2. _The advantage of exchange consists in the raising of the want-gratifying power of goods to both parties._ It generally was assumed by medieval thinkers that if one party to an exchange gained, the other must lose. The mistaken idea prevailed that value is something fixed in the good, and unchangeable. Where the exchange is voluntary (and only that kind is here being considered), it is mutual advantages which make the exchange rational. Many false conclusions on practical questions still result from a failure to grasp this simple truth. It follows from this that the act of exchange is itself useful, for goods having a small importance to men are given a higher importance by being brought into better relations with wants. Merchants, peddlers, traders, and common carriers of all sorts, therefore, are adding to the utility of goods. This idea has been only slowly apprehended, but is now one of the least disputed propositions in economics. [Sidenote: Demand is supply in another aspect] 3. _Barter is the exchange of goods without the use of money._ Either one of the goods traded in cases of barter may be considered as sold, and either one as bought, according as the matter is looked at from the standpoint of the one or the other party to the exchange. Demand, therefore, is supply, and supply is demand when the point of view is shifted from one party to another. The fisherman's demand for venison is expressed in terms of fish; the hunter's demand for fish is expressed in terms of venison. But to the fisherman the venison is the supply offered to him. The term "marginal utility" of a good, therefore, does not refer merely to the demand of the consumer; for it expresses by a single phrase the idea both of demand and of supply. The utility of the goods composing the supply is expressed in terms of the goods that represent demand and vice versa. The only way in which man can give definite, concrete, numerical expression to his desire for goods is to state it in terms of other goods. In expressing numerically, in terms of other objects, an estimate of the utility of an apple, a horse or a house, one inevitably gives expression to a ratio of exchange; demand for one good is the offer of another good. § II. BARTER UNDER SIMPLE CONDITIONS [Sidenote: In isolated exchange the price is not economically fixed]