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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.