[Sidenote: The exchange of any present and future rents results in a rate of time discount] It can not be otherwise in the particular problem of value called capitalization. The first task of scientific study is to state clearly the nature of the problem. In this case it is seen to be the exchange of a present sum of wealth for a series of future rents. Whenever there are income-bearers and buyers and sellers of them, there are the conditions required for the determination of the market rate at which those future incomes shall be discounted. Manufactures and commerce have no peculiar relation to this process. By a flight of scientific imagination we might assume that the stock of indirect agents in the world consisted only of natural food producers, and that this stock and its yield were absolutely unchangeable by man's will or efforts. Each man in such case would have to stand with hands tied, and take the fruits as they matured. Even in such a case there would be capitalization and a rate of discount on future rents. The fruit-tree (that is, the whole future series of fruits) would bear a certain relation to one year's yield; the field would bear a certain relation to its crop. Wherever there are buyers and sellers of more or less durable agents of it matters not what kind or origin, there are present the elements and causes for the fixing of a rate of time discount. [Sidenote: Capitalization of a perpetual uniform series of rents;] 4. _In practical business may be seen innumerable instances of the capitalization of both permanent and limited series of incomes._ The simplest case is the capitalization of an unvarying and supposedly perpetual series of rents. Whatever the rate of time discount prevailing, rents infinitely distant become infinitesimally small when discount is compounded. The present rent is worth most, next year's less, and so on in a decreasing series. [Sidenote: Of a probably increasing series of rents;] But social changes alter rental values, and so far as these changes are foreseen, these anticipated or expected rents are made the basis for present capitalization. Investors and owners alike may foresee that a piece of land used only for agriculture will, within a few years, be taken up for city lots, or will be needed for a factory or as the site of a railroad station. The capitalized value would not in this case be based upon a series of uniform rents each of the amount yielded annually now, but on the progressive series expected. In some cases the physical output of an agent may decline while the price of the product increases. Modern foresters foresee that the selling price of the timber will be greater twenty-five years from now than it is to-day, and they therefore estimate the rental value of the forest on the basis of the future price, thus justifying expenditure that would be unwise if present prices were to continue. [Sidenote: And of a declining or fluctuating series of rents]