Imagine you could carry around one magic question that helped you figure out whether something was right or wrong. Not a grown-up telling you, not a rulebook, just a single question you could ask yourself any time. More than two hundred years ago, a philosopher came up with exactly that, and his idea is still taught in classrooms all over the world today. His name was Immanuel Kant, and his famous question was simple: what if everybody did it?

Meet Immanuel Kant

Immanuel Kant was a German philosopher who lived from 1724 to 1804. He spent almost his entire life in one city and was famous for being incredibly predictable. People joked that you could set your clock by his daily afternoon walk. But inside that quiet, routine life, Kant was thinking some of the boldest thoughts in history, especially about one giant question: how do we know the right thing to do?

His answer was an idea with a fancy name, which he laid out in a book published in 1785: the categorical imperative.

Breaking down the big words

That name sounds scary, so let’s take it apart. An “imperative” is just a command, a rule telling you to do something. “Categorical” means it always applies, with no exceptions and no excuses. So a categorical imperative is a rule you should follow no matter what.

Kant pointed out that most rules we follow are actually “if, then” rules. If you want good grades, then study. If you want to get better at soccer, then practice. Those rules only matter if you want the prize at the end. But Kant believed right and wrong are different. Being honest is not something you should do only if it helps you. It is something you should do, period. That “no matter what” quality is what makes it categorical.

Rule number one: the everybody test

So how do you actually use it? Kant’s most famous version goes like this. Before you do something, imagine that everyone in the world did the exact same thing. If that would wreck everything, then it is wrong.

Take lying. Imagine a world where everybody lied whenever it was convenient. Pretty quickly, nobody would believe anything anyone said. Promises would mean nothing, and the word “truth” would lose all meaning. The whole trick of lying only works because most people tell the truth. So when you picture everyone doing it, lying basically destroys itself. For Kant, that was proof it was wrong.

The same test works for cheating on a test, cutting in line, or stealing. Ask yourself, “what if everybody did this?” If the answer is “the world would fall apart,” you have your answer.

Rule number two: people are not tools

Kant had a second version of his rule that is just as powerful. He said you should always treat people as people, never as just a tool to get what you want.

Here is what he meant. Imagine you pretend to be someone’s friend only so you can copy their homework, and then you ignore them afterward. You are not really treating them as a person with their own feelings. You are using them like a machine that spits out answers. Kant thought that was one of the biggest wrongs of all. Every person, he believed, has worth and deserves respect, and should never be treated as merely something useful to you.

Why this mattered to Kant

Here is the part that made Kant special. He believed that knowing right from wrong does not come from your feelings, or from whether you might get caught, or even from how things happen to turn out. It comes from reason, the thinking part of your mind, and from duty, which means doing the right thing simply because it is right. To Kant, a truly good person helps others not for a reward or a thank-you, but because it is the right thing to do.

Did he get it right?

Not everyone agrees with Kant, and that is part of what makes him so fun to argue about. Some people think his rules are too strict. For example, Kant believed lying is always wrong, with no exceptions at all. But what about a tiny white lie to protect a friend’s feelings, or a lie that could save someone’s life? Many people feel that sometimes bending the rule is actually the kinder choice. Kant would have disagreed, and people have been debating it ever since.

So the next time you are stuck wondering whether something is okay to do, try borrowing Kant’s two questions. First, what if everybody did this? And second, am I treating people as real people, or just using them to get what I want? You might not always like the answer, but you will be thinking exactly like one of the greatest philosophers who ever lived.