Front Page Titles (by Subject) CHAPTER XV: of a measure of value - Principles of Political Economy with some of their Applications to Social Philosophy (Ashley ed.)
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CHAPTER XV: of a measure of value - John Stuart Mill, Principles of Political Economy with some of their Applications to Social Philosophy (Ashley ed.) 
Principles of Political Economy with some of their Applications to Social Philosophy, ed. William James Ashley (London: Longmans, Green and Co., 1909, 7th ed.).
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of a measure of value
§ 1. There has been much discussion among political economists respecting a Measure of Value. An importance has been attached to the subject greater than it deserved, and what has been written respecting it has contributed not a little to the reproach of logomachy, which is brought, with much exaggeration, but not altogether without ground, against the speculations of political economists. It is necessary, however, to touch upon the subject, if only to show how little there is to be said on it.
A Measure of Value, in the ordinary sense of the word measure, would mean something by comparison with which we may ascertain what is the value of any other thing. When we consider farther, that value itself is relative, and that two things are necessary to constitute it, independently of the third thing which is to measure it; we may define a Measure of Value to be something, by comparing with which any two other things, we may infer their value in relation to one another.
In this sense, any commodity will serve as a measure of value at a given time and place; since we can always infer the proportion in which things exchange for one another, when we know the proportion in which each exchanges for any third thing. To serve as a convenient measure of value is one of the functions of the commodity selected as a medium of exchange. It is in that commodity that the values of all other things are habitually estimated. We say that one thing is worth 2l., another 3l.; and it is then known, without express statement, that one is worth two-thirds of the other, or that the things exchange for one another in the proportion of 2 to 3. Money is a complete measure of their value.
But the desideratum sought by political economists is not a measure of the value of things at the same time and place, but a measure of the value of the same thing at different times and places: something by comparison with which it may be known whether any given thing is of greater or less value now than a century ago, or in this country than in America or China. And for this also, money, or any other commodity, will serve quite as well as at the same time and place, provided we can obtain the same data; provided we are able to compare with the measure not one commodity only, but the two or more which are necessary to the idea of value. If wheat is now  40s. the quarter, and a fat sheep the same, and if in the time of Henry the Second wheat was 20s., and a sheep 10s., we know that a quarter of wheat was then worth two sheep, and is now only worth one, and that the value therefore of a sheep, estimated in wheat, is twice as great as it was then; quite independently of the value of money at the two periods, either in relation to those two articles (in respect to both of which we suppose it to have fallen), or to other commodities in respect to which we need not make any supposition.
What seems to be desired, however, by writers on the subject, is some means of ascertaining the value of a commodity by merely comparing it with the measure, without referring it specially to any other given commodity. They would wish to be able, from the mere fact that wheat is now 40s. the quarter, and was formerly 20s., to decide whether wheat has varied in its value, and in what degree, without selecting a second commodity, such as a sheep, to compare it with; because they are desirous of knowing, not how much wheat has varied in value relatively to sheep, but how much it has varied relatively to things in general.
The first obstacle arises from the necessary indefiniteness of the idea of general exchange value—value in relation not to some one commodity, but to commodities at large. Even if we knew exactly how much a quarter of wheat would have purchased, at the earlier period, of every marketable article considered separately, and that it will now purchase more of some things and less of others, we should often find it impossible to say whether it had risen or fallen in relation to things in general. How much more impossible, when we only know how it has varied in relation to the measure. To enable the money price of a thing at two different periods to measure the quantity of things in general which it will exchange for, the same sum of money must correspond at both periods to the same quantity of things in general, that is, money must always have the same exchange value, the same general purchasing power. Now, not only is this not true of money, or of any other commodity, but we cannot even suppose any state of circumstances in which it would be true.
§ 2. A measure of exchange value, therefore, being impossible, writers have formed a notion of something, under the name of a measure of value, which would be more properly termed a measure of cost of production. They have imagined a commodity invariably produced by the same quantity of labour; to which supposition it is necessary to add, that the fixed capital employed in the production must bear always the same proportion to the wages of the immediate labour, and must be always of the same durability: in short, the same capital must be advanced for the same length of time, so that the element of value which consists of profits, as well as that which consists of wages, may be unchangeable. We should then have a commodity always produced under one and the same combination of all the circumstances which affect permanent value. Such a commodity would be by no means constant in its exchange value; for (even without reckoning the temporary fluctuations arising from supply and demand) its exchange value would be altered by every change in the circumstances of production of the things against which it was exchanged. But if there existed such a commodity, we should derive this advantage from it, that whenever any other thing varied permanently in relation to it, we should know that the cause of variation was not in it, but in the other thing. It would thus be suited to serve as a measure, not indeed of the value of other things, but of their cost of production. If a commodity acquired a greater permanent purchasing power in relation to the invariable commodity, its cost of production must have become greater; and in the contrary case, less. This measure of cost is what political economists have generally meant by a measure of value.
But a measure of cost, though perfectly conceivable, can no more exist in fact, than a measure of exchange value. There is no commodity which is invariable in its cost of production. Gold and silver are the least variable, but even these are liable to changes in their cost of production, from the exhaustion of old sources of supply, the discovery of new, and improvements in the mode of working. If we attempt to ascertain the changes in the cost of production of any commodity from the changes in its money price, the conclusion will require to be corrected by the best allowance we can make for the intermediate changes in the cost of the production of money itself.
Adam Smith fancied that there were two commodities peculiarly fitted to serve as a measure of value: corn, and labour. Of corn, he said that although its value fluctuates much from year to year, it does not vary greatly from century to century. This we now know to be an error: corn tends to rise in cost of production with every increase of population, and to fall with every improvement in agriculture, either in the country itself, or in any foreign country from which it draws a portion of its supplies. The supposed constancy of the cost of the production of corn depends on the maintenance of a complete equipoise between these antagonizing forces, an equipoise which, if ever realized, can only be accidental. With respect to labour as a measure of value, the language of Adam Smith is not uniform. He sometimes speaks of it as a good measure only for short periods, saying that the value of labour (or wages) does not vary much from year to year, though it does from generation to generation. On other occasions he speaks as if labour were intrinsically the most proper measure of value, on the ground that one day’s ordinary muscular exertion of one man, may be looked upon as always, to him, the same amount of effort or sacrifice. But this proposition, whether in itself admissible or not, discards the idea of exchange value altogether, substituting a totally different idea, more analogous to value in use. If a day’s labour will purchase in America twice as much of ordinary consumable articles as in England, it seems a vain subtlety to insist on saying that labour is of the same value in both countries, and that it is the value of the other things which is different. Labour, in this case, may be correctly said to be twice as valuable, both in the market and to the labourer himself, in America as in England.
If the object were to obtain an approximate measure by which to estimate value in use, perhaps nothing better could be chosen than one day’s subsistence of an average man, reckoned in the ordinary food consumed by the class of unskilled labourers. If in any country a pound of maize flour will support a labouring man for a day, a thing might be deemed more or less valuable in proportion to the number of pounds of maize flour it exchanged for. If one thing, either by itself or by what it would purchase, could maintain a labouring man for a day, and another could maintain him for a week, there would be some reason in saying that the one was worth, for ordinary human uses, seven times as much as the other. But this would not measure the worth of the thing to its possessor for his own purposes, which might be greater to any amount, though it could not be less, than the worth of the food which the thing would purchase.
The idea of a Measure of Value must not be confounded with the idea of the regulator, or determining principle, of value. When it is said by Ricardo and others, that the value of a thing is regulated by quantity of labour, they do not mean the quantity of labour for which the thing will exchange, but the quantity required for producing it. This, they mean to affirm, determines its value; causes it to be of the value it is, and of no other. But when Adam Smith and Malthus say that labour is a measure of value, they do not mean the labour by which the thing was or can be made, but the quantity of labour which it will exchange for, or purchase; in other words, the value of the thing estimated in labour. And they do not mean that this regulates the general exchange value of the thing, or has any effect in determining what that value shall be, but only ascertains what it is, and whether and how much it varies from time to time and from place to place. To confound these two ideas would be much the same thing as to overlook the distinction between the thermometer and the fire.