Four Kinds of Law

Before dividing law into kinds, Aquinas tells us what law is in general. His definition appears at the end of Question 90 and has four parts. A law, he says, is:

Each piece of this definition is defended in its own Article of Question 90, and each piece rules out something.

Ordinance of reason. A law is not a brute command or an expression of preference. It is something reason can understand and that directs rational beings toward their good. This rules out the idea that a law is just whatever the strongest person says it is.

For the common good. A law is directed at the good of the community, not at the private advantage of the lawmaker or any subgroup. A decree that benefits only the ruler is not really a law.

Made by him who has care of the community. Laws require proper authority. A random person cannot make a law, even a law that would genuinely promote the common good. The lawmaker must stand in a relationship of responsibility to the community the law binds.

Promulgated. A law must be made public. If people cannot know what the law requires, they cannot be bound by it. This condition rules out secret rules and retroactive commands.

This four-part definition is going to do a lot of work in what follows. Keep it in mind. When Aquinas asks whether some purported law is really a law, he is usually checking it against these four criteria.

In Question 91, Aquinas identifies four kinds of law. They form a hierarchy, with each kind depending on the one above it.

Take these one at a time.

The eternal law is the most fundamental kind of law and also the least accessible to us. Aquinas thinks of it as the rational plan by which God governs all creation. Every natural thing behaves in accordance with its nature because that is how God made it; the pattern of that governance, considered from the divine perspective, is the eternal law.

For things without reason, like rocks and plants and animals, participation in the eternal law is automatic. A rock falls, a plant grows toward light, a bird builds its nest: none of these things chooses to follow the eternal law; they simply do what their nature requires. They participate in the eternal law passively, so to speak, by being the kinds of things God made them to be.

Rational creatures participate differently. We can understand the rational order of things, recognize how we ought to act in light of that understanding, and freely choose whether to conform to it. This active, rational participation in the eternal law is what Aquinas calls the natural law.

So the natural law is not a separate law layered on top of the eternal law; it is the eternal law as it applies to us, given our nature as rational creatures. The natural law is what our reason grasps when it thinks carefully about what we are and what our good consists in. Question 94, which we read next, is Aquinas's account of what the natural law actually contains.

Human law is what we ordinarily mean when we use the word "law": the statutes, regulations, and customs enacted by human governments. For Aquinas, human law is necessary because natural law is general, and real communities need specific rules to solve specific problems. Natural law tells us that we should not harm the innocent, but it does not tell us how fast to drive in a school zone or which side of the road to drive on. Human law fills in those specifics.

The key Thomistic claim about human law is that its authority comes from natural law. Human laws bind us in conscience because, and only insofar as, they are derived from natural law. A human law can be derived from natural law in two ways: by conclusion (deducing a specific rule from a general principle, as "do not murder" follows from natural law) or by determination (choosing one permissible specification among several, as choosing to drive on the right rather than the left). Either way, the human law's authority is ultimately borrowed from natural law.

This has a sharp implication. If a human law actually contradicts natural law, it cannot borrow authority from it. Such a law, Aquinas will argue in Question 95, is not really a law at all. We will treat this claim in detail on the next page.

Divine law is the fourth and final category. This is what God reveals directly, through Scripture. Aquinas asks why divine law is needed at all if natural law is already accessible to reason. His answer has several parts. Natural law is accessible in principle but hard to know fully in practice, and revelation corrects our errors. Human beings have a supernatural end (the vision of God) that cannot be known by reason alone, and divine law directs us toward it. And some matters of conscience are internal to us, invisible to human law, which only regulates external conduct; divine law reaches the interior.

For our purposes, divine law is the least important of the four kinds. The ethics we are doing in this course is mostly the philosophical work of reflecting on human nature and reason. Revelation matters to Aquinas's overall system, but most of the action in our reading is at the level of natural and human law.

There is an important philosophical move buried in all of this, and it is worth flagging explicitly. Aquinas takes facts about human nature (what we are, what we need, what our powers are directed toward) and derives moral conclusions from them (what we ought to do). This is how natural law is supposed to work. We reflect on our nature, we see what conduces to our flourishing, and we conclude that we should pursue those things and avoid what undermines them.

Five centuries after Aquinas, the Scottish philosopher David Hume raised a pointed question about exactly this kind of reasoning. Hume noticed that ethical writers often move, without warning, from sentences about what is the case to sentences about what ought to be the case. One moment they are describing human nature or the world; the next they are prescribing what we should do. Hume asked: how does the "ought" get into the argument if it was not already in the premises? If all you start with is facts, how do you end up with values?

This problem is now known as the is/ought distinction, and it is one of the most famous challenges in moral philosophy. If Hume is right, then any argument that tries to derive moral conclusions from purely factual premises contains a logical gap.

Natural law theory is a major target of this critique. Aquinas's defenders have several responses. One is to argue that Aquinas's premises are not purely factual in the Humean sense: when he talks about human nature, he is already talking about nature as directed toward an end, and teleological facts are not the kind of flat, value-free facts Hume has in mind. Another is to argue that the gap Hume identifies is a feature of a certain narrow conception of reason, and that practical reason can indeed move from what we are to what we should do.

You do not need to settle this dispute. You need to see it. Every time Aquinas argues that we should do X because human nature is directed toward X, he is making a move that Hume would challenge. This does not mean the argument fails. It means the argument depends on premises that are themselves contested.