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 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.