Deontology (or Deontological Ethics) is an approach to Ethics that focuses on the rightness or wrongness of actions themselves, as opposed to the rightness or wrongness of the consequences of those actions (Consequentialism) or to the character and habits of the actor (Virtue Ethics).

Thus, to a Deontologist, whether a situation is good or bad depends on whether the action that brought it about was right or wrong. What makes a choice "right" is its conformity with a moral norm: Right takes priority over Good. For example, if someone proposed to kill everyone currently living on land that could not support agriculture in order to bring about a world without starvation, a Deontologist would argue that this world without starvation was a bad state of affairs because of the way in which it was brought about. A Consequentialist would (or could) argue that the final state of affairs justified the drastic action. A Virtue Ethicist would concern himself with neither, but would look at whether the perpetrator acted in accordance with worthy virtues.

Deontology may sometimes be consistent with Moral Absolutism (the belief that some actions are wrong no matter what consequences follow from them), but not necessarily. For instance, Immanuel Kant famously argued that it is always wrong to lie, even if a murderer is asking for the location of a potential victim. But others, such as W.D. Ross (1877 - 1971), hold that the consequences of an action such as lying may sometimes make lying the right thing to do (Moral Relativism).

It is sometimes described as "duty-based" or "obligation-based" ethics, because Deontologists believe that ethical rules bind people to their duty. The term "deontology" derives from the Greek "deon" meaning "obligation" or "duty", and "logos" meaning "speaking" or "study", and was first used in this way in 1930, in the book "Five Types of Ethical Theory" by C. D. Broad (1887 - 1971).

Modern deontological ethics was introduced by Immanuel Kant in the late 18th Century, with his theory of the Categorical Imperative.

Immanuel Kant defined an imperative as any proposition that declares a certain action (or inaction) to be necessary. A hypothetical imperative would compel action in a given circumstance (e.g. if I wish to satisfy my thirst, then I must drink something). A categorical imperative would denote an absolute, unconditional requirement that exerts its authority in all circumstances, both required and justified as an end in itself.

He argued that the "highest good" must be both intrinsically good (good "in itself"), and good without qualification (when the addition of that thing never makes a situation ethically worse). He concluded that there is only one thing that is truly good: a good will chosen out of a feeling of moral duty. From this concept of duty, Kant derived what he called a categorical imperative, a principle that is intrinsically valid (good in and of itself), and that must be obeyed in all situations and circumstances if our behaviour is to observe moral laws. He considered it an unconditional obligation, regardless of our will or desires, and regardless of any consequences which might arise from the action. He also believed that if an action is not done with the motive of duty, then it is without moral value and therefore meaningless.

Kant developed his moral philosophy in three works: "Groundwork of the Metaphysic of Morals" (1785), "Critique of Practical Reason" (1788) and "Metaphysics of Morals" (1797), and he formulated it in three different ways :

• Act only in such a way that you would want your actions to become a universal law, applicable to everyone in a similar situation.
• Act in such a way that you always treat humanity (whether oneself or other), as both the means of an action, but also as an end.
• Act as though you were a law-making member (and also the king) of a hypothetical "kingdom of ends", and therefore only in such a way that would harmonize with such a kingdom if those laws were binding on all others.

Robert Nozick (1938 - 2002) famously points out what has become known as the Paradox of Deontology, that Deontology forbids some acts that maximize welfare overall. The example usually used is that of a trolley hurtling towards five innocent and immobile people at the end of a track, where the only way to stop the trolley and save the five is to throw one innocent bystander in front of the trolley. The Principle of Permissible Harm in Deontology rules out deliberately throwing a person in front of the trolley, but the consequence of that is that five innocent bystanders die (which also contravenes the Principle of Permissible Harm).

Utilitarians like Jeremy Bentham have criticized Deontology on the grounds that it is essentially a dressed-up version of popular morality, and that the objective and unchanging principles that deontologists attribute to natural law or universal reason are really just a matter of subjective opinion.

John Stuart Mill, another 19th Century Utilitarian, argued that deontologists usually fail to specify which principles should take priority when rights and duties conflict, so that Deontology cannot offer complete moral guidance. Mill also criticized Kant's claims for his Categorical Imperative, arguing that it is really just another way of saying that the ends justify the means, which is essentially a consequentialist argument.

Some critics have attempted to show that constraints (e.g. the requirement not to murder, for example) are invariably immoral, but then to show that options (e.g. the right not to give money to charity) without constraints are also immoral.

• Divine Command Theory: a form of deontological theory which states that an action is right if God has decreed that it is right, and that that an act is obligatory if and only if (and because) it is commanded by God. Thus, moral obligations arise from God's commands, and the rightness of any action depends upon that action being performed because it is a duty, not because of any good consequences arising from that action. Therefore, if God commands people not to work on the Sabbath, for example, then people act rightly if they do not work on the Sabbath (but solely because God has commanded it). If they do not work on the Sabbath because they are lazy, then their action is not truly speaking "right", even though the actual physical action performed is the same.
  William of Ockham, René Descartes and the 18th Century Calvinists all accepted versions of this moral theory. William of Ockham went so far as to argue that if God had commanded murder, then murder would indeed have been morally obligatory, and indeed that God could change the moral order at any time on a whim.
  However, Plato's Euthyphro Dilemma asks: "Is an action morally good because God commands it, or does God command it because it is morally good?" It has also been argued that it implies that morality is arbitrary and based merely upon God's whim. It is also possible to question whether the revealed scriptures really state the will of God.

• Natural Rights Theory: the theory which holds that humans have absolute natural rights (in the sense of universal rights that are inherent in the nature of ethics, and not contingent on human actions or beliefs). The theory, espoused by Thomas Hobbes and John Locke among others, originates with the concept of natural justice or natural right of Socrates, Plato and Aristotle. The development of this tradition of natural justice into one of natural law is usually attributed to the Stoics. After the incorporation of the pagan concept of natural law into Christianity by St. Thomas Aquinas, it was Hugo Grotius (1583 - 1645), with his philosophy of international law, who finally freed it from dependence on theology, and allowed its development into what we now refer to as human rights.

• Contractarian Ethics (or the Moral Theory of Contractarianism) claims that moral norms derive their normative force from the idea of contract or mutual agreement. It holds that moral acts are those that we would all agree to if we were unbiased, and that moral rules themselves are a sort of a contract, and therefore only people who understand and agree to the terms of the contract are bound by it. The theory stems initially from political Contractarianism and the principle of social contract developed by Thomas Hobbes, Jean-Jacques Rousseau and John Locke, which essentially holds that people give up some rights to a government and/or other authority in order to receive, or jointly preserve, social order.
  Contractualism is a variation on Contractarianism, although based more on the Kantian ideas that ethics is an essentially interpersonal matter, and that right and wrong are a matter of whether we can justify the action to other people.

• Pluralistic Deontology is a description of the deontological ethics propounded by W.D. Ross (1877 - 1971). He argues that there are seven prima facie duties which need to be taken into consideration when deciding which duty should be acted upon:

  • Duty of beneficence (to help other people to increase their pleasure, improve their character, etc).
  • Duty of non-maleficence (to avoid harming other people).
  • Duty of justice (to ensure people get what they deserve).
  • Duty of self-improvement (to improve ourselves).
  • Duty of reparation (to recompense someone if you have acted wrongly towards them).
  • Duty of gratitude (to benefit people who have benefited us).
  • Duty of promise-keeping (to act according to explicit and implicit promises, including the implicit promise to tell the truth).
  In some circumstances, there may be clashes or conflicts between these duties and a decision must be made whereby one duty may "trump" another, although there are no hard and fast rules and no fixed order of significance.