The radical differences between the moral beliefs of the Western world and those of the new world's natives stretches the whole set of theological, anthropological and legal knowledge of the 16th century. Pope Paul the third had established that the Indians were to be considered fully humans. It was therefore necessary to explain their ignorance of the self-evident principles of natural law, without denying their humani¬ty. The Aristotelian concept of natural slavery seemed to fit the task. Yet, it was soon clear that this concept could have not been employed without dramatically changing it, in order to better fit the new needs. Francisco de Vitoria successfully engaged in this task, providing the European colonial theory with some of its most powerful and long standing features.