The argument that self-ownership entails voluntary slavery is as follows. If individuals possess the right to exclusively control the use of their person, then they also possess the right to voluntarily transfer the right of exclusive control to another person. To transfer this right would be to transfer one’s ownership of one’s self. To be owned by another is to be a slave. Therefore self-ownership entails voluntary slavery.
In chapter 19 of ‘The Ethics of Liberty’, as part of a discussion of legitimate contracts, Rothbard gives the following argument against the possibility of voluntary slavery:
“Suppose that Smith makes the following agreement with the Jones Corporation: Smith, for the rest of his life, will obey all orders, under whatever conditions, that the Jones Corporation wishes to lay down. Now, in libertarian theory there is nothing to prevent Smith from making this agreement, and from serving the Jones Corporation and from obeying the latter’s orders indefinitely. The problem comes when, at some later date, Smith changes his mind and decides to leave. Shall he be held to his former voluntary promise? Our contention—and one that is fortunately upheld under present law—is that Smith’s promise was not a valid (i.e., not an enforceable) contract. There is no transfer of title in Smith’s agreement, because Smith’s control over his own body and will are inalienable. Since that control cannot be alienated, the agreement was not a valid contract, and therefore should not be enforceable. Smith’s agreement was a mere promise, which it might be held he is morally obligated to keep, but which should not be legally obligatory.”
In short, voluntary slavery cannot occur for two reasons. Firstly, an agent cannot alienate their free will and therefore their control over their own body to another agent. Secondly, given the first reason, another agent cannot control another agent’s free will and since control is a requirement for ownership, one agent cannot own another agent as property. Since the transfer of self-ownership cannot occur, voluntary slavery is not a legitimate contract but instead a promise to perform certain actions, and therefore ought not to be enforced by law.
The problem with this argument is that the debate over voluntary slavery is concerned with the transfer of the moral right to control permissible use, that is, the moral right to control which actions are or are not performed. Voluntary slavery is not concerned with the transfer of the psychological capacity to directly control a human body. Clearly one cannot transfer one’s free will to another and one person cannot own another person’s free will. However, these two truths in no way demonstrate that one individual cannot possess the moral right to exclusively control the body of another. In Rothbard’s example the Jones corporation does not own or control Smith’s psychological capacity to control his body, rather they possess the moral right to determine which actions Smith performs and Smith does not possess the moral right to determine which actions that he himself performs. Rothbard’s argument therefore misses the point and so fails.