When in 1550 and 1551, Juan Ginés de Sepúlveda and Bartolomé de Las Casas famously quarreled over the justification of the Spanish conquest of the New World before the Royal and Supreme Council of the Indies at Valladolid, their arguments mostly repeated and reflected a longstanding tradition of the academic discussion of the just war theory. The main tenets of this discussion had been received, collected, expanded and transferred to the specific case of the Conquista during the preceding two generations of Spanish philosophers, theologians and experts in both laws. Particularly the erudite scholar Sepúlveda seems to have given hardly more than a brief sketch of the main tenets that were on the table in the 16th century in the debate of Valladolid, referring to the four key arguments he had assembled and learnedly demonstrated in his ambitiously written, but unpublished dialogue 'Democrates secundus or On the just causes for a war' (at the indios). In this dialogue, the four arguments are presented and defended by a learned Greek, Democrates, who tries to prove the legitimacy of the Spanish conquest and dominion in the New World against a German called Leopold, who echoes pacifist views reminiscent of Erasmus of Rotterdam. The four arguments of the 'Democrates secundus' are: the barbarian status of the indigenous people and their condition of natural slaves, their offenses against Natural Law, the need for humanitarian intervention given the widespread practice of human sacrifice and anthropophagy, and the right of free religious and cultural exchange. As Sepúlveda presents these arguments, he is eager to show that all of them were advanced by philosophers, jurists and theologians of high reputation: Augustine and Thomas Aquinas, Ulpianus and Gerson, and, above all, Aristotle, whose political philosophy is the basis for Sepúlvedas reasoning throughout the dialogue. It was precisely this kind of reasoning that transferred Aristotle's theory of natural slavery to the people of the New World which was responsible for the harsh criticism that Sepúlveda had to face from Las Casas and most other Spanish theorists of the time. Due to this criticism, Sepúlveda's dialogue was never given the royal privilege to be published and it was only in the late 19th century that the first of a very small number of extant manuscripts of the 'Democrates' secundus was rediscovered.