This article intends to analyze three discourses that advocated for slavery abolition in Spanish America: the first one was written in the late seventeenth century, and the last two in the early nineteenth century. By means of an analytical reading, this paper proposes a hypothesis on the relationship between slavery, abolitionism and the discourse of political liberty. It is proved that, while the discourse from seventeenth century followed the principles of Natural Rights to its logical consequences and was able to put forward a revolutionary formula to end slavery, the ones from the nineteenth century ended up concocting a proposal in which the liberty they initially defended was finally negated. Even though these two modern discourses used the principles of Natural Rights, they took a step back from the principle of natural liberty maintaining slavery while protecting the property rights of the masters. They ended up creating a concept of freedom in the form of an oxymoron that allowed them to deny that idea after having pretended to defend it.