The dissertation addresses the impact of the medieval notion of what scholastic theologians termed "invincible ignorance" upon later Spanish attitudes toward and actual treatment of their New World Indian subjects. Once sixteenth-century theologians expanded the range of topics of which "invincible" – and thereby excusable – ignorance could theoretically be had, official Spanish policy towards the pagan and culturally alien Native Americans became noticeably less inhumane and oppressive. This study adds significantly to our knowledge of the interaction between Native Americans and their European conquerors during the first century of Iberian settlement. First, it uncovers the ideological justification for the aforementioned shift in Spain's treatment of its Indian subjects. Second, this study successfully explains why Spanish attitudes towards the American Indians changed at the moment they did. Third, it provides an alternative to the largely discredited but inadequately replaced explanation that Spanish colonial administrators introduced more moderate policies because they increasingly abandoned the position that the Indians were not fully human. This dissertation, moreover, presents a critical contribution to our understanding of the genesis of the concept of individual human rights. As sixteenth-century theologians concluded that insurmountable ignorance constituted valid grounds to excuse some individuals for such "sins" as unbelief, idolatry, and human sacrifice, what became progressively obvious was that no single moral standard could be applied to all human beings, irrespective of upbringing and education. As a result, morality became more subjective and dependent upon the individual circumstances of the actor. Thus, in order to maintain a minimum of justice, what was morally "right" came to be seen in an increasingly direct relation to the individual. Although the connection between moral subjectivity and individual human rights has been well-established in the secondary literature, the underlying issue of invincible ignorance in relation to the problem of colonial conquest has so far not been recognized. Indeed, the very concept of "invincible ignorance" has never been systematically studied. This project reintroduces this critical notion to the center of the conversation.