As Cohen concludes in Chapter 10, "the thesis of self-ownership [SO] cannot be refuted." (244) But he hopes to cast doubt on it and make it less attractive.
Three arguments for SO are analyzed. 1) First, that rejection of SO licenses slavery. Cohen objects that all duties, such as between parents and children, limit freedom. Of course, the duty to care for one's mother, etc. is merely a moral not legal duty enforceable by the state, and Cohen is fully aware of that. He replies that there may be an independent (though indeed non-contractual) political obligation of a citizen to, say, pay taxes. But doesn't that beg the question? Self-ownership precludes such obligations. Hence Cohen must prove that the latter exist, which is a non-trivial task that is not broached in this work. For example, Simmons and Huemer dedicate their entire books to this question and come out against the idea that citizens have political obligations to the government. It may be true that "the socialist constitution requires the state to tax redistributively," but the libertarian point is precisely that such a constitution is unjust for violating the thesis of SO.
Cohen mentions the idea that "the state simply cannot have the particular right [such as to tax] unless it has the comprehensive right over me that betokens slavery." There is much wisdom in this observation. For if the state has the right to tax us, then it is the state and only the state that determines the amount of the tax. Nothing other than public opinion (and perhaps the Laffer curve) prevents the legislators from imposing either a 1% or 99% income tax. The government then effectively owns everything that we produce and alone decides how much to take and how much to let us keep. It follows that individuals are almost fully enslaved, and the only freedoms they enjoy are due to the magnanimous decision of the rulers not to interfere too much. That the slave-master is at times less cruel and demanding than he could be does not take the sting out of being a slave. Or, as Mises thought, "But for the inefficiency of the law-givers and the laxity, carelessness, and corruption of many of the functionaries, the last vestiges of the market economy would have long since disappeared." (HA, 859) Pile on, brother.
Cohen adduces two more arguments in regard to slavery. First, that even many libertarians agree that taxation for the purpose of financing the police is justified. "It is impossible to argue that an hour's labor that ends up as part of somebody's welfare payment is like slavery, while an hour's labor that ends up as part of a policeman's salary is not, when focus is on the condition of the putative slave himself." (235) Now it's true that self-ownership has anarchistic implications as Rothbard amply demonstrates. The problem with natural-law anarchy is that it would only work in the state of pure and uncorrupt nature. But nature in fact fails at least occasionally. The inherent injustice of taxes for law enforcement and a few other essential government services is permitted to a small degree so that the heavens do not fall. Further, taxation for the sake of police, etc., constitutes the absolutely essential taxes which fall far short of Cohen's preferred egalitarian redistribution. His first argument can in fact be brought to bear in favor of SO: "Suppose that you are an innocent person and that I forcibly detain you in a room for five minutes. ... there is a massive normative difference between this brief detention and life-long imprisonment. Brief detention of an innocent person might be justified by, for example, temporary needs of social order, even if life-long imprisonment of an innocent person could never be justified." (231) Very good, then SO survives practically intact the imposition of a small tax by the local government to preserve law and order.
Second, that slavery is as much a problem for libertarianism as it is for statism, because it need not be unlibertarian to allow voluntary slave contracts. As I have suggested, slave contracts are both somewhat self-contradictory and senseless. Another point is that while you can alienate and therefore sell your labor, you cannot alienate your control over your body as such. These are reasons not to recognize slave contracts in a free society. If, however, some such contracts are not unjust, then Cohen's argument is fixed by modifying "slavery" with "coerced / non-contractual" (such as regarding its being morally intolerable).
2) The second argument deals with decreased autonomy under no-SO. By "autonomy," Cohen means "the range of a person choice, as opposed to a feature of a person's character, related to his powers of deliberation and self-control." (236) In response, Cohen invokes his pathetic propertyless proletarians who allegedly lack autonomy under capitalism. I've dealt with this claim earlier. But it's precisely the self-control that is lacking under no self-ownership. This becomes obvious when we consider the situation of a creative artist that Cohen uses. Mises makes the following point: to promote the arts "all that society can achieve... is to provide an environment which does not put insurmountable obstacles in the way of the genius and makes the common man free enough from material concerns to become interested in things other than mere breadwinning." (HA, 155) The plight of a creative man under socialism is twofold. First, socialist citizens are required to worship the totalitarian state. A creative artist becomes a popular rival to the state for the people's affections. This cannot be and usually is not tolerated. In addition, "under a bureaucratic system it is necessary to convince those at the top, as a rule old men accustomed to do things in prescribed ways, and no longer open to new ideas. No progress and no reforms can be expected in a state of affairs where the first step is to obtain the consent of the old men. The pioneers of new methods are considered rebels and are treated as such. For a bureaucratic mind law abidance, i.e., clinging to the customary and antiquated, is the first of all virtues." (Bureaucracy, 67)
Self-ownership then permits creative advance by freeing artists and innovators from the necessity of seeking permission from the authorities to contribute to society.
3) The third argument suggests that absence of self-ownership entails using people in an un-Kantian manner as means rather than ends. Cohen replies that even in the free market people "use" each other quite legitimately: "Of course I treat the ticket-seller as a means when I hand him the money and thereby get him to hand me my ticket. For I interact with him only because he is my means of getting a ticket." (239)
But this is beside the point which is rather that Cohen cannot coercively conscript any Smith into causes for which Smith himself does not care, including helping the disabled or whatever. Cohen would then be using Smith to further ends to which Smith is opposed or at least indifferent to, and Cohen is commandeering Smith's property against his will and imposing a pure cost on him without conferring any corresponding benefit.
Treating one as an end means recognizing that each person has his own ends for which he cares; and that society is supposed to benefit all its members, such that in fact for each individual, "society is the great means for the attainment of [his] earthly ends." (HA, 179) I should naturally rejoice that other human beings exist. But unjust violence tempts me to want to leave society, because unlike social cooperation, state coercion makes me worse off. Smith would prefer it that Cohen and his fellow looters drop dead.
Redistributive taxation ignores the victim's own values and goals and projects, treating him only as a tool to be used by the redistributor for the latter's selfish ends.