In 1785, Immanuel Kant published the Groundwork of the Metaphysics of Morals — a slim book that proposed to derive the foundations of ethics from pure practical reason alone, with no reference to consequences, custom, religious authority, or human nature. The result was the categorical imperative: a moral principle that holds unconditionally for any rational being. 'Act only on that maxim through which you can at the same time will that it should become a universal law.' If your rule of action could not be coherently universalized — if a world in which everyone followed it would be self-defeating or impossible — the action is prohibited. Lying, breaking promises, treating people as mere means: all forbidden by the imperative.
The categorical imperative has several formulations Kant treats as equivalent. The universal-law formulation (above) tests rules by whether they can be universalized. The humanity formulation says: 'Act so as to treat humanity, whether in your own person or in that of another, always as an end and never as a means only.' The kingdom of ends formulation imagines a community of rational beings each legislating universal laws for themselves and one another. The framework is deontological — actions are right or wrong by virtue of their form, not their consequences — which puts Kant in permanent argument with consequentialists (utilitarians, who hold that morality is about maximizing aggregate welfare) and virtue ethicists (who emphasize character rather than rules). The strengths of Kantian ethics are its rigour, its dignity-based grounding (every rational being has intrinsic worth), and its resistance to majoritarian abuse (you cannot torture an innocent for the public benefit on Kantian grounds). The weaknesses are also visible: the prohibition on lying to a murderer asking after your hidden friend has struck most readers as monstrous; the abstraction from consequences sometimes seems to license disasters; the universalization test can be gamed by reformulating maxims at convenient levels of generality.
The human dignity formulation has become the language of international human rights, bioethics, and AI alignment — most ethical arguments against treating people as mere data sources or training-corpus material run on Kantian grounds. The trolley-problem literature (a thought experiment about whether to divert a runaway tram from killing five people to killing one) is essentially a sustained collision between Kantian and utilitarian intuitions. Effective altruism has reopened consequentialist arguments at scale; the philosophical reaction has often been a renewed defense of Kantian dignity-based ethics. Whether ethics can be purely formal (Kant) or must reference outcomes (utilitarianism) is one of the oldest live arguments in philosophy.