Kant's step into metaphysics

What interest could rational agents have in acting lawfully if not the order, stability, and other collective goods that law brings to society? Why should it otherwise matter to them that their actions are lawful? It would matter to them, of course, if acting unlawfully made them liable to punishment. But in that case their interest in acting lawfully would not come from seeing it as a good thing. It would come, rather, from seeing it as the surest way to avoid a bad thing, something they have an interest in escaping. Yet the challenge to an ethics like Kant's that represents lawfulness as the essence of moral action is to explain what could interest rational agents in acting lawfully regardless of how the law is enforced, regardless, that is, of whether it is enforced by threats of punishment or incentives to obey. The question then that confronts a defender of Kant's ethics is why a rational agent should regard an action's being lawful as a condition of its being reasonable to do. If he cannot give an answer to this question, the charge of excessive formalism will stick.

Kant himself was fully aware of the importance of this question. He understood that a person must realize some value through acting lawfully, else making lawfulness a condition of the reasonability of an action would be pointless. It would have no rational basis.