The Categorical Imperative

Kant's master rule: act only according to a principle you could will to become a universal law. Before any action, ask yourself — what if everyone did this? If the answer creates a contradiction, the action is immoral.

Lying fails the test instantly. If everyone lied all the time, language would become meaningless and lies themselves would stop working. A universal law of lying destroys the very concept of lying. That is Kant's signature move: immoral principles self-destruct the moment you imagine everyone following them.