_A Question of Nomenclature._--It may be an open question, as a matter of mere nomenclature, whether these surpluses which are thus traceable to a permanent fund of capital, on the one hand, and to a permanent force of labor, on the other, can with advantage be called rents. In this treatise we do not think it best to employ that nomenclature. What is not uncertain is that these gains are measurable by the same formula that measures the rent of a piece of land. If the essential thing about rent were that it is a material product and consists of a sum of differential quantities, these incomes certainly would be rents. Popular thought, however, attaches another meaning to this term, and we therefore limit ourselves to saying that these differential incomes or surpluses may be determined in amount by the principle of rent. They can be described and measured exactly as the Ricardians described the income of landlords.[1] [1] The term _rent_ has even been applied to surpluses of a psychological kind. Certain gains that men get consist purely in pleasures or in reduced pains or sacrifices, and a few writers have applied to such subjective gains the term _rent_. If a man buys a barrel of flour for five dollars and gets out of it a service that is a hundred times as great as he could get from some other article which he buys for the same amount, this surplus of pleasure may be called, by a figure of speech, "consumers' rent"; and if the essence of rent were the fact that it can be made to take the form of a surplus or difference, the name would be well chosen, though there is danger that by this use of the term science may divorce itself from practical thought and life. If we take all the barrels of flour that a man uses in ten years, there is one which is marginal, because it is worth to the man only enough to offset the sacrifice he incurs in getting it. All the others are worth more. We can arrange them in a scale in the order of their importance, the most necessary one coming first and the least important one last; and we can compare the service which each one renders with that rendered by the last, and measure the surplus of good which each one does to the user. There is here in operation a law of diminishing subjective returns. Early units consumed afford more pleasure than do later ones. There results a series of surplus gains, and the sum of all these surpluses makes a total of net benefit,--is a gain that is not offset by a compensatory sacrifice. The last barrel of flour on the list is worth just what it costs, and all the others are worth more. They give the consumer a surplus of satisfaction for which he pays nothing. The sum of the excesses of service rendered by all the earlier barrels constitutes what has been called the consumers' rent, realized in this case from the entire supply