of Law

The most obvious thing about the moral precepts of the Old Law, perhaps, is that they must come under law in some way. St. Thomas dedicates eighteen questions of his Summa Theologiae to law, so it is clear that we will not be able to give it the attention it deserves. Nevertheless, we can establish the essentials that are necessary for our purposes. The first thing we learn in St. Thomas’ treatment of the subject is that law is an extrinsic principle of human acts (I, q. 90, intro.). He says that it is a certain rule and measure of acts according to which a man is led towards acting or is restrained from doing so (q. 90 a. 1c.). Moreover, the name for law in Latin (lex) comes from the word for binding (ligando) because it obliges a man to act in a certain way. It is defined as “a decree (ordinatio) of reason for the common good, from him who has jurisdiction over the community, having been promulgated.”

As Ralph McInerny pointed out, it is important to recognize that the term “law” is used in so many ways that it could not have the same meaning in every instance. In other words, it’s an analogous term – “that is, a term with an ordered set of meanings, one of which is regulative of the others" (Summa Theologica, Questions 90-97; Lanham, MD: Regnery Publishing, 2001, p. vi.). Although the title of the ninetieth question of the Prima Secundae is “The Essence of Law,” McInerny maintains that Aquinas did not intend to indicate thereby that the notion of law is a univocal one with one meaning only. Rather, he says, the above definition (“a decree of reason,” etc.) is a definition of human or civil law (p. viii). It’s true that it is the “regulative meaning, the primary analogate, of law” (p. ix), but we necessarily negate many notions we associate with the term when we begin speaking about Divine or Eternal Law.

The Notion of Law vis-à-vis Analogy

Seeing that we moderns are not accustomed to speaking about “analogous” terms and “primary analogates,” it is worthwhile taking a moment to grasp their meaning. As a spurious but helpful work of “Aquinas,” called “On the Nature of Accidents” explains, “this is the nature of every analogate, that that of which it is first said will be in the definition (ratione) of all those things which come after it, as health, which is said of animal before it is said of urine or medicine. And therefore the health of an animal falls in the definition of both.” Speaking of of urine as being healthy certainly sounds a bit odd at first, but it doesn't take too much of a stretch of the imagination to grasp the meaning. If we predicate health first of all (as a primary analogate) of an animal, then we will predicate health of urine or medicine in reference to the health of an animal. Moreover, when we give the definition of “healthy urine” we will necessarily include in that definition a reference to the healthy animal whose health is the basis of our calling the urine healthy. The same will apply to the definition of “healthy medicine” because it is only called healthy because it makes an animal healthy.

Understanding this point is essential because St. Thomas goes on to speak of the various kinds of law and we have to understand that all of them, including the Old Law of which the moral precepts are a part, apply first of all to the human, civil law as the primary analogate. That being said, as McInerny argues, it must immediately be pointed out that it is the Eternal Law that “is the source and foundation of all other kinds of law” (ix) for Thomas. This does not mean, however, that our understanding of all kinds of law is based on our understanding of the Eternal Law. The reality is quite the contrary. Because God is pure Spirit and His Eternal Law “which is the type (ratio) of Divine Providence” (I-II, q. 93 a. 5 ad 3) is so far beyond us, we can hardly base our knowledge of other kinds of law off of it. It is much like the fact that because we are not angels, all of our knowledge, “even of things which transcend the senses, originates from the senses” (SCG, I. 12). In other words, all of our knowledge has to not only be proportioned to us, but it has to begin with principles that are more known to us as human creatures. It is for this reason that the kind of law that is the most imperfect is the one we begin with.

The Eternal Law

We may need to begin our understanding of law with a rather human understanding of it, but that does not stop St. Thomas from speaking of Eternal Law before any of the others. The above discussion, therefore, will need to be kept in mind. When we think of a law being promulgated or published, for instance, we do not tend to think of a promulgation made from all Eternity by the Divine Word or by the publication of the Book of Life. Nevertheless, that is the only way it is possible to say that the Eternal Law was promulgated (I-II, q. 91 a. 1 ad 2). Moreover, we tend to assume that if a law is promulgated, it must be made known to someone besides the lawgiver, but this was not possible in the case of the Eternal Law. Even if we have to make many negations in order to come to some understanding of the Eternal Law, however, the positive thing is that we can have knowledge of it. For instance, it certainly is a decree or ordinance of reason because we know that “the whole community of the universe is governed by Divine Reason” (I-II, q. 91 a. 1 c.). Moreover, as St. Thomas explains, we can know that all laws are derived from it:

“Since then the Eternal Law is the plan of governing in the Supreme Governor, it is necessary that all the plans of governing in the inferior governors be derived from the Eternal Law. But these plans of inferior governors are all other laws besides the Eternal Law. Therefore all laws, in so far as they partake of right reason, are derived from the Eternal Law” (I-II, q. 93 a. 3).


It remains true that the Eternal Law is a rather obscure concept. However, because it comes first in honor (as being the very “law of God” and the source of all other kinds of law; see I-II, q. 93 a. 5 c.) and not first in our knowledge about law, there is no reason to be surprised if we cannot have clear and distinct knowledge of it.

The Natural Law

 St. Thomas speaks next of the natural law, which is a “participation of the Eternal Law” (I-II, q. 91 a. 2 c.) that is proportioned to the capacity of a rational creature.” (I-II, q. 91 a. 4 ad 1). This “light of the intellect implanted in us by God” (prologue to the Expositio in 2 Praecepta Caritatis et in 10 Praeceptis Legis) helps us to make moral decisions. From the precepts of the natural law (the first of which is “the good ought to be done and pursued and evil avoided"; I-II, q. 91 a. 2 ad 1), man proceeds to particular conclusions as from “general and indemonstrable principles." When these particular determinations also meet the essential requirements of law, they are called human laws (q. 91 a. 3c.). Because human laws are based on the principles of the natural law, the latter is one of its rules or measures (I-II, q. 95 a. 3c.).

Divine Law

Although human law is ultimately derived from the Eternal Law, and the natural law is “nothing else than an imprint on us of the Divine light” (I-II, q. 91 a. 2c.), Thomas says the Divine Law was necessary as well. The primary reason is that the natural law (like human law, which is proximately derived from it), is only directed towards a natural end. And “although the end be last in the order of execution, yet it is first in the order of the agent’s intention” (I-II, q. 1 a. 1 ad 1) and it influences everything one does. As St. Thomas pointed out, the power of the first intention, “which was in respect of the last end, remains in every desire directed to any object whatever, even though one’s thoughts be not actually directed to the last end” (I-II, q. 1 a. 6 ad 3). For this reason, it makes an inestimable difference whether or not someone is directed towards the happiness that Aristotle spoke of (which was limited to an imperfect happiness that had to be qualified by the phrase “happy as men;” see, SCG 3.48 & I-II, q. 3 a. 2 ad 4) or the eternal happiness consisting of the beatific vision. The Divine Law, consisting of the Old and the New Law (I-II, q. 91 a. 5 s.c.), was given so as to direct man to the latter. The Old Law did so by way of fear as a kind of pedagogue (cf., Gal. 3:24) that prepared the Jews for the coming of Christ, whereas the New Law does so by way of love (I-II, q. 91 a. 5 s.c.).

In order to direct people to the beatific vision, the Divine Law provides the proper rule and measure of human law (something the natural law, which is directed towards a natural end, could not do). The Divine Law could also make laws that pertain to interior dispositions since God is able to know and judge the heart. This is important because true virtue must surpass mere external rectitude.  As a consequence, the precepts of the Divine Law are primarily geared towards helping people adhere to God (SCG 3.115). Finally, only a divine law could forbid all sins, whereas if human law were to do so, much good would be hindered. Because men are subject to the disordered impulse of sensuality called the “fomes,” though, it is necessary for all sins to be censured. For all of these reasons, St. Thomas tells us (I-II, q. 91 a. 4 c.), the Divine Law was necessary.

Moral Precepts of the Old Law in Particular

We are now ready to look more distinctly into the moral precepts of the Old Law. The most salient point is that they are from the law of nature (I-II. q. 60 a. 5 s.c.). Indeed, they can be said to belong (pertinent) to it (see, I-II, q. 99 a. 2 ad 1) and yet, they belong to the Old Law as well. The reason is that although the law of nature and the Old Law are distinct from each other, they are not altogether different. In fact, just as “grace presupposes nature, so is it fitting (oportet) that the Divine Law presupposes the natural law” (I-II, q. 99 a. 2 ad 1). Accordingly, there is a lot of overlap between the two and the moral precepts of both of these laws are in many cases practically identical. The coinciding area, however, is always one dimensional in the sense that while the Old Law can manifest the precepts of the natural law, the reverse is not even a possibility. Accordingly, St. Thomas says the Old Law “contained” the precepts of the law of nature and when it did, the Gentiles, too, were bound to observe them inasmuch as they pertain to the natural law (I-II, q. 98 a. 5c.).

So the Old Law both “added some precepts of its own” (ibid.) to the natural law and presupposed them. In order to better distinguish between the two laws and see how the Old Law could add to the natural law, we will consider the criteria given by St. Thomas for judging whether or not something belongs to the latter. Essentially, if a man is naturally inclined to something - either by rectitude or by knowledge (i.e., apparently, by will or by reason; see I-II, q. 94 a. 4 c.), he is under the influence of the natural law. There are some common principles that humans are equally inclined by nature to hold in their reason and, consequently, to volitionally pursue. The difficulty that arises, though, is that unlike the common principles of the speculative reason (such as the principle of non-contradiction), those of the practical reason pertain to contingent matters. They can, therefore, be more difficult to know - at least due to bad customs of one's society (which was the case with the Germani who didn't grasp the truth that stealing was wrong; see, I-II q. 94 a. 4c.). Besides, there are often various considerations to make when applying them. As a consequence, the proper conclusions that flow from them do not retain the same definitive weight as the principles and they may admit of exception.

Marriage and the Education of Offspring

For example, the inclination to educate offspring (whose good is “the principal end of marriage”) seems to belong to the natural law as a primary precept - although this does not mean, of course, that every individual is bound to marry or educate offspring (III Suppl., q. 41 a. 2c; q. 65 a. 1; I-II, q. 94 a. 2c). Determining how this precept applies, however, can be challenging. For instance, “the law prescribing one wife was framed not by man but by God” and it “was imprinted on the human heart” (III Suppl., q. 65 a. 2c.). Such a law is clearly perfectly adapted to the good of the offspring, so it must be bound up with the natural law in some way. Nevertheless, Thomas maintains that polygyny (polygamy in which a man has more than one wife as distinguished from its antonym polyandry) is only contrary to the natural law in regard to its secondary precepts because it does not entirely hinder the primary end of marriage. The principle the Summa gives for making a determination in regard to these secondary precepts is as follows:

If an action… be in any way improportionate to the secondary end, or again to the principal end, as rendering its attainment difficult or less satisfactory, it is forbidden, not indeed by the first precepts of the natural law, but by the second which are derived from the first even as conclusions in speculative matters receive our assent by virtue of self-known principles” (III Suppl. q. 65 a. 1 c.).

The result is that polygyny is not necessarily entirely contrary to the good of the offspring and the rule against it is only binding “in the majority” of cases (ibid., a. 2c.). The fact that “all animals that are used to copulate have a natural instinct to resist another’s intercourse with their consort” and to freely copulate at will (which is not possible “if many males have access to one female;" SCG 3b, ch. 124) suggests that the inclination to avoid polygyny is very basic to man, but there can be exceptions.

This example is emblematic of how challenging it can be to apply, by means of a practical syllogism, first principles or precepts to contingent matters. That is not to say that there are not moral absolutes, of course; even if people do not know them or improperly apply them, they remain objectively true and valid. Moreover, the general precepts of the natural law “can nowise be blotted out from men’s hearts” (I-II, q. 94 a. 6c.).

Natural Love of God?

Another example of a natural law precept is the one pertaining to man’s inclination to love God. As I-II, q. 94 a. 2 makes clear, man naturally has a kind of gradation of inclinations that leads him to do good. There are some that he has in common with all substances, others that he has in common with the animals (to which Ulpian’s law applies), and finally, those had in common with all men in “according to the nature of reason, which is proper to him.” It is due to this latter category that the desire to not only “know the truth about God” (I-II, q. 94 a. 2c.) but to love him more than oneself arises; neither inanimate substances nor irrational beasts are even capable of such an inclination.

Aquinas argues that man’s basic desire to love (diligere) God more than himself belongs to the natural law even though there are very similar precepts that are clearly included in the Divine Law. He says, first of all, that man knows he is subject to a Higher Being and in need of assistance from Him. Regardless of what he specifically knows about this Higher Being that all call “God,” it pertains to his nature to know that it is fitting to honor Him (therefore, even the duty of offering sacrifice to him belongs to the natural law).

Moreover, a part is naturally inclined to sacrifice itself for the good of the whole. This is the reason man naturally raises his hand to defend his head (since the head is more essential to the whole body) and why citizens often willingly exposes themselves to danger for the good of a whole society. Since God is the good of the whole universe, the result is that it is connatural to man to not only sacrifice to Him but also to love Him more than himself (I, q. 60 a. 5c.). It's true that healing grace is somehow necessary to love God in this way (I-II, q. 109 a. 3c.), but it remains a capacity of nature.

Divine Law and Natural Law

The Divine Law, St. Thomas says, both presupposes these natural desires and builds upon them. It was necessary, in part, because the law of nature as regards loving God was obscured because of sin. Even if the natural precepts can be known naturally, then, the Old Law reinforced them by positively revealing them via the Commandments (which are precepts of natural law; II-II, q. 147 a. 4c.) and other precepts, as well. Hence it is written in Deuteronomy, “And now, Israel, what doth the Lord thy God require of thee, but that thou fear the Lord thy God, and walk in His ways and love Him, and serve the Lord thy God, with all thy heart and with all thy soul” (10:12). This passage both reinforces the natural law and builds upon it in virtue of the intense interiority of love that is required, and so it is a prime example of the way that a natural law precept which is contained by the Old Law can be extrapolated upon so as to better lead men to its end; viz., “the justification of men” (I-II, q. 107 a. 2c.).

Moral, Judicial and Ceremonial Precepts of the Old Law

Having enquired into the relationship of Old Law moral precepts vis-à-vis those of the natural law, we can now enquire into the relationship the various precepts of the Old Law have among themselves. The basic division of these precepts is into moral precepts, ceremonial precepts and judicial precepts. One helpful principle to keep in mind is that the moral precepts are specifically “about acts of virtue” that make a man good so as to “establish man in friendship with God” (I-II, q. 99 a. 2c.). It is true that by breaking the other precepts, friendship with God would be precluded or at least hindered, but they are not directed principally to that end.  

Regarding the relationship between the moral precepts and the ceremonial precepts, St. Thomas says that the ceremonial precepts help determine how to act upon them by means of “certain external works.” (I-II, q. 101 a. 1c.). Because the moral precepts were means of directing men to God, the ceremonial ones were also. The ceremonial precepts, though, stipulated certain acts of worship in particular because “man is directed to God by the worship due to Him" (ibid.). These acts of worship were intended both towards the worship of God and “to foreshadow Christ” (I-II, q. 102 a. 3 s.c.). They were needed, in part, to hinder men from acts of idolatry (I-II, q. 101 a. 3). and were merely “a shadow of things to come” (see Col. 2:17 and I-II, q. 107 a. 2c.) since the reality is found in Christ. Finally, they are distinct from the moral precepts of the Old Law in that the former are binding at all times, “a contrary custom not withstanding” (Quodlibetal q. 4 a. 3c.) whereas the ceremonial precepts only have force in virtue of their Divine institution (I-II, q. 100 a. 11c & q. 104 a. 1 ad 3). That is, the moral precepts pertain to good morals by their very nature because they either make up the Decalogue, which is based on the natural law, or are reducible to it (I-II, q. 100 a. 11c.) whereas the ceremonial precepts, which were not binding before being instituted, do not.

The judicial precepts can also be distinguished from the moral precepts inasmuch as they “derive their force from their institution alone” (ibid.) and do not pertain to good morals by their very nature (I-II, q. 100 a. 1c & 104 a. 4 ad 2). They are, however, like the moral precepts in that they are ultimately derived from reason (I-II, q. 99 a. 4 ad 2) even if they do not “derive their force from the mere dictate of reason” as the moral precepts do (I-II, q. 104 a. 1 ad 2, emphasis added). They are necessary because the general natural law precepts that pertain to justice - whereby a man is “directed to his neighbor” (I-II, q. 101 a. 1c.) - have to be determined more particularly.

Relationship of the Precepts of the Old and New Laws

Turning now to the way the Old Law’s moral precepts relate to the New Law, a fundamental point is that the Old law in general is imperfect. Consequently, the New Law, which is perfect, supplies what was lacking in it (I-II, q. 107 a. 2c.). Both are directed to man’s justification, but the Old Law did not provide the means for its fulfillment as the New Law does. Therefore, even though the New Law is more difficult to keep insofar as it explicitly directs interior dispositions, it is accompanied by the grace that enables it to be kept and thus it is subjectively less burdensome (I-II, q. 107 a. 4c.).

The moral precepts of the Old Law may be the same in many cases inasmuch as the Commandments and the precepts necessarily following from them were in no way repudiated in the New Law. However, the means of inducing men to keep them is entirely different. The Old Law used temporal promises and threats to lead people to keep them due to the imperfection of the people (I-II, q. 99 a. 6c.) and can, therefore, be said to have restrained the hand, but not the will. On the other hand, the New Law “is said to restrain the will” because it is able to instill grace into men’s hearts (I-II, q. 107 a. 1 ad 2).


In sum, the consideration of the various precepts leads one to marvel at the providential intricacies of the economy of salvation. The natural law, implanted by God in the hearts of men by nature, guides all people to do good and avoid evil. To be sure, because man's nature has been wounded because of the Fall, people needed the Old Law to direct them more surely to God. The ceremonial and judicial precepts, therefore, applied the principles of natural law and the moral precepts expounded upon and made explicit the law of nature. Both the precepts of natural law and those of the Old Law, however, disposed man to the coming of Christ and the reception of grace. In this way, people were ultimately led to the grace of becoming sons and daughters of God (Gal. 3:26; Rom. 8:14) who hope for the bliss of everlasting life.


Ryan J Brady

Subsequent to a few semesters of study at Thomas Aquinas College, Dr. Brady graduated from La Salle University in Philadelphia with a B.A. in Religion. After receiving a Masters degree in Systematic Theology from Christendom Graduate School (where he was the valedictorian) he defended his doctoral dissertation “Aquinas on the Respective Roles of Prudence and Synderesis vis-à-vis the Ends of the Moral Virtues” with distinction and received his Ph.D. in Systematic Theology.