We all know about the synthetic a priori and the analytic a priori. Let me introduce a new logical category: the desperate a priori. The desperate a priori arises when one wants to imagine that all moral rules and all law can be spun out from some logical axiom, such as "self-ownership." Inevitably, one will hit cases where one's chain of logical deductions doesn't get one where one wants to be. At that point, the "desperate a priori" enters the scene: it consists in the frantic effort to show that the result one wants, which is seemingly denied by the plain meaning of one's axioms, is actually necessitated by those very axioms!
Rothbard, for instance, wants to derive all law from the principle of self-ownership*. But when he comes to voluntary slavery, he is forced to pause. The principle of self-ownership would seem to require that we permit voluntary slavery: after all, an important component of ownership is alienability. If I really do own myself, I damn well ought to be able to sell myself! But Rothbard (quite rightly!) finds the idea of legal slavery unpalatable. So what does he do? Why, he simply declares voluntary slavery, an institution that existed for thousands of years, to be logically impossible!
"The concept of 'voluntary slavery' is indeed a contradictory one, for so long as a laborer remains totally subservient to his master's will voluntarily, he is not yet a slave since his submission is voluntary; whereas, if he later changed his mind and the master enforced his slavery by violence, the slavery would not then be voluntary." (And, in the same way, no one can voluntarily take a flight on a plane, since if, halfway across the Atlantic, you demand to be let off, the airline will force you to finish the trip.)
So there you go, problem solved: we don't need to deal with the legal status of voluntary slavery because it is impossible! Apply broom to problem, lift rug, problem solved!
If Rothbard had not been so anxious to sweep this problem out of sight, he might have noticed that, in the process of doing so, he had created a teeny difficulty for his "system." Rothbard writes, "The right of property implies the right to make contracts about that property: to give it away or to exchange titles of ownership for the property of another person." Therefore, if I owned my will I could sell it, and, since Rothbard says I can't, then I must not own it. Then, per Rothbard, the only alternatives are "(1) the 'communist' one of Universal and Equal Other-ownership, or (2) Partial Ownership of One Group by Another - a system of rule by one class over another." So it turns out that my will is either owned by everyone or by some ruling elite!
* The whole concept of "self-ownership" is of course nonsense. I saw a wonderful way of putting this on a bulletin board: "Calling [one's relationship to one's self] 'ownership' would be like letting ids file lawsuits against egos."