Immanuel Kant thought morality could not depend on what you want or what happens to be useful. It must be binding regardless of desire or consequence. The categorical imperative is his name for the supreme principle of morality, and it comes in several formulations. The most famous says: act only on that maxim which you can at the same time will to be a universal law. Test your reason for acting by asking whether everyone could act on it.

Kant contrasted categorical imperatives with hypothetical imperatives. A hypothetical imperative tells you what to do if you want a certain end: if you want to be healthy, exercise. A categorical imperative commands unconditionally. The second formulation says: treat humanity, whether in yourself or others, always as an end, never merely as a means. The third speaks of a kingdom of ends, where rational beings legislate moral law together. Kant believed these formulations expressed the same principle. Lying, cheating, and breaking promises fail the universalizability test, because a world where everyone did them would undermine the very practices they depend on.

Kant's formulations

• Universal law. Act only on maxims you could will as universal.
• Humanity. Treat persons as ends, not merely means.
• Kingdom of ends. Act as a legislator in a community of rational beings.

The categorical imperative grounds deontological ethics, which holds that some acts are wrong regardless of their consequences.