Plato’s other error concerning money, as above observed, was that it need possess no intrinsic value for domestic use. He looked upon gold and silver as causes of degeneration in state and individual,[200] and would therefore have put a ban on them for use within the state.[201] To his mind, a mere state fiat was sufficient to give currency and value.[202] This doctrine has also often recurred in the history of economic thought, as in Ruskin and the Greenback party of a generation ago.[203] The error, however, was not so grave in Plato’s case, for he, at least, recognized the need of the precious metals for international purposes.[204] Moreover, in his proposed state of such limited extent, the problem would have been far simpler, and he would have distinguished between actual conditions and possibilities in Greece and his admittedly more or less utopian ideal. EXCHANGE Exchange in Greek economy held a very minor place, compared with its dominant importance in modern theory. It was discussed chiefly in a negative manner, as the object of the moral and aristocratic prejudice of Greek thinkers. We find, however, some appreciation of its true place in the economic life of a state. Plato divides trade, ἀλλαγή or ἀγοραστική, into αὐτοπωλική, which sells its own products, αὐτουργῶν, and μεταβλητική, which exchanges the products of others. He further divides the latter into καπηλική, the exchange within the state, which he calls one-half of all the exchange, and ἐμπορική, foreign commerce.[205] He finds its origin in the division of labor, and in the mutual interdependence of men and states.[206] He understands the necessity of the reciprocal attitude in international, as well as in private, exchange, and thus has a clearer insight than the mercantilists and some modern statesmen. A state must raise a surplus of its own products, so as to supply the other state from which it expects to have its own needs satisfied.[207] Since a tariff on imports played little part in Greek life, except in so far as it was imposed for sumptuary or war purposes,[208] the perplexing modern problem of the protective tariff scarcely came within the horizon of Greek thinkers. Plato would prohibit the import of certain luxuries, as a moral safeguard. He divides merchandise into primary and secondary products, and would not permit the import of the latter.[209] Elsewhere, however, he legislates against imposts upon either imports or exports, though unconscious of the significance of his suggestion.[210] He appreciated something of the function of exchange in society. It performed a very important service, as a mediator between producer and consumer.[211] Like money, it served to equalize values, and thus acted as an aid to the satisfaction of needs.[212] When limited to this primary function, it was of advantage to both parties to the exchange,[213] and merchants and retailers had then a real part in the production of values.[214]