determines the value of one, it must determine the value of all, as, by hypothesis, all sacks were alike, and therefore all interchangeable. Thus we obtain the universal formula for the valuation of goods in quantity. The value of a quantity of similar goods is the value of the marginal good multiplied by the number of the goods. To follow the illustration out. If another sack gets lost, the marginal utility is found to have been that of the making of spirits; if still another, the feeding of poultry. Finally, suppose Crusoe to be reduced to the one sack. Then the satisfying of all lesser wants is out of the question; the losing of it means death to him; the marginal utility and the highest utility are one. Again, suppose Crusoe as merchant bargaining, say, with the Spaniards. If he have five sacks, he will sell one at a low rate; if he have four, he will ask a higher price; if he have only one, he will not part with it for any money. Extend this to the phenomena of an industrial community. The five sacks represent a larger supply than the four, the four than the three, and so on; and, as the supply decreases, the value of the single sack rises. Now one of the commonest phenomena of a market is that, _ceteris paribus_, increase of supply brings down value and decrease of supply sends it up. To put it in terms of our theory: When the quantity of any good produced is increased, the good is put to lower levels of use; the last want supplied determines the last satisfaction; and this last satisfaction determines the value of all the stock. Here we have the explanation of the old Paradox of Value. If any commodity is available in such quantity that all possible wants for that commodity are supplied, and yet there is a surplus of the commodity, the marginal utility is zero, and the value of the entire stock is nil. And it is also explained how diamonds have a high value compared with bread. The quantity of diamonds available is never sufficient to satisfy more than a fraction of the desire for them: the marginal utility, then, is high. Bread again is, happily, to be had everywhere at a comparatively small expenditure of labour, and the immense supply as compared with the limited wants, puts the marginal utility low. CHAPTER VI DIFFICULTIES AND EXPLANATIONS A chapter may be devoted to answering certain doubts which naturally arise in the reader’s mind, and to disentangling some complications which hide the working of our fundamental law. I. Some goods are perishable, some durable; some are single goods, some are groups of separable elements; and, of these groups again, some are composed of homogeneous, some of very heterogeneous elements. Consequently there is a difference in the way in which goods give off their use, and the marginal utility is not always perfectly obvious.