The form of imperative contrasting with hypothetical imperatives is the categorical imperative. This type of imperative purports to be true, or as is sometimes said, valid, apart from any presupposed end or purpose to be achieved by acting as it commands. This is what the statement quoted above implies (Kant 1977, 25/4:414). Hence, if a categorical is valid, then it is valid universally, for everyone, regardless of his or her private ends. To Kant’s way of thinking, moral requirements can be expressed only in categorical imperatives. We ought to obey moral requirements regardless of our personal projects, or of what may be most convenient for us.

The formula of universal law. Kant began the first section of the Groundwork with the concept of “good will,” and by analysis derived the supreme principle of duty. That is, reasoning from the concept of good will he arrived at the principle showing how to tell whether actions are morally right or wrong. In Groundwork II he did something similar, starting this time with the concept of a “categorical imperative.” The principle he arrived at here, known as “The Formula of Universal Law,” is the same as in the previous derivation: Act only in accordance with that maxim through which you can at the same time will that it become a universal law (Kant 1997, 31/4:21). Most of the time, in discussions of Kant’s ethics, the categorical imperative is understood as the formula of universal law. It is worth bearing in mind, however, that in Groundwork II Kant derived a few other principles, besides the formula of universal law; and he thought of these also as versions or expressions of the categorical imperative. He thought there is only one categorical imperative, in fact. So technically, the formula of universal law is not the same as the categorical imperative, even if it is the most widely discussed version of that imperative, and even if it is what most people think of as the categorical imperative.

Structure of the derivation. An imperative is a command or similar statement expressing that an action is necessary. An imperative can also command that it is necessary to omit an action. A categorical imperative would be one expressing the necessity of doing (omitting) an action regardless of any ends or purposes to be achieved. From these concepts Kant reasons that if there is a true categorical imperative, then all it can command is, as he had said earlier, in Groundwork I, “conformity to universal law.” The structure of the derivation of the categorical imperative in Groundwork II is quite similar to the derivation presented in Groundwork I. It has already been explained that by the idea of conformity to universal law he did not mean conformity to any particular laws that apply for everyone, universally. He meant rather that the only thing that can make an action or omission categorically necessary is its conformity to the whole (universal) system of possible actions. That is, if the action’s maxim could be a law in such a system—if everyone could act that way—then it conforms to universal law and is morally permissible. If not, then the action would be prohibited by the categorical imperative. It was seen earlier how the principle of duty, the formula of universal law, would reject the maxim of making a lying promise to borrow money. The categorical imperative thus makes it necessary not to act that way—regardless of any ends or purposes to be gained by doing so.

Extension to the law of nature. Immediately after stating the formula of a categorical imperative in this way, Kant offered a clarifying extension of the idea:

the universal imperative of duty can also go as follows: act as if the maxim of your action were to become by your will a universal law of nature.

The first of three points worth making about this extension is that it makes it easier to see how Kant thought of both morality and nature, respectively, as coherent systems of laws. As explained earlier in discussing his idea of conformity to universal law, natural objects, and the laws by which they operate, must be conceived as conforming to nature as a coherent system of laws—otherwise, they would be impossible. Something similar is true for actions: their maxims must conform to a coherent system of laws—known as “laws of freedom”—in order for them to be “morally possible.” The second point is that the extension of the formula of universal law presents that formula as a family of formulae, a family of two, to be precise. We shall see a similar extension later, for another version of the categorical imperative, known as the “formula of autonomy.” The third point, finally, is that Kant’s invoking the analogy between the system of laws to which human actions ought to conform and the system of laws to which natural objects must conform prepares his readers to think analogically when applying the categorical imperative. In the several paragraphs following the law-of-nature extension of the formula of universal law he will present a set of examples, some of which make more or less explicit reference to nature.

Synthetic a priori. Kant claims that the categorical imperative is a “synthetic a priori proposition.” This means in part that we can know the categorical imperative—that one ought to act only on maxims we can will to become universal laws—independently of experience or any sensory observations. That is the a priori part. It means also that the statement is not true by definition. Just by knowing the definition of “ought” it is not possible to know how one ought to act. That is the synthetic part. Knowledge of the categorical imperative’s truth or validity therefore presents a special kind of problem. Usually when we know something synthetic, something that is not just a matter of definition, we know it by experience. But the categorical imperative is supposed to be known a priori, so experience is not helpful here. In Kant’s theoretical philosophy he dealt extensively with the problem of explaining the synthetic a priori knowledge of nature we have, and that was a very difficult problem to solve. He will try later, in the deduction in Groundwork III, to explain our synthetic a priori knowledge of the categorical imperative’s validity. He will say there that the concept of freedom of the will is the key to this explanation. But for the balance of Section II Kant will continue elaborating the categorical imperative, with a succession of different formulae, and sets of examples, until he arrives at a concept he thinks is crucial for making the transition from the categorical imperative to freedom of the will. This concept is “autonomy.”

Empty formalism. One of the traditional complaints about the categorical imperative is that is a purely formal principle. This means partly that it has nothing to do, exactly, with empirical (material) conditions in the world, such as consequences of obeying the principle. But it also means that it is indifferent between one social structure and another. For example, the categorical imperative can tell us why stealing is wrong—because there would not be any property to steal if everyone were expected to act that way. But it cannot tell us whether private property is right or wrong. The charge that the categorical imperative is an empty formalism is something to the effect that it is really only a kind of logical test whose results depend on the “matter” or content tested. So it is really the content itself that determines rightness and wrongness, not the categorical imperative. This line of criticism has been closely associated with German philosopher G.F.W. Hegel (see Sedgwick 1996; Wood 1989). Most Kant scholars are convinced, however, that Hegel has not understood the categorical imperative very well. Probably the best line of response to the above criticism is to say the following: that the categorical imperative can tell us how a system of private property can be structured so that it is right; that it is up to those who will participate in the system to decide if they will recognize private property; in which case they ought to obey the categorical imperative and maintain their system of property so that it is right; and if they do, then their taking something that doesn’t belong to them is wrong; otherwise their taking the same thing need not be wrong.