Some acts are wrong no matter what good comes from them. That is the core claim of deontology, an ethical theory that grounds morality in duties and rules rather than consequences. Lying, breaking a promise, or killing an innocent person is prohibited, even if the outcome would be better. The word comes from the Greek deon, meaning duty. Immanuel Kant is the most influential deontologist, though the tradition includes others, including W. D. Ross and his theory of prima facie duties.

Kant argued that moral requirements are categorical. They do not depend on what you want or what will happen. They apply to all rational beings. His categorical imperative tests maxims by asking whether they could be universal laws and whether they treat humanity as an end. Ross softened the rigor. He held that we have several prima facie duties, such as fidelity, gratitude, and non-maleficence, and that when they conflict, we must judge which is more stringent in the situation. Critics of deontology point to the problem of conflicting duties and the difficulty of applying absolute rules to messy circumstances. Defenders reply that consequences matter, but they cannot justify everything.

Deontological themes

• Duty. Moral requirements bind regardless of outcomes.
• Rules. Some acts are prohibited or required.
• Respect for persons. Treat individuals as ends, not merely means.
• Intentions. The moral quality of an act depends partly on the agent's will.

Deontology contrasts with utilitarianism, which evaluates acts by their consequences, and with virtue ethics, which focuses on character.