"And how do you know that you're mad?," the Cat asks Alice in Lewis Carroll's books. The Cat explains that a dog is not mad because it shows anger by growling and happiness by wagging its tail. This set of behaviors is considered normal for a dog. Alice agrees with this perspective, which sets the stage for the Cat to compare human and animal behaviors.

The Cat then points out that it behaves oppositely, growling when it is pleased and wagging its tail when angry. Based on this, the Cat concludes that it must be mad because its actions contradict the usual signs of happiness and anger, highlighting how perceptions of madness can vary based on behavior. The dialogue explores the relativity of normalcy and madness.