Natural slavery

Natural slavery is term used by Aristotle in the Politics to express the belief that some people are slaves by nature, contrasting them with those who were slaves solely by law or convention.

Aristotle's discussion
In book I of the Politics, Aristotle addresses the questions of whether slavery can be natural or whether all slavery is contrary to nature and whether it is better for some people to be slaves. He concludes that

"those who are as different [from other men] as the soul from the body or man from beast—and they are in this state if their work is the use of the body, and if this is the best that can come from them—are slaves by nature. For them it is better to be ruled in accordance with this sort of rule, if such is the case for the other things mentioned."

It is not advantageous for one to be held in slavery who is not a natural slave, Aristotle contends, claiming that such a condition is sustained solely by force and results in enmity.

Influence
In 16th century Spain, theologian Juan Ginés de Sepúlveda defended the position of the New World colonists, claiming that the Amerindians were "natural slaves." He wrote this in Democrates alter de justis belli causis apud Indios (A Second Democritus: on the just causes of the war with the Indians). Although Aristotle was a primary source for Sepúlveda's argument, he also pulled from various Christian and other classical sources, including the Bible.

Sepúlveda was opposed in the Valladolid debate by Bartolemé de las Casas, bishop of Chiapas. De las Cases countered that Aristotle's definition of the "barbarian" and the natural slave did not apply to the Indians, who were fully capable of reason and should be brought to Christianity without force or coercion.