 The Kantian tradition puts enormous weight on the idea that morality must be principled and that the ultimate principle of morality must be one we can know a priori. According to Kant, the moral law as applied to imperfect agents who are subject to temptations, provides what he called a “categorical imperative”—an imperative whose rational authority is not dependent on the agent’s contingent ends. Kant provided several formulations of the categorical imperative. The so-called “universal law” formulation holds that one must always act so that one’s maxim could at the same time be willed as a universal law. The humanity formulation holds that one must always act so as to treat humanity, whether in one’s own person or that of another, always as an end and never merely as a means. The Kantian tradition emphasizes common sense moral ideas like respect and dignity, and provides a distinctive interpretation of the role of universalizability in moral thought. On some readings of Kant, the moral law must itself be constitutive of being a rational agent at all. This idea has, in turn, been enormously influential, especially in the late twentieth and early twenty-first century. 