How Does Logic 101 (#28): Modus Tollens Work?

Modus Tollens is a rule that helps us know something isn’t true by looking at what happens if it were true.

Imagine you have a toy robot that only sings when its battery is full. If the robot sings, we can say its battery is full. That’s like regular logic. But Modus Tollens turns this around: if the robot does not sing, then we know for sure its battery is not full.

Let’s make it even simpler:

You have a friend who only eats pizza on weekends. If your friend eats pizza, you can guess it's the weekend. But Modus Tollens says: if your friend does not eat pizza, then it must be a weekday, because that's when they don’t eat pizza.

So, in logic terms:

• If A happens, then B happens.
• If B does not happen, then A does not happen either.

This is like turning a rule inside out to figure things out. It’s a helpful tool for solving clues and making smart guesses, just like your friend figuring out what day it is by looking at their snack!