In the case of _different periods in the same country_, though we have not, as in the case of two different countries, the test of an actual exchange, we can still use labour as the measure. We must allow for the higher profits of the earlier period; and on the (Ricardian) principle that profits and wages vary inversely, though corn wages have risen, profits have in proportion fallen, and the total value of the produce measured by its power of purchasing labour must be the same,[590] the purchased labour then representing the producing labour plus the then rate of profits. From Ricardo’s dogma it seems (to Malthus) to follow directly that the value of labour is constant.[591] Taking the labour they will purchase as the best measure of the value of the precious metals, as of anything else, we have light on one of the pressing questions of the day (in 1823), the causes of the changing value of money. The causes affect not the labour but the money, and they are of two kinds. The first Malthus describes as a primary or necessary cause, namely, the variation in profits depending on the (Ricardian) theory of the interlocking of wages and profits, and the (Malthusian) theory of the relation of profits to rent. Dear corn due to difficult cultivation would lower profits, and would alter the value of money, but only in relation to raw, not in relation to manufactured produce, or at least (from the effects of Ricardo’s principle of the inverse variation of wages and profits) not to the same extent. But the second, which is a “secondary and incidental” class of causes, affects both raw and manufactured goods, and is often enough to completely dwarf the effects of the primary cause;[592]—it is the general commercial situation of a country,—“the fertility and vicinity of the mines, the different efficiency of labour in different countries, the abundance or scarcity of exportable commodities, and the state of the demand and supply of commodities and labour compared with” the precious metals.[593] The efficiency of labour and a prosperous commerce, with a great consequent demand for corn and labour, are often more powerful in making bullion cheap than agricultural productiveness and high profits in making bullion dear and corn cheap. During the war—say from 1790 to 1814—we had an instance of this, and since the war—say from 1814 to 1823—we have had a clear instance, he thinks, of the converse.[594]