You’d think de Jaca and de Moirans would be world famous, but the record of their struggle was lost for 300 years. Even today, a Google search on their names yields very little. From an article by Liliana Obregón:
During their lengthy captivity, de Jaca and de Moirans wrote in their defense that the institution of slavery violates natural law, divine law and the law of nations. By making reference to biblical texts and religious authorities, they carefully refute the arguments that justified slavery and conclude that it was a "manifest robbery of the negroes’ freedom." Basing their argument on Saint Thomas' doctrine of restitution, de Jaca and de Moirans demanded compensation as the only way of redeeming—in part—the “terrible sins” committed by all who had participated in the slave trade. De Jaca wrote "these negroes, and their ancestors, are free, not only as Christians, but also in their native land. And as such, … the obligation exists to restore their freedom, but also, in pursuit of justice, to pay them what they would have inherited …, what would have enriched them, the lost time, the labor and the damages that they have suffered… for their enslavement and personal service…".
Most significantly, de Moirans and de Jaca concluded that the tragedy of slavery was not based on an erroneous theological interpretation, nor on the innocent acceptance of a just cause, but rather on the intentional deafness and blindness of all of those who participated in some aspect of the trade, be they as vendors or recipients of slaves. The two Capuchins repeatedly noted that there were enough legitimate arguments against the trade in human beings for all to have rejected it. Aristotle's just cause theory— it was widely known— did not apply to the case of the enslaved Africans brought to the Indies. The only possible conclusion was that the participants in the slave trade were acting in bad faith or, at a minimum, that so much injustice had blinded those who could otherwise have denounced and impeded it. De Jaca affirms "If the professors, theologians, confessors, religious men had not been silent dogs in the Indies, then iniquity and injustice would not have developed so enormously and without remedy."
The rest of this fascinating article is here.