thing varied permanently in relation to it, we should know that the cause of variation was not in it, but in the other thing. It would thus be suited to serve as a measure, not indeed of the value of other things, but of their cost of production. If a commodity acquired a greater permanent purchasing power in relation to the invariable commodity, its cost of production must have become greater; and in the contrary case, less. This measure of cost, is what political economists have generally meant by a measure of value.
But a measure of cost, though perfectly conceivable, can no more exist in fact, than a measure of exchange value. There is no commodity which is invariable in its cost of production. Gold and silver are the least variable, but even these are liable to changes in their cost of production, from the exhaustion of old sources of supply, the discovery of new, and improvements in the mode of working. If we attempt to ascertain the changes in the cost of production of any commodity from the changes in its money price, the conclusion will require to be corrected by the best allowance we can make for the intermediate changes in the cost of the production of money itself.
Adam Smith fancied that there were two commodities peculiarly fitted to serve as a measure of value: corn, and labour. Of corn, he said that although its value fluctuates much from year to year, it does not vary greatly from century to century. This we now know to be an error: corn tends to rise in cost of production with every increase of population, and to fall with every improvement in agriculture, either in the country itself, or in any foreign country from which it draws a portion of its supplies. The supposed constancy of the cost of the production of corn depends on the maintenance of a complete equipoise between these antagonizing forces, an equipoise which, if ever realized, can only be accidental. With respect to labour as a measure of value, the language of Adam Smith is not uniform. He sometimes speaks of it as a good measure only for short periods, saying that the value
