A more difficult case is presented by the phenomenon of “capitalised value.” A quarry or mine which will be worked out in fifty years is valued at a sum much less than the sum of its fifty annual outputs. These annual outputs are seen in a perspective of value diminishing according to their remoteness in time. Say that the first year’s output is £100, the second (at an interest rate of 5%) will _now_ be worth only £95.23, the third, £90.70, and so on. Adding these together, we obtain a sum which is very much less than £100 × 50, and we express it—conveniently if somewhat misleadingly—by saying that the capital value is so many years’ purchase of the annual rent. In other words, to determine the marginal utility of a durable good involves a calculation of the agio on present goods as against future.[11] [11] The difficult subjects of capital value and of interest on durable goods are fully treated in Böhm-Bawerk’s _Positive Theory of Capital_. See particularly p. 339. II. One must guard against an easy misunderstanding of the expression Lowest Use. Most goods permit of two or more entirely distinct kinds of use: a book, for instance, may be read, or it may be used to light a fire. On the principles just laid down, one might think that it is the latter which determines the value of the book. There are two mistakes here. The first will be seen on referring to the terms of our cardinal proposition. It is the least use to which a good is put, and is, of course, _economically_ put, that decides—not the possible uses to which it may be put. If we were valuing two exactly similar copies of one book, and if the _only_ uses to which these copies could be put were, to be read or to be burned, then the value of each would be waste-paper value.[12] But this is an almost inconceivable supposition. Books are made to be read, and to enumerate lighting of fires among the possible _uses_ of a book is to make the mistake already alluded to—of not being clear as to what is the good that is being valued. [12] Just as the nutritive value of the horse competed with its draught value during the siege of Paris. The second and more important mistake is that here we are presenting a case which is essentially different from the typical one given in last chapter. In the case of the peasant we are valuing one of a stock of five similar goods, and concluded that the use to which the fifth sack was put determined the value of the five. In other words, we had a _stock_ of goods competing for employment. Now we have employments competing for one good, and, where a good or stock of goods is not sufficient for all possible employments, of course the only economical possibility is that the highest use, and so the highest marginal utility, should decide the value.