Deontology is the ethical tradition that judges the morality of actions based on their adherence to rules, duties, or obligations rather than their consequences. Its most prominent philosopher is Immanuel Kant, whose Groundwork of the Metaphysics of Morals (1785) established the foundations of modern duty-based ethics. Kant's project was to discover a supreme moral principle that could be binding on all rational agents regardless of their personal desires or circumstances. This led him to distinguish between hypothetical imperatives, which are conditional commands of the form "if you want X, do Y," and categorical imperatives, which are unconditional demands that command action regardless of any personal inclination.
The categorical imperative is Kant's central moral principle, expressed through several formulations. The first, the Formula of Universal Law, instructs agents to act only according to maxims that they could will to become universal laws without contradiction. The second, the Formula of Humanity, requires that we treat humanity, whether in our own person or in that of another, always as an end and never merely as a means. The third, the Formula of the Kingdom of Ends, calls on us to act as if we were legislating members of a moral community in which all rational beings are simultaneously lawmakers and subjects of the law. Together, these formulations emphasize the dignity of rational agency and the universalizability of moral reasoning.
Central to Kant's framework is the concept of the good will, which he regards as the only thing that is good without qualification. A good will is one that acts from duty, that is, from respect for the moral law itself rather than from inclination, self-interest, or fear of consequences. Kant draws a careful distinction between acting from duty and acting merely in accordance with duty: a shopkeeper who charges fair prices only to avoid lost customers acts in accordance with duty but lacks moral worth, whereas one who charges fair prices because doing so is right acts from duty and possesses genuine moral worth. For Kant, duty (Pflicht in German) represents the necessity of acting from reverence for the moral law, and only actions performed from such reverence have true moral value. This duty-centered approach has shaped not only ethical theory but also discussions of rights, justice, and political legitimacy.