On the division of labor, Aristotle adds little to Plato’s and Xenophon’s theory. He agrees with Xenophon against Plato that it implies a necessary distinction between the work of men and women.[631] He also applies the principle more extensively, so as to include all nature, whereas Plato seems to limit its application to man. Nature (ἡ φύσις) he observes, does not produce things like the Delphian knife, in a poverty-stricken manner (πενιχρῶς) to serve many purposes, but each for a single purpose (ἓν πρὸς ἓν).[632] Like Plato, he makes the principle of reciprocity (τὸ ἴσον ἀντιπεπονθός) out of which the division of labor arises, the saving element in the state.[633] He is also fully as emphatic in his application of the law to politics and citizenship.[634] SLAVERY We have seen that the references to slavery in Xenophon and Plato are incidental, and reveal a certain unconscious naïveté as to the actual social problem involved. By Aristotle’s day, however, the criticisms of the Sophists had shaken the foundations of all traditional institutions, and their thesis that slavery is contrary to nature had become through the Cynics a prominent social theory.[635] The thought on the subject had crystallized into two leading doctrines—one including benevolence in justice, and hence denying the right of slavery; and the other identifying justice with the rule of the stronger, and hence upholding slavery as based on mere force.[636] The practical Aristotle, an upholder of slavery, not from tradition, but through conscious belief in its economic necessity, thus takes his stand midway between the two opposing theories. He champions the old view of natural slavery, but rejects the basis of mere force for that of morality and benevolence.[637] His thesis is that slavery is a natural and necessary relation in human society, not accidental or conventional. The slave, being property, which is a multitude of instruments (ὀργάνων πλῆθος), is an animate instrument (ὄργανον ἔμψυχον) conducive to life (πρὸς ζωήν).[638] He is just as necessary to the best life of the citizen as are inanimate instruments, and will be, until all tools work automatically, like the mythical figures of Daedalus or the tripods of Hephaestus.[639] The slave is a servant in the realm of action (πρᾶξις), not of production (ποίησις). He is not a producer of commodities (ποιητικός), but of services (πρακτικός),[640] and just as property is merely a part or member (μόριον) belonging wholly to something else, so the slave, as property, belongs entirely to his master, and has no true existence apart from him.[641] From these facts, the whole nature and power of the slave are evident. One who, though a human being, is merely property is a natural slave, since he is naturally not his own master, but belongs to another, in whom he finds his true being.[642] As Barker has observed, this conclusion of the first part of Aristotle’s argument