Having defined the concept of de jure possession, Kant immediately makes what is for him, by this late stage in the unfolding of his system, an entirely predictable move. He asks for a deduction of the concept of purely de jure possession of an external object [what he calls, parenthetically, possessio noumenon). That is to say, he seeks to show that the concept finds legitimate employment, indeed must find legitimate employment, within the realm of experience.
Put as simply and clearly as I am able, Kant's argument for the possibility of de jure possession is this: Since I am a phenomenal being - since I am, in other words, a rational agent that manifests its agency in the realm of appearance - my will at least potentially requires the cooperation of nature for the fulfillment of its purposes, whatever they may turn out to be. Even if I adopt the extreme stoicism of an Epictetus, seeking only virtue and not the powers or pleasures of the world, nevertheless I shall find myself compelled to employ some portions of nature as means to my ends.
But the laws of nature are such that I can use a portion of nature as a means to my ends only by appropriating it, and thereby excluding others from a like appropriation of those same portions. In short, for the accomplishment of what I will, for the enactment of my maxims, I require property.
Were I an incorporeal being, not manifesting my agency in space and time, I might have neither the need for, nor indeed the possibility of, property. Imagine, for example, that I were merely a noumenal rational agent whose acts consisted in the contemplation of pure ideas or the endless elaboration of the relationships among abstract logical constructs. In that case, my appropriation of modus ponens or the law of the excluded middle would in no way exclude others, for the contemplation of a timeless truth of logic does not require that others be denied its use or enjoyment. But because we are phenomenal beings whose agency is manifested in space and time, my appropriation at least potentially excludes you.
Kant's argument thus far can be summarized in a series of conditionals, preceded by a declarative assertion that comes as close as he thinks possible to the flat claim that I am a rational agent. The argument looks like this:
1. Insofar as I act, I must assume, though I cannot know it, that I am a rational agent, which is to say, that I am free. [This is the conclusion of the Third Section of the Foundations.]
2. If I am a rational agent, then willing an end, I necessarily will the means. [This, Kant has persuasively argued in the Second Section of the Foundations, is analytic.]
3. If I will the means to the fulfillment of my ends, then, as a phenomenal being - one whose agency is manifested in the realm of appearance - I must legitimately appropriate, which is to say take de jure possession of - portions of the spatiotemporal realm as means.
4. If I must take de jure possession of some portion of the spatio-temporal realm as means, then it must be possible for me to do so. What is more, there must be, in principle, no portion of the natural world that it is not possible for me to possess, inasmuch as there is nothing in the nature of willing as such that places limits on what might, according to the laws of nature, serve as means to my ends.
From all of which it follows that de jure possession must be possible.
The question remains, however: How is de jure possession possible? What is required for such legitimate possession to be actual? Locke, of course, had begged this question in the Second Treatise of Government by simply asserting the right of property as a truth revealed by the natural light of reason. But Kant, correctly in my view, recognizes that this is a radically unsatisfactory grounding for the right of property. Instead, like both Hobbes and Rousseau before him, Kant grounds the right to property in a prior mutual agreement, or social contract, among all those who, in the pursuit of their ends, may come into conflict with one another in the appropriation of portions of nature.
Legitimate ownership involves the exclusion of others from the use and enjoyment of a portion of nature, an exclusion that may be instituted by force if necessary. Such a use of force, Kant argues, if in all consistency universalized, entails mutual constraints among all the members of a society - where society here can be understood quite simply as the totality of persons who, in the pursuit of their ends, are likely to interfere with one another. So Kant concludes that the possibility of property entails the existence of a social contract.
And now Kant concludes his argument in a strikingly bold and imaginative fashion. The central text is §8 of the First Chapter of the First Part of the Rechtslehre. Here it is in its entirety:
When I declare (by word or deed), "I will that an external thing shall be mine," I thereby declare it obligatory for everyone else to refrain from the object of my will. This is an obligation that no one would have apart from this juridical act of mine. Included in this claim, however, is an acknowledgment of being reciprocally bound to everyone else to a similar and equal restraint with respect to what is theirs. The obligation involved here comes from a universal rule of the external juridical relationship [that is, says the translator, the civil society]. Consequently, I am not bound to leave what is another's untouched if everyone else does not in turn guarantee to me with regard to what is mine that he will act in accordance with exactly the same principle. This guarantee does not require a special juridical act, but is already contained in the concept of being externally bound to a duty on account of the universality, and hence also the reciprocity, of an obligation coming from a universal rule.
Now, with respect to an external and contingent possession, a unilateral Will cannot serve as a coercive law for everyone, since that would be a violation of freedom in accordance with universal laws. Therefore, only a Will binding everyone else - that is, a collective, universal (common), and powerful Will - is the kind of Will that can provide the guarantee required. The condition of being subject to general external (that is, public) legislation that is backed by power is the civil society. Accordingly, a thing can be externally yours or mine only in a civil society.
[And now, Kant concludes with a dramatic flourish:]
Conclusion: If it must be de jure possible to have an external object as one's own, then the subject must also be allowed to compel everyone else with whom he comes into conflict over the question of whether such an object is his to enter, together with him, a society under a civil constitution.
The conclusion of the argument prior to this point, you will recall, was that de jure possession must be possible, or, as Kant puts it in the text before us, "it must be de jure possible to have an external object as one's own." Combining this with the conclusion of the argument just quoted, we can now conclude that it must be allowed to compel others to enter with one into a society under a civil constitution. Since this necessity is universal, it of course follows that others have an equal right to compel me to enter civil society.