The objective of this article is to better understand the workings of expertise by turning to history. To borrow David Kennedy’s expression, how do “struggles of articulation” take place? How do some experts succeed in imposing their own vision of the world and in giving out solutions to respond to ongoing crises? To reflect on these questions, we will closely look at the legal-theological debates that took place at the turn of the sixteenth century on the legitimacy of the transatlantic slave trade. We will examine how the first members of the School of Salamanca—the very ones who are considered to be the “fathers of international law”—were able to place the slave trade in the perspective of an ethic or law of commerce.