(3.) There is a third case, intermediate between the two preceding, and rather more complex, which I shall at present merely indicate, but the importance of which in political economy is extremely great. There are commodities which can be multiplied to an indefinite extent by labor and expenditure, but not by a fixed amount of labor and expenditure. Only a limited quantity can be produced at a given cost; if more is wanted, it must be produced at a greater cost. To this class, as has been often repeated, agricultural produce belongs, and generally all the rude produce of the earth; and this peculiarity is a source of very important consequences; one of which is the necessity of a limit to population; and another, the payment of rent. In case (3) articles like agricultural produce have a very great power to satisfy desires, and if scarce would have a high value. So far as U is concerned, here also, as in case (2), exchange value might mount upward to almost any height, but it can go no higher than D permits. In commodities of this class, affected by the law of diminishing returns, the tendency is for D to increase, and so for exchange value to rise, as indicated by the dotted lines above that of the exchange value. [Illustration: Same as before.] § 3. Commodities limited in Quantity by the law of Demand and Supply: General working of this Law. These being the three classes, in one or other of which all things that are bought and sold must take their place, we shall consider them in their order. And first, of things absolutely limited in quantity, such as ancient sculptures or pictures. Of such things it is commonly said that their value depends on their scarcity; others say that the value depends on the demand and supply. But this statement requires much explanation. The supply of a commodity is an intelligible expression: it means the quantity offered for sale; the quantity that is to be had, at a given time and place, by those who wish to purchase it. But what is meant by the demand? Not the mere desire for the commodity. A beggar may desire a diamond; but his desire, however great, will have no influence on the price. Writers have therefore given a more limited sense to demand, and have defined it, the wish to possess, combined with the power of purchasing.(207) To distinguish demand in this technical sense from the demand which is synonymous with desire, they call the former _effectual_ demand.