The institution of slavery is, in modern times, almost universally condemned and abhorred. Much of the ancient world, however, regarded slavery as an indisputable fact of life. While it had its critics, the institution of slavery was never seriously challenged in the ancient world. Even cynical and stoic philosophers, who ostensibly believed in the brotherhood of mankind, were more or less mute in their opinion. Aristotle, in Politics, qualifies and defends certain forms of the institution of slavery, identifying two distinct forms of slavery, one illegitimate, one legitimate. According to Aristotle, superiority and inferiority are inherent in all systems and relationships, and human slavery is a case parallel to many in nature. The soul rules the body like a master rules a slave, and reason governs the appetites like a king governing a city. It is natural and advantageous for the soul to rule the body, just as it is better for animals to be ruled by man, and the relationship of male to female is a relationship of superior to inferior.
The relation of master and slave is simply a manifestation of the law of subordination. A so-called "natural" slave is, according to Aristotle, one who does not possess the full use of reason. The natural slave participates in reason to the extent that he perceives of it, but he does not himself have use of it. He therefore lacks in his capacity to perform civic duty and lead an ordered life. He compensates, however, by being possessed of a body suited for labor. Nature intends also to erect a physical difference between the bodies of freemen and those of the slaves, giving the latter strength for the menial duties of life, but making the former upright in carriage and...useful for the various purposes of civic life (Cohen, Michelle 1999) Aristotle's rhetoric depicts slavery as a mutually beneficial relationship, symbiotically cooperative in its ideal manifestation. The master envisions tasks the slave cannot reason himself, and the slave executes tasks the master cannot execute. Aristotle does not advocate or justify brutality in the master/slave relationship. Whenever the relation is in accordance with nature's ordinance, there is mutual helpfulness and friendship between master and slave. A mutually beneficial association does not, however, manifest in cases when the master/slave relation does not follow nature's ordinance.
In his discussion of the household, Aristotle identifies the relationship of master and slave (along with husband/wife and parent/child) as fundamental to this most basic and naturally occurring form of human association. The slave, Aristotle says, is an "animate instrument of property" that exists for the sole purpose of facilitating household management (Pg. 1253). Slaves are essential for performing the myriad variety of mundane tasks that by and large comprise daily living. Importantly, the genuine slave is not simply a metaphorical extension of its master's body--a "helping hand" as it were--but instead must also be understood as one who lacks any sort of identity outside of this relationship. The slave is literally a "part" of its master who "belongs entirely to him and has no life or being other than that of so belonging" (Pg. 1254).
The function and nature of the slave is thus clear. Nonetheless, the crucial question of who should count as a slave remains open. What is at issue here, of course, is a matter of justification. Under what conditions should we allow one person to own another? An immediate option, and one that Aristotle repeatedly rejects, is the conventional route: slaves are those persons whose subjugation is legally sanctioned. In other words, it is up to each society to enact a "slave code"--that is, a set of laws that determine those classes of people whose enslavement is deemed permissible. On this view, slavery is simply a product of cultural circumstance. Anyone who is either unwise enough or unlucky enough to run of a particular society's edicts is doomed to a life of involuntary servitude.
The problem with the conventional response is that in basing slavery solely on "legal sanctions and superior power," it completely ignores the question of justice (Pg. 1255). For instance, is slavery as a result of capture in war morally permissible? On the conventionalist account, so long as one's society recognizes the relevant convention, it obviously is. Aristotle, however, belittles this response as overly simplistic and morally problematic. Slavery as a result of capture in battle, he argues, is only permissible insofar as the war waged was just. If not, then enslaving the vanquished is simply an unjust consequence of an unjust war (P 1255).
The deeper difficulty that this example highlights, and the reason Aristotle ultimately dismisses a conventionalist justification of slavery as ethically inadequate, is the capricious and arbitrary fashion with which it determines the relationship of master and slave. On such an account, the precise problem is that anyone can count as a slave, "anyone of the highest rank could be captured and sold into slavery," and such a state of affairs is, from a moral point of view, extremely unsettling, since "no one would say that a person who does not deserve to be in a condition of slavery really ought to be a slave" (P 1255).