When it is permissible to take men captive, according to moral justice

In those places where custom sanctions the captivity and slavery of men, this ought to be limited primarily, if we have regard to moral justice, in the same way as in the case of property; with the result that, in fact, such acquisition may be permitted so far as the amount of either an original or derivative debt allows, unless perhaps on the part of the [captured] men themselves there is some special crime which equity would suffer to be punished with loss of liberty. To this degree, then, and no further, he who wages a lawful war has a right over the captured subjects of the enemy, and this right he may legitimately transfer to others.

Furthermore in this case also, it will be the task of equity and goodness to employ those distinctions which were noted above, when we discussed the question of killing enemies.

Treatment of slaves according to the moral power of justice

Now in the first place, it must here be noted that that right, which originates in a kind of surety on behalf of the state, can nowhere extend so widely as the right which arises [when, as a result of a crime, the criminal becomes a slave] as a penalty. Hence a certain Spartan said that he was a prisoner, not a slave; for, if we regard the question properly, this general right against prisoners captured in a lawful war is equivalent to that right which masters have over those who, under constraint of poverty, have sold themselves into slavery; only the misfortune is even more to be pitied of those who have met this fate not by their own particular act, but through fault of their rulers. … This servitude, then, is a perpetual obligation of services for maintenance that is likewise perpetual.