is called the total utility of the commodity. If there were but one umbrella in the world, the exchange value of its total utility would be what the most delicate person would pay for it on a very wet day sooner than go without it. But practically, thanks to the law of indifference, the most delicate person pays no more than the most robust: that is, both pay alike the exchange value of the utility of the last umbrella produced--or of the final utility of the whole stock of umbrellas. These terms--law of indifference, total utility, and final utility--though admirably expressive and intelligible when you know beforehand exactly what they mean, are, taken by themselves, failures in point of lucidity and suggestiveness. Some economists, transferring from cultivation to utility our old metaphor of the spreading pool, call final utility “marginal utility.” Either will serve our present purpose, as I do not intend to use the terms again. The main point to be grasped is, that however useful any commodity may be, its exchange value can be run down to nothing by increasing the supply until there is more of it than is wanted. The excess, being useless and valueless, is to be had for nothing; and nobody will pay anything for a commodity as long as plenty of it is to be had for nothing. This is why air and other indispensable things have no exchange value, whilst scarce gewgaws fetch immense prices. These, then, are the conditions which confront man as a producer and exchanger. If he produces a useless thing, his labor will be wholly in vain: he will get nothing for it. If he produces a useful thing, the price he will get for it will depend on how much of it there is for sale already. If he increases the supply by producing more than is sufficient to replace the current consumption, he inevitably lowers the value of the whole. It therefore behooves him to be wary in choosing his occupation, as well as industrious in pursuing it. His choice will naturally fall on the production of those commodities whose value stands highest relatively to the labor required to produce them--which fetch the highest price in proportion to their cost, in fact. Suppose, for example, that a maker of musical instruments found that it cost him exactly as much to make a harp as to make a pianoforte, but that harps were going out of fashion and pianofortes coming in. Soon there would be more harps than were wanted, and fewer pianofortes: consequently the value of harps would fall, and that of pianofortes rise. Since the labor cost of both would be the same, he would immediately devote all his labor to pianoforte-making; and other manufacturers would do the same, until the increase of supply brought down the value of pianofortes to the value of harps. Possibly fashion then might veer from pianofortes to American organs, in which case he would make less pianofortes and more American organs. When these, too, had increased