What Makes a Law a Good Law? Thomas Aquinas’s Big Idea

Imagine you’re in the middle of a heated argument with your parents about a house rule. Maybe it’s about screen time, or chores, or when you have to be home. You might say something like, “That rule isn’t fair!” And they might say, “It’s the rule, so you have to follow it.”

But here’s a strange question: What if a rule isn’t fair? Does it even deserve to be called a rule? Or is it just someone bossing you around, pretending to have authority they don’t really have?

A 13th-century philosopher named Thomas Aquinas spent a lot of time thinking about this. He lived in a world of kings, popes, and crusades, but he was trying to answer a question that still matters in every family, school, and country: What makes a law actually good—not just powerful?

The Strange Thing About Human Beings

Before we get to laws, Aquinas noticed something weird about people. Unlike rocks or trees or even most animals, human beings can choose what to do. A rock doesn’t decide to fall. A tree doesn’t decide where to grow. But you—every morning you wake up and make choices. You could do your homework or play a game. You could be kind to someone or ignore them. Nothing forces you either way.

This might not seem strange, but think about it: if you’re truly free to choose, then there must be some standard for good choices and bad choices that isn’t just “whatever you feel like.” If everything were equally fine, there’d be no point in choosing at all. Aquinas thought that the very fact that we deliberate—that we think about what to do before doing it—shows we believe some things are worth pursuing and others aren’t.

He called these worth-pursuing things “basic goods.” They’re not goods because someone says so. They’re goods because any reasonable person, once they understand them, sees that they’re valuable. Knowledge is like that. So is life itself, friendship, marriage and raising children, and being a reasonable person. You don’t need to be convinced that knowing things is better than being ignorant—you just get it. Aquinas said these goods are “self-evident.” Not in the sense that everyone already knows them (babies don’t), but in the sense that once you understand what they are, you can’t honestly say they don’t matter.

Here’s an example he gives. When you were little, you probably asked a lot of questions. “Why is the sky blue?” “What’s for dinner?” “How do cars work?” And when someone gave you a real answer, you felt satisfied. When they didn’t, you were disappointed. At some point, you had an insight: this thing called “knowledge” is a general possibility you can pursue throughout your life. And you also realized: it’s good to know things. Not just useful—good. That insight, Aquinas says, is the beginning of practical reasoning. It’s the first tiny step from “what is” to “what ought to be.”

Where “Ought” Comes From

This leads to a big claim. Many people, then and now, say you can’t get an “ought” from an “is.” That is, just because something is a certain way doesn’t mean it ought to be that way. If humans naturally compete for resources, that doesn’t mean we should compete for resources. Nature isn’t a moral guide.

Aquinas agrees with this—partly. He says the basic goods aren’t discovered by looking at nature and then deciding to value them. Instead, you first grasp that something is good (knowledge, friendship, life), and then you understand that human nature includes a tendency toward those goods. The “ought” comes first, logically speaking. This is a subtle point, but it matters because it means Aquinas isn’t making the mistake of saying “it’s natural, so it’s good.” He’s saying something closer to “it’s good, so it makes sense to call it natural.”

The First Commandment of Thinking

Aquinas says there’s one principle that underlies all practical reasoning: “Good is to be done and pursued, and bad avoided.”

This sounds obvious, maybe even empty. But notice what it doesn’t say. It doesn’t say “do whatever makes you happy” or “maximize pleasure and minimize pain.” It says good is to be done. The hard work is figuring out what actually is good—and that’s where the basic goods come in.

Also notice the grammar. “Is to be done” isn’t a command like “do good!” It’s more like a statement of what makes sense, given what goodness is. Aquinas compares it to the principle of non-contradiction: you can’t honestly believe both that something is true and that it’s false. Similarly, once you understand that something is genuinely good, you can’t honestly treat it as if it didn’t matter. The “ought” is built into the recognition of the good.

From Goods to Morality

So you’ve got these basic goods: life, knowledge, friendship, marriage, practical reasonableness, and (Aquinas adds) knowing and relating to God. These are things worth pursuing for their own sake. But they don’t tell you how to pursue them. You could pursue knowledge by studying physics, or by reading history, or by learning to cook. You could pursue friendship by being loyal to your friends, or by making new ones, or by helping someone in need.

Morality, for Aquinas, is about how you integrate your pursuit of all these goods. You can’t pursue them all at once all the time. You have to make choices. And here’s the key moral principle Aquinas arrives at: Love your neighbor as yourself.

This isn’t just a nice sentiment. If the basic goods are genuinely good for anyone—not just for you—then you have no reason to treat your own pursuit of them as more important than anyone else’s. Knowledge is good whether you have it or your friend has it. Life is good whether it’s your life or a stranger’s. So morality requires that you not cut yourself off from the good of others. You must treat their well-being as part of your own concern.

Aquinas thinks this principle is actually a summary of the basic goods. When you understand that knowledge is good, you understand that it’s good for anyone who can have it. The “for anyone” part is the seed of love of neighbor. It doesn’t need to be proven—it’s just what it means to grasp a basic good.

Rules, Not Just Feelings

Now we get to something many people find surprising about Aquinas. He believed that some actions are always wrong, no matter what. Not “usually wrong” or “wrong unless you have a good reason.” Always wrong. He calls these “intrinsically wrongful” acts.

For example: intentionally killing an innocent person. Aquinas says this is always wrong. You can’t justify it by saying “but it led to good results.” The reason, he thinks, is that intentionally killing someone treats the basic good of life as something that can be traded away, sacrificed for other goods. But if life is a basic good—good in itself, not just as a means to something else—then you can’t coherently treat it as a mere tool for achieving other goods.

This is a hard idea, and philosophers still argue about it. But here’s Aquinas’s logic: when you choose to kill an innocent person as a means to some end, you’re saying “this person’s life is less important than my goal.” But if the life in question is a human life—the same kind of life as your own—you’re contradicting yourself. You’re saying your goals matter more than someone else’s existence, but you have no non-arbitrary reason for that preference.

The same reasoning applies to lying. Aquinas defines lying as saying something you believe to be false, with the intention of asserting it as true. The wrongness, he says, isn’t mainly about deception—it’s about the division within yourself between what you really think and what you present yourself as thinking. You’re using your own mind and speech to create a false image of yourself, and that treats the good of truth (and of honest relationship) as something you can discard when convenient.

But What About Self-Defense?

What if someone is attacking you, and the only way to stop them is to use force that might kill them? Aquinas says this is different from murder. If you’re defending yourself, your intention is to stop the attack, not to kill. If the attacker dies as a side-effect of your legitimate self-defense, that’s tragic but not wrong.

The difference is in what you’re trying to do. In one case, you’re trying to kill someone (even if you have a reason). In the other, you’re trying to stop an attack, and you foresee that death may result. This distinction became famous later as the “principle of double effect,” but Aquinas just thought it was obvious: moral rules apply to what you intend, not just to what you foresee will happen.

This is why he thinks soldiers in a war can kill enemy combatants—they’re trying to stop the enemy, not just kill people—but they can’t intentionally kill civilians. And why a doctor can’t kill a patient to harvest organs for transplant, even if that would save more lives. The intention makes the act what it is.

Laws and the People They Rule

Now we finally get to Aquinas’s big question about law itself. He defines law as: an ordinance of reason for the common good, made by the authority responsible for the community, and promulgated (made known).

Each part matters. It must be an ordinance of reason—not just someone’s whim. If a ruler makes a law out of greed or cruelty, it’s not really a law; it’s an act of violence pretending to be law. It must be for the common good—not for the ruler’s benefit, or for one group’s benefit at the expense of others. It must be made by proper authority—not just anyone who feels like giving orders. And it must be promulgated—people have to know about it, or they can’t be expected to follow it.

This was radical in Aquinas’s time (and still is in some places). It means that a law can be legally valid—passed by the right people in the right way—but morally invalid. If a law is unjust, Aquinas says, it “is not so much a law as an act of violence.” You have no moral obligation to obey it.

But wait: shouldn’t you follow laws to keep society peaceful? Aquinas thinks about this. He says that if the injustice is small and obeying would prevent greater harm, you might have a duty to obey anyway. But if the law commands something truly evil—like killing the innocent—you must disobey. No exceptions.

What About Government Itself?

Aquinas didn’t think democracy was the only good form of government. He thought the best constitution was a “mixed” one: a single good ruler (like an elected king), advised by a few wise people (aristocracy), chosen by many citizens (democracy). What mattered wasn’t the form but whether the rulers governed for the common good.

He also believed that political authority is limited in several ways:

It’s limited by morality. Rulers can’t do whatever they want. They’re subject to the same moral rules as everyone else—no murder, no theft, no lies.
It’s limited by law. Even the supreme ruler should follow the laws. If they don’t, they’re a tyrant, and the people have a right to resist.
It’s limited by purpose. The state’s job is to maintain justice and peace, not to make everyone virtuous. You can’t force people to be good. The law should only punish external acts that harm others, not private thoughts or vices.
It’s limited by other communities. For Aquinas, the Church is another “complete community” with its own authority over spiritual matters. The state can’t tell the Church what to do, and the Church can’t run the state.

And here’s a surprise: Aquinas thought that even in a perfect world without any crime or sin, there would still need to be government. Why? Because people have different ideas about how to do things, and you need authority to coordinate them. Even saints need traffic lights.

The Hard Cases: Tyranny and Resistance

When is it okay to overthrow a government? Aquinas says tyrants—rulers who govern for their own benefit rather than the common good—forfeit their right to rule. Their “laws” aren’t really laws. The people can resist them, even by force.

But he’s cautious. Overthrowing a government is dangerous and often makes things worse. He says there’s a presumption in favor of just putting up with bad rulers, unless the oppression is truly unbearable. Revolution can be justified, but only as a last resort, and only if the new government will genuinely serve the common good.

He also says something that might bother us today: he thought heretics (people who rejected the Church’s teachings after having accepted them) could be punished by the state, even executed. This is an uncomfortable part of his thought. But it’s worth noting that his reasoning was based on the circumstances of his time, and later thinkers in his tradition (including the Catholic Church itself) have rejected this view, arguing that Aquinas’s own principles about conscience and free choice actually support religious freedom.

So What?

Aquinas’s big idea is that laws aren’t just commands backed by force. They’re supposed to be reasonable—appealing to our minds, not just threatening our bodies. A good law gives you a reason to follow it, beyond just fear of punishment. When you understand why a law is good—when you see that it serves the common good—you can follow it freely, as a rational person, not just as a subject.

This is why, when you argue with your parents about a rule, you’re not just being difficult. You’re doing philosophy. You’re asking: does this rule serve the good of our family? Is it fair to everyone? Does the person making it have real authority? These are exactly the questions Aquinas thought we should ask about every law.

And the most important thing he teaches is that power doesn’t make right. A law can be powerful—backed by police and courts and armies—and still be wrong. The ultimate test of a law isn’t whether it can be enforced. It’s whether a reasonable person, thinking about the common good, could freely choose to follow it.

That’s a standard worth arguing about.

Appendices

Key Terms

Term	What it does in this debate
Basic goods	The fundamental things (like life, knowledge, friendship) that give us reasons to act; they don’t need to be justified by anything else
Common good	The well-being of a whole community, not just the sum of individual benefits—like the way friendship creates something shared that’s more than just two people each getting what they want
Conscience	Not a little voice inside you, but your own practical reasoning making judgments about what’s right and wrong
Determinatio	The act of turning a general principle into a specific rule; like deciding the speed limit on a road—it has to be reasonable, but many different numbers would be fine
Intention	What you’re actually trying to do, as opposed to what you foresee happening as a side-effect; for Aquinas, this is what makes an act the kind of act it is
Intrinsically wrongful	An act that’s always wrong, no matter the circumstances or consequences, because of what you’re intending to do
Natural law	The set of moral principles that any rational person can figure out by thinking about what’s good for human beings
Practical reason	Thinking about what to do, as opposed to thinking about what’s true (which is “theoretical reason”)
Prudentia	The master virtue of practical wisdom—knowing what’s good in general and also knowing how to apply it in your specific situation

Key People

Thomas Aquinas (1225–1274) — A Dominican friar and philosopher who tried to merge the ideas of the ancient Greek philosopher Aristotle with Christian theology; he was considered radical in his own time and was even condemned by some Church authorities after his death, but later became one of the most influential thinkers in Western philosophy.

Things to Think About

Aquinas says some acts are always wrong, including lying. But what about the classic case: a Nazi soldier asks if you’re hiding Jews in your attic, and you are. Is it okay to lie? If not, what should you do? If so, does that mean lying is sometimes okay after all?
Aquinas argues that you can’t get an “ought” from an “is”—but then he says basic goods are “self-evident.” How do you know something is genuinely good? Is there a way to argue with someone who says knowledge or friendship doesn’t matter?
If the state should only punish external acts that harm others, what about hate speech? Does it harm people? Should it be illegal? What counts as “harm” in the way Aquinas means it?
Aquinas says that even in a perfect world, you’d need government just to coordinate people’s different good ideas. Do you think that’s true? Wouldn’t people just naturally cooperate? Or is that naive?

Where This Shows Up

School rules and family rules — Every time you argue about whether a rule is fair, you’re using Aquinas’s basic test: does this rule serve the common good, and does the authority making it have any real right to?
Political debates — Arguments about whether a law is “constitutional” or “right” often turn on Aquinas’s distinction between a law that’s legally valid and one that’s morally valid.
International law — The idea that some acts (like genocide or torture) are wrong everywhere, regardless of what any country’s laws say, comes from the natural law tradition Aquinas helped develop.
Medical ethics — The “principle of double effect” Aquinas introduced is still used to think about hard cases like whether a doctor can give pain medication that might hasten death, or whether a pregnant woman can be treated for cancer if the treatment might harm the fetus.