Aquinas’ definition of law is very brief and straight-forward. Most lawyers and even college students will at least have heard tell of it. It reads: “Law is an ordination of reason, by the proper authority, for the common good, and promulgated.” Many things are stated and implied in this brief, compact sentence.

When each of these four elements of the definition is spelled out and understood, the reason why Aquinas stands at the heart of things legal becomes clear. One must take a stand for or against him. If we stand against him, as most do, we lock ourselves into a series of logically declining steps that ultimately, in the real order when carried out, end with no law at all, only with decrees arbitrarily issued against a background of nothingness.

The first thing that is to be noted is where the discussion of law is located in the order of things. In the Second Part of the First Part of the Summa Theologiae, Aquinas explains what he calls “external” principles of human action. Certain things, especially the existence and needs of others, must be considered in any of our actions. So we are talking about human action which follows from our given minds and wills. We receive minds and wills from nature. We do not create them ourselves.

The fact that we have powers of the soul like reason, will, and desire, that we can acquire virtues and vices, all belong to interior principles of action. Any adequate understanding of law presupposes this background.

An external principle of action means that some norm or being that influences us from outside of ourselves must be taken into consideration in our actions. We do not make or create what we deal with but we must take account of it. Thus, if some natural (do not kill) or civil (drive forty miles an hour) law exists with regard to a proposed action, we are to take it into consideration. We then act for or against the written or unwritten law that we find.

It will be noted in the beginning that no indication of a penalty is found in Aquinas’ definition. The issue of sanction only arises if the law is not observed. Those definitions that define law wholly in terms of sanctions or penalties for non-observance do not get at the heart of law. Laws do have consequences if they are broken. Indeed, they have consequences when they are observed. That is why we observe them. These consequences must also be intelligible, because in order to legitimately be a law at all, any law must be comprehensible to those it concerns.

A law is an intelligible statement of reason about what ought or ought not to be done. It should make clear what it is we are talking about, what it is we want observed. This is why the letter of the law is important. It is up to the law-giver to state precisely what he means in obliging us to a given law. In this sense, law is mind speaking to mind. In short, the person who obeys the law does so because he understands it if the legislator makes it intelligible.

This is why those who live by law also live by reason. Such people can understand and follow what is reasonable even if they did not themselves make the law. They do not need to be forced to act or act reluctantly for fear of punishment. When reason is followed both by the law-giver and by him who is to observe the law, coercion is not necessary.

In his methodology, Aquinas maintains that we do not fully understand a thing until we understand the objections to it. These objections must be known to and stated fairly and directly by the law-giver. The most famous objection to Aquinas’ understanding of law is taken from the famous Roman Code of Laws. It maintains that law is essentially a statement of will, not reason. In will-based or voluntarist legal systems (also theorized as “legal positivism”), the law is not a statement of reason but of will, the will of the legislator or ruler. If the latter does not have to obey reason, he can make laws indifferent to the distinction between good and evil.

Voluntarist legal theories, such as those in Roman Law or the Koran are found increasingly in the legal systems of other nations. They follow the voluntarist legal consequences of Hobbes’ Leviathan in which all power was placed in the ruler to enforce all laws whatever they were. If a ruler decides a law in a completely voluntarist system, there can be no such thing as an unjust law.

So, it follows naturally that in a rational system of law, unreasonable laws are unjust. The citizen can appeal over the head of the ruler to a standard of reason and justice to which the ruler is also obliged to obey. The concept of a limited government is found in these premises. Both the ruler and the ruler, including the courts, are subject to the same reason to which they must appeal to decide the validity of their respect positions. It is this principle that gives moral weight to our actions under the law.

The second element in the definition has to do with who is authorized to make the law. Every state must have a known center, whatever it is called, charged with the responsibility of making the law. Here we recognize that many conflicting reasons, good and bad, may be proposed as to why we, as a citizenry bound together, should or should not do this or that. We need to designate some individual or group to decide which of many proffered reasons, is the one that is to be followed and enforced, if necessary.

This deciding is what legislatures are for. They decide what law is to be observed and for what reason. In the classical political separation of powers, the legislature (whatever it be called) decides what law is to be the guiding one. The executive is to carry out and enforce the details of the law, while the judiciary is to judge who does or does not observe the law in particular cases.

Aquinas held that the end of the law is the common good, not a particular good. The common good is not some kind of universal plan into which all must fit. It means rather the fostering of the good in each person freely to come forth. Justice, both distributive and commutative, is the basic political category as it seeks to guarantee to each what is due. Law cannot easily mandate or even properly grasp the possibility of civic and personal friendship that makes human life most worth living, but law can offer a framework to order and pursue those goods.

Political living at its best and at its worst indicates that something beyond justice and politics exists in the universe. Not all crimes committed by men are punished and not all good deeds are rewarded.

Because of this, the promulgation of the law is more important than at first might be realized. Those citizens who observe the law do so as persons with intelligence and free will. If they do not understand the reason of the law, then they only obey the law because it is enforced—and this is a particular challenge in our regime of extremely complex administrative law. There can never any moral and spiritual unity among men in such a system. The harmony envisioned by Aquinas includes the citizens’ understanding of what they are about in the city in obeying its laws.

Such are the essentials of Aquinas’ conception of law. He goes into greater detail in later questions. One thing that needs to be added from these latter discussions has to do with this question: Can we expect all citizens to observe the law rationally? Aquinas says that the law is made for the generality of men, the majority of whom are not perfect. This is certainly a practical and common sense observation.

Law thus must be carefully stated and gradually improved. The law should concern itself primarily with those issues, the denial of which makes civic life impossible. Thus we need laws about murder, theft, and marriage. Many other things are to be left to individual citizens to work out among themselves. For, to try to legislate everything implies that we have the mind of the gods and can foresee all contingencies. Aquinas still offers us a pointed reminder that if law is to remain law, it must always respect its own limits.