[221] This enquiry must constitute the first part of the theory of exchange value, since it is quite beyond dispute that cost in any form can influence exchange value only by influencing supply. Value will rest at the level of cost only when the supply of the good is at just the proper point. When the supply is at any other point, as in the case of monopolies, value no longer rests at the level of costs. But value is still determined by certain other influences. The description of these is the first problem. [222] If the good be of a kind held by consumers in stocks, it should go without saying that instead of a buyer being entirely excluded by a rise of price, only the marginal increments of his purchases may be excluded. [223] Employing Professor Marshall’s terminology we would say that the “social demand schedule” is a _resultant_ from combining all the “individual demand schedules.” [224] See his _Introduction to the Theory of Value_, p. 37. [225] See his _Theoretische Socialökonomik_, 1895, p. 282. [226] As pointed out in section 2 of this chapter. [227] Or at any rate, if not by decreasing their marginal utility, by decreasing their marginal price equivalent. See the section just preceding. [228] Explanation of the principles in accordance with which the various classes of production goods share in the value of the product is but a part of the theory of distribution viewed in a particular way. [229] The use of some kinds of production goods cannot be increased without increasing to the same extent the use of certain other kinds in the same production, but it can be shown that this does not change the general principle of the case. [230] When a single-use production good is short-lived instead of durable, so that it receives its value from its product in one payment, instead of a series of payments in time, we do not call its value return a “rent.” Its value is nevertheless “price-determined” in the same sense as the rents just discussed and belongs to the same category as these rents. The term “price-determined rent” has, among professed followers of Ricardo (such as Professor Marshall who holds to Ricardo’s theory in the main), come to mean the income to a durable single-use production good. In the most unfortunate terminology of the Ricardian school—which the writer believes can be traced back to their ultimately false philosophy of value—a “price-determined rent” is one which “does not enter into price.” But the leading exponents of present-day Ricardian doctrine are now agreed, it seems, that when a production good is capable of more than one application—as land to wheat or fruit or pasture—its rent _does enter_ into the price of its product. Therefore they mean by a price-determined rent, not the rent of such a good, but the rent of a single-use production good.