Kant is well known for his view that within a state, even one that falls short of the ideal of justice that is required by morality itself, there can be no constitutional right to forcible resistance against perceived injustices, only the freedom of citizens to criticize the government and petition for redress and a corresponding duty of the government to reform itself in the direction of a more fully just republic.¹ Both together no doubt constitute an ongoing and never-ending process because of the imperfections of human beings, including those in positions of power. The ground for this position is his view that the purpose of a state is to make the rights and possessions of its citizens determinate and secure and that in order to accomplish the latter, there must be a monopoly of coercive power within an executive branch, even though in Kant’s view, sovereignty lies in the legislature, as the expression of the ultimate right of the people, and the executive is in principle only an agent of the legislature: a state that does not maintain a monopoly of coercion in its executive but allows another locus of potential coercion either within its legislature or in the people without parliamentary representation against all branches of government cannot accomplish the legitimate and morally necessary ends of the state and is thus not a genuine state at all – but we all stand under a duty to be part of a just state. In relations among sovereign² territorial states, Kant holds that there is no room for the coercive enforcement of rights by the analogue of an international executive both for the a priori reason that the existence of such a coercive force would undermine the monopoly on coercion by the executives within the several states and for the empirical reason that a world-wide government could not remain a republic but would inevitably degenerate into a world-wide despotism. Opposite grounds, same outcome: because of the impossibility rather than the necessity of international coercion, the only legitimate way for different states to address disputes over their rights is through means similar to the only permissible means for the redress of injustices within states – through the rights of nations to address one another peacefully concerning their desires and disputes and the duties of nations to seek resolutions for their disputes and progress toward greater international justice peaceably rather than through force. Neither intranationally nor internationally can justice ultimately be enforced through anything except good will, at least on the part of rulers within states and between states. Within states, what we might call the mechanism of juridical and penal institutions that is designed to ensure compliance with law through aversive incentives rather than sheer respect for the moral law must in the end be operated by individuals who themselves cannot be coerced into doing the right thing so must be motivated by morality itself, while at the international level, the institutions of conflict resolution do not have enforcement powers so can ultimately be enforced only by the good will of the sovereign states involved, however that is achieved.

The foundation of Kant’s theory of both national and international justice is of course the Universal Principle of Right. Although stated in the form of a definition, this principle expresses the imperative that each agent must be allowed as much scope for freedom of action as is compatible with an equal scope for freedom for any agent with whom the former might interact: “Any action is right if it can coexist with everyone’s freedom in accordance with a universal law, or if on its maxim the freedom of choice of each can coexist with everyone’s freedom in accordance with a universal law” (MS, RL Introduction, section C, 006:230).³ This principle might seem to be a strictly a priori principle, derived either from the universal law formulation of the categorical imperative or directly from the concept of freedom itself as the foundation of morality,⁴ but it presupposes the empirical assumption that the freedom of action by any human agent as the expression of her freedom of choice can limit the freedom of choice in her freedom of action by any other through the interaction of their bodies; such interactions can limit not merely the expression of an agent’s choice in external action but also constrain their ability even to set ends, since an agent cannot rationally set an end for herself for which she does not believe she has adequate means given the choices made by others, among other possible factual conditions.⁵ That human beings have bodies that can interfere with each other in this way is not something that is known a priori. Rather than consisting solely of a priori principles valid for all rational beings, as does Kant’s groundwork for morals, Kant’s metaphysics of morals, including the metaphysical foundations of right, explores the implications of the a priori principle of morality for rational beings in our embodied condition, something of which we have only empirical knowledge, just as Kant’s metaphysics of nature explores the implications of the foundational principles of theoretical philosophy for creatures who can only detect matter by motion.⁶ Other empirical facts on which Kant’s doctrine of right depends, or more precisely the more concrete facts that constitute the general fact that the freely chosen actions of human beings can limit the freedom of each other, are that human bodies need a place on the earth to occupy, that their subsistence depends on the use and consumption of objects other than their own bodies that can only be produced on a piece of the earth and that the ground that human beings have to occupy is finite, the surface of a sphere any part of which is in principle accessible from any other, with no natural division of this surface into determinate parts antecedently assigned to particular individuals. A further empirical assumption that Kant makes throughout his practical philosophy – it is already assumed in the transition from the fundamental principle of morality to the categorical imperative as the presentation of the moral law to human beings in the form of an imperative, that is, a constraint – is that human beings are not always naturally impelled to adhere faithfully to all the dictates of morality and can thus reasonably assume that neither they themselves nor their fellows will always conform to the requirements of justice out of respect for the moral law alone. All of these assumptions, it is safe to say, although contingent and only empirically known, are incontrovertible.