Aristotle, Politics

[1255b] [1] for they assume that just as from a man springs a man and from brutes a brute, so also from good parents comes a good son but as a matter of fact nature frequently while intending to do this is unable to bring it about.

It is clear therefore that there is some reason for this dispute, and that in some instances it is not the case that one set are slaves and the other freemen by nature; and also that in some instances such a distinction does exist, when slavery for the one and mastership for the other are advantageous and just, and it is proper for the one party to be governed and for the other to govern by the form of government for which they are by nature fitted, and therefore by the exercise of mastership, while to govern badly is to govern disadvantageously for both parties （for the same thing is advantageous for a part and for the whole body or the whole soul, and the slave is a part of the master—he is, as it were, a part of the body, alive but yet separated from it; hence there is a certain community of interest and friendship between slave and master in cases when they have been qualified by nature for those positions, although when they do not hold them in that way but by law and by constraint of force the opposite is the case）.

And even from these considerations it is clear that the authority of a master over slaves is not the same as the authority of a magistrate in a republic, nor are all forms of government the same, as some assert. Republican government controls men who are by nature free, the master's authority men who are by nature slaves; and the government of a household is monarchy （since every house is governed by a single ruler）, [20] whereas statesmanship is the government of men free and equal. The term ‘master’ therefore denotes the possession not of a certain branch of knowledge but of a certain character, and similarly also the terms ‘slave’ and ‘freeman.’ Yet there might be a science of mastership and a slave's science—the latter being the sort of knowledge that used to be imparted by the professor at Syracuse （for there used to be a man there who for a fee gave lessons to servants in their ordinary duties）; and indeed there might be more advanced scientific study of such matters, for instance a science of cookery and the other such kinds of domestic service—for different servants have different functions, some more honorable and some more menial, and as the proverb says, “ Slave before slave and master before master.¹
” The slave's sciences then are all the various branches of domestic work; the master's science is the science of employing slaves—for the master's function consists not in acquiring slaves but in employing them. This science however is one of no particular importance or dignity: the master must know how to direct the tasks which the slave must know how to execute. Therefore all people rich enough to be able to avoid personal trouble have a steward who takes this office, while they themselves engage in politics or philosophy. The science of acquiring slaves is different both from their ownership and their direction—that is, the just acquiring of slaves, which is akin to the art of war or that of the chase. Let this then stand as our definition of slave and master.