Front Page Titles (by Subject) The Standard Unit of Value. - Money and the Mechanism of Exchange
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The Standard Unit of Value. - 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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The Standard Unit of Value.
It is essential, in the first place, to decide clearly what we mean by a standard unit of value. This must consist of a fixed quantity of some concrete substance, defined by reference to the units of weight or space. Value may seem to some people to be a purely mental phenomenon, and a pound would then have to be defined, as Lord Castlereagh asserted, by a sense of value. But we might as well define a yard by a sense of length, or a grain by a sense of weight. Just as every quantity in physical science is defined by reference to some concrete standard specimen, so, if we are to measure and express value at all, we must fix upon definite quantities of one or more definite and unchangeable commodities for the purpose.
The expression standard unit of value, will indeed be almost inevitably misunderstood as implying the existence of something of fixed value. As we have seen, however (p. 11), value merely expresses the essentially variable ratio in which two commodities exchange, so that there is no reason to suppose that any substance does for two days together retain the same value. All that a standard of value means is, that some uniform unchangeable substance is chosen, in terms of which all ratios of exchange may be expressed and calculated, without any regard whatever to the feelings or mental phenomena which the commodities produce in men. For reasons already stated, one or other of the metals, gold, silver, or copper, has usually been considered most suitable for constituting the standard substance.
The absolute weight or magnitude of the unit of money is a matter of little or no importance, provided that all people agree upon the same unit, and that it be permanently and exactly defined, and afterwards adhered to. Before the English yard was fixed, it would not have mattered whether it was a few inches longer or shorter; it does not matter, indeed, whether the inch, the foot, the furlong, or the mile is the unit, provided that one of them is definitely fixed, and the others referred to it by known ratios. So, it is really indifferent whether we regard the pound troy of standard gold, or the ounce, or the fixed number of grains in the sovereign, as our standard. It is only requisite that every contract expressed in money shall enable us to ascertain exactly how much standard gold is due from one person to another.
M. Chevalier and some other continental economists have argued elaborately in favour of a universal standard unit of value, coinciding with the metric system of weights. They wish the unit of value to be ten grams of gold exactly, and seem to think that there is some magical efficacy in the correspondence of money and weights. This correspondence might perhaps be a slight convenience to those bullion dealers who have to calculate the metallic value of coins before melting or exporting them, or to those mint officials who have to adjust and test the weights of coins; to all other persons it would be a matter of complete indifference. Those who use coins in ordinary business need never inquire how much metal they contain. Probably not one person in ten thousand in this kingdom knows, or need know, that a sovereign should contain 123.27447 grains of standard gold. Besides, if we agree to accept a precise metrical quantity of one metal as our standard, the weights of the coins composed of other metals will be complicated fractional amounts, to be determined with reference to the accidental market values of the metals.
All we can say, then, is that the standard unit of value is some entirely arbitrary weight of the standard metal, the exact amount of which, being a matter of indifference on general grounds, should be fixed as seems most convenient in reference to the habits of nations or other accidental circumstances.