Ricardo's most famous work is his "Principles of Political Economy and Taxation" (1817). He advanced a labor theory of value:  The value of a commodity, or the quantity of any other commodity for which it will exchange, depends on the relative quantity of labour which is necessary for its production, and not on the greater or less compensation which is paid for that labour. Ricardo's note to Section VI:  Mr. Malthus appears to think that it is a part of my doctrine, that the cost and value of a thing be the same;—it is, if he means by cost, "cost of production" including profit. Ricardo contributed to the development of theories of rent, wages, and profits. He defined rent as "the difference between the produce obtained by the employment of two equal quantities of capital and labor." Ricardo believed that the process of economic development, which increased land utilization and eventually led to the cultivation of poorer land, principally benefited landowners. According to Ricardo, such premium over "real social value" that is reaped due to ownership constitutes value to an individual but is at best a paper monetary return to "society". The portion of such purely individual benefit that accrues to scarce resources Ricardo labels "rent". In his "Theory of Profit", Ricardo stated that as real wages increase, real profits decrease because the revenue from the sale of manufactured goods is split between profits and wages. He said in his "Essay on Profits", "Profits depend on high or low wages, wages on the price of necessaries, and the price of necessaries chiefly on the price of food." Between 1500 and 1750 most economists advocated Mercantilism which promoted the idea of international trade for the purpose of earning bullion by running a trade surplus with other countries. Ricardo challenged the idea that the purpose of trade was merely to accumulate gold or silver. With "comparative advantage" Ricardo argued in favour of industry specialisation and free trade. He suggested that industry specialization combined with free international trade always produces positive results. This theory expanded on the concept of absolute advantage. Ricardo suggested that there is mutual national benefit from trade even if one country is more competitive in every area than its trading counterpart and that a nation should concentrate resources only in industries where it has a comparative advantage, that is in those industries in which it has the greatest competitive edge. Ricardo suggested that national industries which were, in fact, profitable and internationally competitive should be jettisoned in favour of the most competitive industries, the assumption being that subsequent economic growth would more than offset any economic dislocation which would result from closing profitable and competitive national industries.