Aristotle describes a natural slave in his book Politics as "anyone who, while being human, is by nature not his own but of someone else…" Aristotle also states "he is of someone else when, while being human, he is a piece of property; and a piece of property is a tool for action separate from its owner."  Based on this quote, Aristotle defines natural slavery in two phases. The first part is the natural slave's existence and characteristics. The second part is the natural slaves in society and how they interact with his or her master. According to Aristotle, natural slaves' main features include being pieces of property, tools for actions, and belonging to others. 
Aristotle's work has come under controversy and criticism in recent years, with many scholars agreeing that "…the formulation of Aristotle's account of slavery is riddled with inconsistency and incoherence."  Other scholars have argued that the state of natural slavery is ultimately alterable, since Aristotle's conception of nature is as well. 
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.
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 Bartolomé de las Casas, bishop of Chiapas. De las Casas 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.
- Wayne Ambler, "Aristotle on Nature and Politics: The Case of Slavery," Political Theory 15, no. 3 (Aug. 1987): 390-410.
- Aristotle, Politics, 1254b16–21.
- Karbowski, Joseph (2013). "Aristotle's Scientific Inquiry Into Natural Slavery". Journal of the History of Philosophy. 51.3.
- Dobbs, Darrell (1994). "Natural Right and the Problem of Aristotle's Defense of Slavery". The Journal of Politics. 56.1.
- Rodriguez, Philippe-André (2016). "L'impérialisme institutionnel et la question de la race chez Aristote". European Review of History. 23.4.
- Aristotle, Politics, 1254b16–21.
- Aristotle, Politics, 1255b11–15.
- Crow, John A. The Epic of Latin America, 4th ed. University of California Press, Berkeley: 1992.
- Bonar Ludwig Hernandez, ""The Las Casas-Sepúlveda Controversy: 1550-1551"," Ex Post Facto: Journal of the History Students at San Francisco State University 10 (2001): 95–104.