It is therefore indispensable that all things which can be exchanged should be capable of comparison, and for this purpose money has come in, and comes to be a kind of medium, for it measures all things and so likewise the excess and defect; for instance, how many shoes are equal to a house or a given quantity of food. As then the builder to the shoemaker, so many shoes must be to the house (or food, if instead of a builder an agriculturist be the exchanging party); for unless there is this proportion there cannot be exchange or dealing, and this proportion cannot be unless the terms are in some way equal; hence the need, as was stated above, of some one measure of all things. Now this is really and truly the Demand for them, which is the common bond of all such dealings. For if the parties were not in want at all or not similarly of one another’s wares, there would either not be any exchange, or at least not the same. And money has come to be, by general agreement, a representative of Demand: and the account of its Greek name νομισμα is this, that it is what it is not naturally but by custom or law (νόμος), and it rests with us to change its value, or make it wholly useless. Very well then, there will be Reciprocation when the terms have been equalised so as to stand in this proportion; Agriculturist : Shoemaker : : wares of Shoemaker : wares of Agriculturist; but you must bring them to this form of proportion when they exchange, otherwise the one extreme will combine both exceedings of the mean:[18] but when they have exactly their own then they are equal and have dealings, because the same equality can come to be in their case. Let A represent an agriculturist, C food, B a shoemaker, D his wares equalised with A’s. Then the proportion will be correct, A:B::C:D; _now_ Reciprocation will be practicable, if it were not, there would have been no dealing. Now that what connects men in such transactions is Demand, as being some one thing, is shown by the fact that, when either one does not want the other or neither want one another, they do not exchange at all: whereas they do[19] when one wants what the other man has, wine for instance, giving in return corn for exportation.