I quoted Saint Thomas' reply:
Reply to Objection 2. Considered absolutely, the fact that this particular man should be a slave rather than another man, is based, not on natural reason, but on some resultant utility, in that it is useful to this man to be ruled by a wiser man, and to the latter to be helped by the former, as the Philosopher states (Polit. i, 2). Wherefore slavery which belongs to the right of nations is natural in the second way, but not in the first.
Reply to Objection 2. Considered absolutely, the fact that this particular man should be a slave rather than another man, is based, not on natural reason, but on some resultant utility, in that it is useful to this man to be ruled by a wiser man, and to the latter to be helped by the former, as the Philosopher states (Polit. i, 2). Wherefore slavery which belongs to the right of nations is natural in the second way, but not in the first.
