The Practical Law

In the Critique of Practical Reason, Kant argues that there is one and only one maxim of action suitable to ground morality. This maxim is referred to in his Groundwork for a Metaphysics of Morals as the "categorical imperative," and is best known by that name, although in the Critique of Practical Reason he prefers to refer to it as the Fundamental Law of Pure Practical Reason. The law is that one should "so act that the maxim of your will could always hold at the same time as a principle in a giving of universal law."

Much of the Analytic section of the Critique of Practical Reason is devoted to showing that the categorical imperative is the only possible moral law. It is argued that the law-giving force of the moral law must stem from its mere form—that is, its universalizability—alone, because if it stemmed from the content, the law could only hold for those who cared about that content and not universally.

Popular pages: Critique of Practical Reason