On Liberty by John Stuart Mill Chapter 5 Page 29

least endurable, to him, and his good is on the whole best provided for by allowing him to take his own means of pursuing it. But by 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. He is no longer free; but is thenceforth in a position which has no longer the presumption in its favor, that would be afforded by his voluntarily remaining in it. The principle of freedom cannot require that he should be free not to be free. It is not freedom, to be allowed to alienate his freedom. These reasons, the force of which is so conspicuous in this peculiar case, are evidently of far wider application; yet a limit is everywhere set to them by the