What does “Categorical Imperative” mean? Kant's Attempt to Build Morality Out of Reason Alone

Some philosophical phrases survive because they sound authoritative even when few people know what they mean. "Categorical imperative" is one of those phrases. It often appears in ordinary language as shorthand for a rule that must be followed no matter what. That usage is not entirely wrong, but it misses both the force and the ambition of the idea. Immanuel Kant did not mean merely that some duties are strict. He meant that morality itself must rest on a kind of rational law that applies to every person simply because they are rational beings.