What Makes Something Good? Thomas Aquinas on God, Ethics, and Why We Exist

Imagine you’re walking home from school and you see someone drop their wallet. Nobody else notices. You could pick it up and keep it, or you could run after them and return it. Most people would say you should return it—that it’s the right thing to do. But why? What makes something right or wrong in the first place? And what would it mean to live a truly good life?

These are the kinds of questions that Thomas Aquinas spent his life trying to answer. He lived in the 13th century—a time when most people in Europe couldn’t read, when universities were just being invented, and when almost everyone believed the universe was a mysterious place governed by God. But Aquinas wasn’t content to just accept what he was told. He wanted to understand why things are the way they are. He wrote millions of words trying to figure it out, and his ideas are still debated by philosophers today.

The Puzzle: Why Is There Anything at All?

Here’s a strange thing philosophers noticed: everything that exists seems to have a cause. Your parents caused you to exist. The food you ate caused you to have energy. The sun caused the plant to grow. But if everything has a cause, then what caused the universe? And what caused that cause? You could keep going backward forever—but does that make sense?

Aquinas thought not. He argued that there must be something that sets everything else in motion without itself needing to be set in motion. Something that causes everything else to exist but doesn’t need a cause itself. He called this thing God. And he wasn’t just guessing—he gave five different arguments, which he called the “five ways,” trying to prove that such a being must exist.

Here’s the basic idea. Imagine you’re standing in a line of dominoes. Each domino falls because the one before it knocked it over. You trace the chain backward: domino A knocked over domino B, which knocked over domino C, and so on. But if there’s no first domino, none of the others would ever fall. There has to be a starting point. For Aquinas, the universe is like that line of dominoes, and God is the first push.

But—and this is important—Aquinas didn’t think you could prove everything about God using reason alone. Reason can tell you that a first cause exists, but it can’t tell you that this being is a Trinity, or that Jesus was both human and divine, or many other things that Christians believe. For those, Aquinas said, you need revelation—what God has chosen to tell people directly. Philosophy and faith, he thought, work together. Philosophy can take you partway, but faith takes you the rest of the way.

The Strange Thing About God

Once Aquinas argued that God exists, he had to figure out what God is like. And here things get weird.

Aquinas said we can’t really know what God is. We can only know what God is not. God isn’t a body. God isn’t made of parts. God doesn’t change. God isn’t limited. This is called “negative theology”—you approach God by stripping away everything you can say that’s false.

But wait, you might think: doesn’t Aquinas spend a lot of time saying positive things about God? Like that God is good, or wise, or powerful? Yes, he does. But he had a clever way of explaining how that works.

When you say “Socrates is wise,” you mean that Socrates has the quality of wisdom. But when you say “God is wise,” you can’t mean the same thing, because God isn’t a being who has wisdom as one quality among many. God is wisdom itself. The word “wise” is being used in two different but related ways. Aquinas called this “analogical” language. It’s not exactly the same meaning (univocal) and not totally different (equivocal). It’s somewhere in between. When we say God is good, we mean that whatever goodness we see in the world exists in God in a much higher way—but we can’t really grasp what that “higher way” is.

This part gets complicated, but here’s what it accomplishes: it keeps Aquinas from pretending he understands more than he does. He’s honest about the limits of human thinking. We can know that God exists and that God is good, but we can’t fully comprehend what that goodness is like.

What Are We? Soul and Body

Aquinas had a very specific idea about what human beings are. He thought we’re not just bodies, and we’re not just minds trapped in bodies. We’re a single thing—a unified combination of form and matter.

For Aquinas, everything in the physical world is made of two principles: form and matter. “Form” is what makes something the kind of thing it is—what makes a cat a cat rather than a rock. “Matter” is the stuff that thing is made of. But in living things, the form is something special: it’s the soul. Your soul isn’t a ghost living inside your body. It’s the principle that makes your body alive and makes it your body. Your soul and your body aren’t two separate things stuck together. They’re two aspects of one single thing: you.

But here’s where it gets interesting. Aquinas thought that human beings have one part that can’t be explained by matter alone: our ability to think abstractly. When you think about the concept of “justice” or “triangle,” you’re not thinking about any particular just act or any particular triangle. You’re thinking about something universal. Aquinas argued that this kind of thinking couldn’t be done by a physical organ—not even the brain. The brain can process images of individual cats, but it can’t grasp the universal concept of “cat.” Therefore, he concluded, the human intellect must be immaterial—and if it’s immaterial, it can’t be destroyed when the body dies.

This creates a puzzle. If you’re a combination of soul and body, and your body dies, what happens to you? Aquinas was very clear: in this life, you are the whole combination. Your soul isn’t the complete you. So when your body dies and your soul goes on existing without it, are you still you? Philosophers still argue about this. Some say Aquinas thought you wouldn’t really survive until your body was resurrected. Others say he thought the soul alone was enough to keep you going. What’s clear is that Aquinas believed in bodily resurrection—that eventually, souls would be reunited with their bodies.

How Do We Know What’s Good?

Aquinas thought that the ultimate goal of human life is happiness. But he didn’t mean the kind of happiness you get from eating ice cream or winning a video game. He meant the deep, lasting fulfillment that comes from doing what humans are made to do.

And what are humans made to do? For Aquinas, the answer was: understand. He thought that our highest activity is intellectual contemplation—trying to understand the world and its creator. This sounds very academic, but think about it differently. Have you ever had a moment where you suddenly understood something that had been confusing you? That feeling of clarity and satisfaction? Aquinas thought that kind of activity, pursued at its highest level, is what happiness really is. The fullest happiness, he believed, would come only in the afterlife, when we could directly contemplate God. But even in this life, people who devote themselves to understanding truth are as happy as anyone can be.

Now, here’s a problem. If everything we do is aimed at our own happiness, doesn’t that make us selfish? What about loving other people? What about loving God?

Aquinas’s answer is subtle. He says that what’s actually best for us—what will truly make us happy—is not pursuing our own pleasure but understanding and loving what’s truly good. And what’s truly good, ultimately, is God. The more we understand God’s goodness, the more we naturally love God. And when we love God, we also love what God loves—including other people. So pursuing your own true happiness doesn’t lead away from loving others; it leads toward it. It’s like the difference between someone who craves candy and someone who craves vegetables: both are pursuing what they desire, but one desire leads to health and the other doesn’t. Your job as a moral person is to shape your desires so that you want what’s actually good for you.

Where Does Right and Wrong Come From?

Aquinas had a theory about where morality comes from. He called it “natural law.”

The basic idea is simple: God created the universe with a certain design. Just as a good watchmaker creates a watch that runs correctly, a good God creates creatures that function well when they act according to their nature. So what’s good for a human being is what allows a human being to flourish—to function well as the kind of creature we are.

How do we figure out what that is? Aquinas said we have something called “synderesis”—a basic built-in ability to recognize the first principles of morality. The most fundamental one is: do good and avoid evil. That’s obvious, but it’s also empty—it doesn’t tell you what counts as good. To figure that out, Aquinas said, we need to look at our natural inclinations. Humans naturally want to stay alive. They naturally want to have children and raise them. They naturally want to know the truth, especially about God. They naturally want to live in society with others.

These inclinations, Aquinas thought, point the way to what’s good for us. Since we naturally want to live, preserving life is good. Since we naturally want to raise children, educating the young is good. Since we naturally want to know the truth, learning is good. Since we naturally want to live together, justice and fairness are good.

This doesn’t mean every natural impulse is good—we also have impulses that lead us astray. But the basic inclinations, properly understood, give us the starting points for moral reasoning. From there, we use our reason to work out detailed rules for how to live.

This theory has some appealing features. It doesn’t depend on a holy book (though Aquinas thought God gave us those rules too, through divine law). It says that anyone, anywhere, can use their reason to figure out basic moral truths. And it ties morality to human flourishing—being good isn’t just following arbitrary rules; it’s about becoming the kind of person who lives well.

The Big Argument That Never Ends

You might think that after 700 years, philosophers would have settled whether Aquinas was right. They haven’t. Here are some of the questions people still argue about:

Does God really exist? Aquinas’s arguments have been criticized by many philosophers. David Hume, an 18th-century philosopher, argued that just because everything in our experience has a cause doesn’t mean the universe itself has a cause. Immanuel Kant, another famous philosopher, argued that the idea of a “first cause” doesn’t make logical sense—you can’t prove that there must be a starting point. Other philosophers have defended versions of Aquinas’s arguments. This debate is still alive.

Is natural law just “natural” or is it cultural? Critics point out that what people think of as “natural” varies a lot across cultures and time periods. Aquinas thought everyone naturally wanted certain things, but is that really true? People in different times have had very different ideas about family, work, and even what counts as knowledge. If natural law depends on what’s natural for humans, and what’s natural changes, then maybe natural law isn’t as fixed as Aquinas thought.

Can happiness really be the foundation of ethics? Some philosophers worry that making everything about your own happiness is selfish, no matter how you define happiness. Others defend Aquinas by saying that wanting to be happy doesn’t mean being selfish—you can want your own happiness and also genuinely care about others.

What does freedom mean? Aquinas believed that humans have free will—that we can genuinely choose between options. But he also believed that God knows everything that will happen and that everything happens according to God’s plan. How can both be true? This is called the problem of free will and divine foreknowledge, and philosophers still argue about whether Aquinas had a good answer.

Why Bother?

You might wonder: why should a 12-year-old in the 21st century care about what some medieval monk thought? Here’s why.

First, Aquinas’s questions are still our questions. What makes something right or wrong? What would a truly good life look like? Is there a purpose to existence? These aren’t just academic questions—they’re questions you face every time you make a choice about how to treat someone or how to spend your time.

Second, Aquinas shows how to think carefully about big questions without pretending you have all the answers. He’s confident about what he believes, but he’s also honest about the limits of reason. He doesn’t say “I’ve proved everything.” He says “Here’s what reason can show, and here’s where faith has to take over.”

Third, Aquinas’s ideas are still alive in our world. When people argue about human rights—saying that certain things are just wrong because they violate human nature—they’re using ideas that Aquinas helped develop. When people argue about whether morality requires God or can be based on reason alone, they’re continuing debates that Aquinas was part of.

And finally, Aquinas reminds us that thinking carefully about difficult questions is itself a kind of good life. He thought that the highest human activity was trying to understand—and that includes trying to understand what it means to live well. That’s a project that never gets finished, and that anyone can join.

Appendices

Key Terms

Term	What it does in the debate
Natural law	The idea that basic moral rules can be discovered by reasoning about human nature, not just from religious texts
Form and matter	Two fundamental principles of physical things: form makes something the kind of thing it is, matter is the stuff it’s made of
Soul	In living things, the form that makes them alive; for humans, also an immaterial principle that can think abstractly
Analogical language	Using words to describe God that aren’t exactly the same as when we use them for people, but aren’t totally different either
First cause	The thing that starts the chain of causes without needing a cause itself; for Aquinas, this is God
Happiness (beatitudo)	Not just feeling good, but the deep fulfillment that comes from doing what humans are made to do

Key People

Thomas Aquinas (1225–1274): A medieval Italian philosopher and theologian who tried to combine Christian faith with the philosophy of Aristotle, and wrote massive works that are still studied today. He argued that reason and faith can work together to understand God and morality.
Aristotle (384–322 BCE): An ancient Greek philosopher whose ideas about science, ethics, and metaphysics were hugely influential on Aquinas. Aquinas wrote commentaries on his works and adopted many of his basic concepts (like form and matter).

Things to Think About

If you disagree with Aquinas’s proof that God exists, can you explain why it doesn’t work? Or do you think it’s valid? Try to argue the other side as well as you can.
Aquinas thinks that pursuing your own true happiness will lead you to love others more, not less. But can you think of cases where doing what’s truly good for you might conflict with what’s good for someone else? How do you resolve that?
If natural law says we can figure out right and wrong just by reasoning about human nature, does that mean people who disagree about morality are just bad at reasoning? Or could there be multiple reasonable answers?
Aquinas says our ultimate happiness comes from understanding God. But what if you don’t believe in God? Does his theory of happiness still have anything useful to say?

Where This Shows Up

Human rights debates: Arguments about whether certain rights (like the right to life or freedom) are universal come from natural law thinking. When people say “this is just wrong, no matter what the law says,” they’re using ideas Aquinas helped develop.
Discussions about “the meaning of life”: Every time you hear someone argue that happiness comes from understanding and doing what you’re meant to do, you’re hearing an echo of Aquinas’s ideas.
Ethics classes and philosophy books: Modern philosophers who defend “virtue ethics” (the idea that being good is about having good character, not just following rules) are continuing a tradition that Aquinas was a major part of.
Religious debates about science and faith: Aquinas’s view that faith and reason can work together—rather than being in conflict—is still a major position in discussions about religion and science.