Self-ownership, or the property of your own person, is a metaphor for the right to exclusive control of your own body and life. It captures some important intuitions: for example, that you should have a right to end your life as they see fit, that no one should be enslaved and that you generally have a right to decide what to do with your own life. As such it supports the idea of personal autonomy. For some, it also supports the right to abortion and it invalidates taxation.
Others even believe that self-ownership implies a right to sell your own body and life, just as you have a right to sell your other property. If that’s the case, then you have a right to sell yourself into slavery.
However, if self-ownership is understood as merely a metaphor for autonomy then there can’t be a right to sell yourself into slavery. Autonomy, or any other value for that matter, can’t be made to include the seeds of its own destruction. In other words, autonomy can’t include the right to autonomously abdicate your autonomy. Take this quote from Mill:
The ground for thus limiting his power of voluntarily disposing of his own lot is apparent, and is very clearly seen in this extreme case. … [B]y selling himself for a slave, he abdicates his liberty; he foregoes any future use of it beyond that single act. He, therefore, defeats in his own case, the very purpose which is the justification of allowing him to dispose of himself. (source)
If you insist that values or rights should be made to include their own negation, you’ll end up in Absurdistan. Democracies, for example, should then include the possibility to vote democracy away. Freedom should include the freedom to create totalitarian government. Tolerance should include tolerance of intolerance and of the forces intent on destroying tolerance. I don’t think we want to go there.
So, autonomy must include certain limits if it’s not to collapse under its own weight. This means that it’s legitimate to deny the moral value of – and perhaps even to forbid – autonomous actions that forfeit autonomy. Just like democracy is limited and suppresses anti-democratic movements and votes, and just like tolerance is limited and excludes tolerance of intolerance.
More on self-ownership here.