Front Page Titles (by Subject) Value expresses Patio of Exchange. - Money and the Mechanism of Exchange
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Value expresses Patio of Exchange. - William Stanley Jevons, Money and the Mechanism of Exchange 
Money and the Mechanism of Exchange (New York: D. Appleton and Co. 1876).
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Value expresses Patio of Exchange.
We must now fix our attention upon the fact that, in every act of exchange, a definite quantity of one substance is exchanged for a definite quantity of another. The things bartered may be most various in character, and may be variously measured. We may give a weight of silver for a length of rope, or a superficial extent of carpet, or a number of gallons of wine, or a certain horsepower of force, or conveyance over a certain distance. The quantities to be measured may be expressed in terms of space, time, mass, force, energy, heat, or any other physical units. Yet each exchange will consist in giving so many units of one thing for so many units of another, each measured in its appropriate way.
Every act of exchange thus presents itself to us in the form of a ratio between two numbers. The word value is commonly used, and if, at current rates, one ton of copper exchanges for ten tons of bar iron, it is usual to say that the value of copper is ten times that of the iron, weight for weight. For our purpose, at least, this use of the word value is only an indirect mode of expressing a ratio. When we say that gold is more valuable than silver, we mean that, as commonly exchanged, the weight of silver exceeds that of the gold given for it. If the value of gold rises compared with that of silver, then still more silver is given for the same quantity of gold. But value like utility is no intrinsic quality of a thing; it is an extrinsic accident or relation. We should never speak of the value of a thing at all without having in our minds the other thing in regard to which it is valued. The very same substance may rise and fall in value at the same time. If, in exchange for a given weight of gold, I can get more silver, but less copper, than I used to do, the value of gold has risen with respect to silver, but fallen with respect to copper. It is evident that an intrinsic property of a thing cannot both increase and decrease at the same time; therefore value must be a mere relation or accident of a thing as regards other things and the persons needing them.
The Functions of Money
We have seen that three inconveniences attach to the practice of simple barter, namely, the improbability, of coincidence between persons wanting and persons possessing; the complexity of exchanges, which are not made in terms of one single substance; and the need of some means of dividing and distributing valuable articles. Money remedies these inconveniences, and thereby performs two distinct functions of high importance, acting as—
In its first form money is simply any commodity esteemed by all persons, any article of food, clothing, or ornament which any person will readily receive, and which, therefore, every person desires to have by him in greater or less quantity, in order that he may have the means of procuring necessaries of life at any time. Although many commodities may be capable of performing this function of a medium more or less perfectly, some one article will usually be selected, as money par excellence, by custom or the force of circumstances. This article will then begin to be used as a measure of value. Being accustomed to exchange things frequently for sums of money, people learn the value of other articles in terms of money, so that all exchanges will most readily be calculated and adjusted by comparison of the money values of the things exchanged.