The apparent contradictions soon shew themselves. It is evident that the exchange value of anything depends on its utility, since no mortal exertion can make a useless thing exchangeable. And yet fresh air and sunlight, which are so useful as to be quite indispensable, have no exchange value; whilst a meteoric stone, shot free of charge from the firmament into the back garden, has a considerable exchange value, although it is an eminently dispensable curiosity. We soon find that this somehow depends on the fact that fresh air is plenty and meteoric stones scarce. If by any means the supply of fresh air could be steadily diminished, and the supply of meteoric stones, by celestial cannonade or otherwise, steadily increased, the fresh air would presently acquire an exchange value which would gradually rise, whilst the exchange value of meteoric stones would gradually fall, until at last fresh air would be supplied through a meter and charged for like gas, and meteoric stones would be as unsaleable as ordinary pebbles. The exchange value, in fact, decreases with the supply. This is due to the fact that the supply decreases in utility as it goes on, because when people have had some of a commodity, they are partly satisfied, and do not value the rest so much. The usefulness of a pound of bread to a man depends on whether he has already eaten some. Every man wants a certain number of pounds of bread per week, no man wants much more; and if more is offered he will not give much for it--perhaps not anything. One umbrella is very useful: a second umbrella is a luxury: a third is mere lumber. Similarly, the curators of our museums want a moderate collection of meteoric stones; but they do not want a cartload apiece of them. Now the exchange value is fixed by the utility, not of the most useful, but of the least useful part of the stock. Why this is so can readily be made obvious by an illustration. If the stock of umbrellas in the market were sufficiently large to provide two for each umbrella carrier in the community, then, since a second umbrella is not so useful as the first, the doctrinaire course would be to ticket half the umbrellas at, say, fifteen shillings, and the other half at eight and sixpence. Unfortunately, no man will give fifteen shillings for an article which he can get for eight and sixpence; and when the public came to buy, they would buy up all the eight and sixpenny umbrellas. Each person being thus supplied with an umbrella, the remainder of the stock, though marked fifteen shillings, would be in the position of second umbrellas, only worth eight and sixpence. This is how the exchange value of the least useful part of the supply fixes the exchange value of all the rest. Technically, it occurs by “the law of indifference.” And since the least useful unit of the supply is generally that which is last produced, its utility is called the final utility of the commodity. The utility of the first or most useful unit