[249] The reader should bear in mind that the “theory of price,” in which exchange value is explained according to the utility theory of value, involves so comparison whatever of the satisfactions of different persons. See _ante_, § 4. [250] This kind of affirmation is, however, very common, and hence the presumption is that it has a legitimate meaning. [251] Suppose a machine is destroyed in the making of 100 units of a certain product. Then the total labor cost of each of these units contains ¹⁄₁₀₀ of the total labor cost of this machine. This is explained by Ricardo and by recent followers of Ricardo, as for instance by Professor Macvane in his text book. Another earlier machine was partly used up in making this first one. Perhaps it contributed ¹⁄₁₀₀₀₀ of its total labor cost to this first. Then each of our products contains in its total labor cost ¹⁄₁₀₀ of ¹⁄₁₀₀₀₀ of the total labor cost of the machine of the second generation back. [252] Using capital here in the sense of means of production that are themselves products of labor. We will go so far in our illustration as to suppose that the land has never had labor expended upon it to drain it, or in any other way to “fix an element of capital in it.” [253] In this illustration the marginal cost is five times as high as the “total cost” or total average cost of a bushel, but this ratio could have no significance even if the data of our illustration were approximately true with respect to the _direct_ labor cost of wheat on good land, for we have eliminated from the real total labor cost all of what Ricardo called the “indirect” labor cost, by eliminating capital. [254] Ricardo frequently supposed his doses to consist of sums of money expended by the farmer, or to consist of increments of money capital. These doses of money, however, would be expended for capital goods and labor power conjointly. [255] Traced in the fifth chapter of the present essay. [256] That is to say, when we affirm that in our theory we can disentangle the specific product of labor, we mean that entrepreneurs in practical effect do ascertain the marginal product of labor in making up their labor forces. [257] Sometimes “pure capital.” [258] _Distribution of Wealth_, p. 119. [259] See _ante_, chap. vii, §§ 4 and 5. [260] Adopting the view of the income of capital taken by Professor Clark and advocated with so much force by Professor F. A. Fetter. [261] By this phrase we mean always _in relative proportion_, so that the value of A is to that of B, as the cost of A is to that of B. [262] Put in proximate and practical language, the amount of ore that can be taken profitably from a mine depends jointly on the price of the ore at the surface and the wages of labor.