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
