"Political Economy," is defined by Mr. M'Culloch, to be "the science of the laws which regulate the production, distribution, and consumption of those articles or products which have exchangeable value, and are either necessary, useful, or agreeable to man. Many very useful articles, such as air, light, and water, under some circumstances, have no exchangeable value, and are not included in the term wealth. Whenever labour is required to produce a commodity it receives, and most commodities, which are the product of labour, possess the quality of exchangeable value, and are included under the term wealth; commodities not produced by labour, and which no labour is required to obtain, do not possess exchangeable value. To this doctrine, land forms a remarkable exception. Labour improves and fertilizes it; but it possesses, in most cases, exchangeable value independent of the labour vested in it; and in all cases more exchangeable value than is measured by that labour. How land comes to form this exception, will be hereafter explained; but as all consideration of land, with its varied degrees of fertility, will be expressly excluded from this Work; as exchangeable value is, in all other cases, given by labour, the science of which I am to treat, is strictly and exclusively confined to labour and its products.