A connected and comprehensive grasp of principles was the great achievement of Adam Smith;(28) for, although the “Wealth of Nations” was naturally not without faults, it has been the basis of all subsequent discussion and advance in political economy. In Books I and II his own system is elucidated, while Book IV contains his discussion of the Agricultural School and the attacks on the mercantile system. Seeing distinctly that labor was the basis of all production (not merely in agriculture), he shows (Books I and II) that the wealth of a country depends on the skill with which its labor is applied, and upon the proportion of productive to unproductive laborers. The gains from division of labor are explained, and money appears as a necessary instrument after society has reached such a division. He is then led to discuss prices (market price) and value; and, since from the price a distribution takes place among the factors of production, he is brought to wages, profit, and rent. The functions of capital are explained in general; the separation of fixed from circulating capital is made; and he discusses the influence of capital on the distribution of productive and unproductive labor; the accumulation of capital, money, paper money, and interest. He, therefore, gets a connected set of ideas on production, distribution, and exchange. On questions of production not much advance has been made since his day; and his rules of taxation are now classic. He attacked vigorously the balance-of-trade theory, and the unnatural diversion of industry in England by prohibitions, bounties, and the arbitrary colonial system. In brief, he held that a plan for the regulation of industry by the Government was indefensible, and that to direct private persons how to employ their capital was either hurtful or useless. He taught that a country will be more prosperous if its neighbors are prosperous, and that nations have no interest in injuring each other. It was, however, but human that his work should have been somewhat defective.(29) A new period in the history of political economy, however, begins with Adam Smith. As Roscher says, he stands in the center of economic history.