The first formula is a completely formal test, requiring that maxims can be willed as universal laws, that is, as laws for all rational beings. By contrast, the second is not purely formal, for it introduces the notion of “humanity”: we are required to respect the freedom and ability of each person to make his or her own decision. Probably the best-known version of the second formula is this: “Act so that you treat humanity, whether in your own person or in the person of any other, always as an end and never as a means only” (429; see 433).

In Chapter 1, we discussed Kant's protest against tyranny, in which the ruler uses his subjects merely as instruments to carry out his will. By contrast, Kant offered a political theory of the people, based on the moral Universal Principle of Justice: everyone has a fundamental dignity simply as a person, and no one has the moral right to interfere with the lawful freedom of others or to use them merely for his or her own purposes. This is just what the second formula enjoins.

Kant's stress in the second formula on the intrinsic equal value of each person does, in fact, enunciate a fundamental moral, political, and religious principle presupposed in the ordinary moral judgments of nearly everyone today. It has a majesty that can so fire the moral sensibilities of his readers as to make the second formula seem both obviously right and the most appealing version of the Categorical Imperative.