Hence the price of a commodity and the ratio between its supply and demand mutually condition each other. On the height of the price depends, in great part, how many purchasers shall resolve to make an effectual demand; but, at the same time, to what amount of cost of production, sellers shall extend their supply.(659) We can speak of an equilibrium between supply and demand only when the former corresponds with the _wish_ of those who are ready to make good the full cost of production. (_Malthus._) It has been asked, indeed, whether it were more natural and better that demand should precede supply or supply demand.(660) But the inquiry is an illogical one, when expressed in so general a manner, since supply and demand are only two sides of the same transaction. But, we may say that in the case of indispensable goods, the want of them (demand) is always felt sooner than the excess of them (supply), and that in the case of goods which may be dispensed with, including, originally, money, the reverse is true. Besides, a person engaging in the production of any kind of goods, can, as a rule, only seldom directly investigate the relation between supply and demand. Generally, he can do no more than compare the market price of the commodity with the cost at which he can produce it. Many mistakes are inevitable here; but the making of them is the necessary sacrifice which must be endured to purchase the more than counterbalancing advantages of free competition.(661) Section CXII. Exceptions.