The Great Catechism

What, then, under these circumstances is justice? It is the not exercising any arbitrary sway over him who has us in his power, nor, by tearing us away by a violent exercise of force from his hold, thus leaving some colour for a just complaint to him who enslaved man through sensual pleasure. For as they who have bartered away their freedom for money are the slaves of those who have purchased them (for they have constituted themselves their own sellers, and it is not allowable either for themselves or any one else in their behalf to call freedom to their aid, not even though those who have thus reduced themselves to this sad state are of noble birth; and, if any one out of regard for the person who has so sold himself should use violence against him who has bought him, he will clearly be acting unjustly in thus arbitrarily rescuing one who has been legally purchased as a slave, whereas, if he wishes to pay a price to get such a one away, there is no law to prevent that), on the same principle, now that we had voluntarily bartered away our freedom, it was requisite that no arbitrary method of recovery, but the one consonant with justice should be devised by Him Who in His goodness had undertaken our rescue. Now this method is in a measure this; to make over to the master of the slave whatever ransom he may agree to accept for the person in his possession.