We have already reminded ourselves that Kant's ethics is the key to his entire philosophy. How, then, does his Jurisprudence relate itself to the Ethics?
Political laws define duties, as do moral laws; and all duties are ultimately moral. Respect for the sacredness of personality is implied in all duties. In this sense the moral seems ready to shoot through all branches of the political, and to dominate the scene entirely. But there is an important difference in the nature of the motive implied in the two—a difference so important, indeed, that it results in constituting two fields of vastly different character—Ethics and Jurisprudence. Ethical motivation is internal. It is the expression of our conscious respect for duty, and ultimately of our reverence for the absolute value of rational personality. Like the quality of mercy, it may not be constrained. Political motivation, on the other hand, is external. It seizes upon any influence which is likely to be effective in inducing the people to carry out the required act, whatever be their inward state of mind. Any club is good enough to beat a dog, it is said; and likewise any motive that will give results is available to sustain a political law. Fear of death, fear of incarceration, fear of a money fine—these are characteristic political motives. They are external to the real merits of the act. Further, they all have this common quality, that they arise from the application of external force. In general, then, political duties are such as may be made effective by force, and the field of jurisprudence is the field of enforceable law; whereas the very essence of a moral act would be impaired if it were performed from any such external motive.
(E. L. Hinman, "Kant's Philosophy of Law," The Monist 35 [April 1925]: 280-95, at 282-3 [italics in original])
