5 The system flourished even among the most civilized peoples, among the Greeks and among the Romans, with whom the few imposed their will upon the many; and this power was exercised so unjustly and with such haughtiness that a crowd of slaves was regarded merely as so many chattels-not as persons, but as things. They were held to be outside the sphere of law, and without even the claim to retain and enjoy life. "Slaves are in the power of their masters, and this power is derived from the law of nations; for we find that among all nations masters have the power of life and death over their slaves, and whatever a slave earns belongs to his master."(6) Owing to this state of moral confusion it became lawful for men to sell their slaves, to give them in exchange, to dispose of them by will, to beat them, to kill them, to abuse them by forcing them to serve for the gratification of evil passions and cruel superstitions; these things could be done, legally, with impunity, and in the light of heaven. Even those who were wisest in the pagan world, illustrious philosophers and learned jurisconsults, outraging the common feeling of mankind, succeeded in persuading themselves and others that slavery was simply a necessary condition of nature. Nor did they hesitate to assert that the slave class was very inferior to the freemen both in intelligence and perfection of bodily development, and therefore that slaves, as things wanting in reason and sense, ought in all things to be the instruments of the will, however rash and unworthy, of their masters. Such inhuman and wicked doctrines are to be specially detested; for, when once they are accepted, there is no form of oppression so wicked but that it will defend itself beneath some color of legality and justice. History is full of examples showing what a seedbed of crime, what a pest and calamity, this system has been for states. Hatreds are excited in the breasts of the slaves, and the masters are kept in a state of suspicion and perpetual dread; the slaves prepare to avenge themselves with the torches of the incendiary, and the masters continue the task of oppression with greater cruelty. States are disturbed alternately by the number of the slaves and by the violence of the masters, and so are easily overthrown; hence, in a word, come riots and seditions, pillage and fire.
Source: In Plurimis (Vatican.va)