José de Acosta, De procuranda Indorum salute (Salamanca, 1588)

A Jesuit who had spent his working life in Peru printed a six-book manual telling the ministers and officers of the Indies what they might and might not do. Its most quoted page grades the peoples of the world into three classes of barbarians. Its least quoted pages rule that the Indians cannot be conquered, owe no tribute for their own soil, and must be paid for their labour — and that the encomenderos, the magistrates and the parish priests are the ones sinning against it.