The basis of this law Plato finds in the fact of the diversity of natures, which fits men for different tasks.[158] In this he differs from Adam Smith, who believes that the differences of natural talents in men are much less than is generally supposed. Smith makes the propensity to barter the source of specialization, which, in turn, is based on the interdependence of men. He thus considers the diversities in human nature to be the effect rather than the cause of the division of labor.[159] Plato, however, is probably nearer the truth, since the very reason for mutual interdependence is diversity of nature.[160] The advantages of specialization, according to Plato,[161] are four, as stated above. It enables one to accomplish more work with greater ease, more skilfully, and at the proper season. The second and fourth of these are not mentioned by Adam Smith, but he notes the resulting increase in opulence for all the people, and the development of inventive genius. He also observes that the division of labor causes the growth of capital, and that this in turn increases specialization.[162] Of course Plato could not appreciate the important fact of the influence of the division of labor on the development of inventive genius, since he lived before the age of machinery. Plato is also a forerunner of Adam Smith in his recognition of the fact that the division of labor depends for its advance upon a great increase in the size and complexity of the state.[163] It means a multiplication of trades, a development of industry,[164] the entrance of the retail trader (κάπηλος),[165] and the invention of money as a means of exchange.[166] The necessity of the division of labor between states is also recognized. It is impossible to establish a city where it will not be in need of imports (ἐπεισαγωγίμων). International trade therefore arises, and with it are born the merchant (ἔμπορος) and the sailor class, together with all those who are engaged in the labor of the carrying trade.[167] Thus Plato, the idealist, and reputed enemy of trade and industry, develops them directly out of the basal principle of his _Republic_. He appreciates the necessity of a full-fledged industry and commerce to the existence even of a primitive state, and his hostility to them is actually directed only against what he terms their unnatural use.[168] Moreover, in his opinion, one function of the division of labor should be to limit them to the performance of their proper tasks, and keep them from degenerating into mere money-making devices. It should also result in limiting such vocations to the less capable classes since the rulers should be artisans of freedom.[169]