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
