From Murray Rothbard's Ethics of Liberty, "Property Rights and the Theory of Contracts":
Let us pursue more deeply our argument that mere promises or expectations should not be enforceable. The basic reason is that the only valid transfer of title of ownership in the free society is the case where the property is, in fact and in the nature of man, alienable by man. All physical property owned by a person is alienable, i.e., in natural fact it can be given or transferred to the ownership and control of another party. I can give away or sell to another person my shoes, my house, my car, my money, etc. But there are certain vital things which, in natural fact and in the nature of man, are inalienable, i.e., they cannot in fact be alienated, even voluntarily.
Specifically, a person cannot alienate his will, more particularly his control over his own mind and body. Each man has control over his own mind and body. Each man has control over his own will and person, and he is, if you wish, “stuck” with that inherent and inalienable ownership. Since his will and control over his own person are inalienable, then so also are his rights to control that person and will. That is the ground for the famous position of the Declaration of Independence that man’s natural rights are inalienable; that is, they cannot be surrendered, even if the person wishes to do so.
Or, as Williamson Evers points out, the philosophical defenses of human rights "are founded upon the natural fact that each human is the proprietor of his own will. To take rights like those of property and contractual freedom that are based on a foundation of the absolute self-ownership of the will and then to use those derived rights to destroy their own foundation is philosophically invalid".
Hence, the unenforceability, in libertarian theory, of voluntary slave contracts.Read on.