Shortly after deriving the third of the three propositions, from the two preceding, Kant proceeded to derive the “supreme principle of duty,” known also as the “categorical imperative.” But a number of commentators have been convinced that there is a gap in his reasoning. In the quotation above (Kant 1997, 15/4:402), he inserts a “that is,” which suggests that the statements preceding and following would be logically equivalent. But it has been hard to see how they are.

Structure of the derivation. The extended discussions of the good will and acting with moral worth have led to this conclusion: that someone acting from duty would not be guided by the prospect of bringing about some object or condition to which she is attracted by inclination; she would be guided in acting by respect for a principle. Kant now ingeniously takes this conclusion as signifying the content of that guiding principle—what it says—and so a formula by which it can be determined that the action is a duty. He says, roughly, that since the will of someone acting from respect for duty is deprived of any object she might aim at bringing about, nothing remains to serve as the content of the principle guiding her except the idea of conformity to universal law. Then he says by way of clarification, “that is,” and the familiar formula of the principle of duty follows: I ought never to act except in such a way that I could also will that my maxim should become a universal law.

The alleged gap. But how do we get from the requirement that actions conform to a law that is universally valid to the principle of duty? That is the question that has bothered a number of recent commentators on this derivation. Aune claims that knowing whether actions conform to universally valid laws is not the same as knowing whether their maxims can be willed to become universal law. So these ideas cannot be logically equivalent. He cites W.D. Ross as an example of a moralist who would agree with Kant that actions ought to conform to principles that are universally valid. But Ross did not agree with the Kantian principle of duty; and his view does not seem contradictory (Aune 1979, 28-34, see also Ross 1954, 45). Wood charges that it is fine to tell us that our actions must conform to universally valid laws, except that Kant failed to tell us what universally valid laws their are. So we have no way of knowing whether his principle of duty is one, or if it is somehow supposed to summarize them all (Wood 1999, 78-81).

Conformity to universal law in nature. If Kant’s idea of “conformity to universal law” is not properly understood, then his argument deriving the principle of duty will not be understood either. The mistake made by those critical of the derivation is to assume that “conformity to universal law” means obedience to some particular law(s) that everyone ought to obey. But that is not what Kant meant. He meant something more like conformity to a “rule of law,” or conformity to the constraints of a legal system. This can be inferred from a passage in his Prolegomena to any Future Metaphysics, published two years prior to the Groundwork. His concern there was about the conformity of objects in the world to “nature” as a system of laws. He posed the following question:

How is it possible to cognize a priori the necessary conformity to law of things as objects of experience, or: How is it possible in general to cognize a priori the necessary conformity to law of experience itself with regard to all of its objects?  (Kant 2002, 91/4:296)

Here he is applying the concept of conformity to universal law to objects of nature. He is asking, essentially, what gives us confidence a priori that nature is a law-governed system? Science does not discover that nature is a law-goverened system; rather, science presupposes this. Scientists draw conclusions about what causes what based on the idea that nature in general is a coherent system of laws. He clarifies the point as follows:

There are many laws of nature that we can know only through experience, but lawfulness in the connection of appearances, i.e., nature in general, we cannot come to know through any experience, because experience itself has need of such laws, which lie a priori at the basis of its possibility.  (Kant 2002, 111/4:318-19)

The general answer Kant gives to the questions he asked above invokes the idea of the unity of consciousness. His answer is that because the system of laws of human experience is subjective, although universal, the condition of the systematic unity of nature is “one consciousness”:

we are not acquainted with nature except as the sum total of appearances . . . and so we cannot get the laws of [the connection of appearances] from anywhere else except the principles of their connection in us, i.e., from the conditions of necessary unification in one consciousness....  (Kant 2002, 111/4:319)

Conformity to universal law in action. Notice that in the second of the passages just quoted Kant directs attention from particular laws of nature to nature’s systematic regularity, or lawfulness: “lawfulness in the connection of appearances.” By the idea of conformity to universal law in the derivation of the principle of duty Kant does not refer to conformity to particular laws that are universally valid, as a number of critics have supposed. He refers instead to systematic lawfulness in human action. He refers to the coherence of an action with a system of laws regulating human conduct in general. If the action’s maxim, as a lawlike principle, coheres with the formal requirements of such a system, then it is consistent with duty; otherwise, the action is wrong, morally forbidden (see McCarty 2010). That is how the Kantian principle of duty follows from the idea of actions’ conformity to universal law. In the second section of the Groundwork we find confirmation of this point when Kant himself draws the analogy between conformity to universal law in nature and in human action:

Since the universality of law in accordance with which effects take place constitutes what is properly called nature in the most general sense (as regards its form) – that is, the existence of things insofar as it is determined in accordance with universal laws – 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.  (Kant 1997, 31/4:421)

This takes us next to the controversial concept of an action’s maxim: the form of the action by which its conformity to universal law can be determined.