We can now see that in the text of the Rechtslehre, Kant finally provides what was missing from the argument of the Foundations, namely a demonstration that such obligations as faithful promising are not hypothetically, but are rather categorically, imperative.
Kant's argument is open to criticism at every stage, of course, though it is nonetheless, in my estimation, extremely interesting. Before sketching some of those criticisms, let me call attention once again to an implication of the argument that goes directly counter to the most common impression concerning Kant's ethical theory. Kant is generally viewed as a rigorist, an objectivist, a universalist, a theorist who claims to be able to demonstrate a priori the universal validity of very powerful moral principles that are binding on all agents regardless of their nature or circumstances. And this impression is quite correct. But readers of Kant almost always draw the natural conclusion that he believes these moral principles to be binding on us even in the absence of a legitimately established state. In the familiar language of liberal political theory, Kant is seen as siding with Locke on the question whether the moral law is binding on agents in a state of nature. But it should now be clear that Kant's argument actually entails the opposite conclusion. The procedural obligation to enter into a social contract is certainly binding upon agents in a state of nature, but the moral principles enacted into law by the Legislature of a state thus established are binding only after, and on the condition of, the establishment of the legitimate state.
Kant clearly does not intend this consequence of his moral theory. Quite to the contrary. But his failure to provide a satisfactory theory of obligatory ends forces anyone wishing to embrace his moral theory to rely upon the legislation of the legitimate state as the only source of morally binding substantive principles of practical reason.
And this leads us ineluctably to the question sketched above, whether a legitimately established state, based upon a unanimous social contract and committed to embodying the fundamental principle of justice in its laws, is thereby constrained to a single structure of justice that thus constitutes the objective, universal, unconditionally binding system of principles of practical reason? This, I take it, is essentially the question originally posed by John Rawls in the earlier versions of his theory, before he drained it of its force and interest by an endless series of ad hoc adjustments, concessions, baroque elaborations, and qualifications.
If the answer to the question is yes - if, let us say, a legitimately established republic of rational agents in search of principles compatible with the fundamental postulate of justice must necessarily come upon and agree to Rawls' Two Principles of Justice - then by combining Kant's argument and Rawls', we would have a very powerful defense indeed of a universal system of moral principles. The argument, in a nutshell, would go like this:
1. Rationality as such entails consistent willing.
2. Consistent willing, for a phenomenally appearing agent, entails the possibility of property.
3. The possibility of property entails the necessity of establishing a state through a social contract.
4. A legitimate state composed of rational agents will necessarily enact one and only one set of fundamental principles of justice, namely the Two Principles of Justice.
5. Therefore, we are all, as phenomenally appearing rational agents, obligated universally and unconditionally to form legitimately grounded political communities with those with whom we come into contact, and in those communities to enact the Two Principles of Justice as the fundamental laws governing our interactions.
There are three stages in this argument: the derivation of the requirement of consistent willing from an analysis of what it is to be an agent; the deduction of the necessity of entering a social contract from an analysis of the preconditions of property; and the demonstration that reason dictates one and only one system of principles as the content of a legitimate state.
My own view is that the first stage in the argument, which as I have indicated we find in the pivotal portion of the Second Section of the Foundations, is essentially sound. To be an agent is to be moved by reasons, and the logic of reasons requires consistency of willing. What counts as a good reason for me necessarily counts as a good reason for any agent in relevantly similar circumstances.
The third stage of the argument, I am convinced, is mistaken. Neither Rawls nor anyone else has, to my knowledge, made a convincing case for the claim that free and equal rational agents must, insofar as they are rational, coordinate on a single system of substantive principles regulating their interactions with one another.
It is the second stage that I find especially interesting, in part, I confess, because I have managed to make it clear to myself only recently. Can the material circumstances of human existence - our spatio-temporality, our dependence for the pursuit of our ends on inanimate nature - ground an argument that we have a standing procedural obligation to attempt rational community with those agents with whom we interact? Can we, as Kant claims, compel such rational community?
I think we can conclude immediately that we cannot compel others to enter with us in a social contract, for the essence of such an agreement is that it represents mutual willing, and that implies that it is voluntary. But am I required, simply by the constraints of rational consistency, to seek such community with others, or is it consistent with my status as an agent to adopt nothing more than an instrumental stance vis-a-vis those with whom I interact? Is this, after all, the real content of the injunction to treat humanity as an end and not simply as a means?
I honestly don't know. My pre-philosophical inclination is to believe that the answer is yes, that there is something about construing myself as a rational agent that requires me, in all consistency, to attempt to achieve rational community with others whom I consider to be agents as well - though I do not consider myself bound, should they reject that attempt, to treat them as I would have agreed to treat and be treated, had we actually achieved rational community. But at this point, I do not see an argument that can make that conclusion plausible.