The Wealth of Nations by Part 1 Chapter 5 Page 20

nearly in the same condition. In the meantime the temporary and occasional price of corn may frequently be double, one year, of what it had been the year before, or fluctuate, for example, from five and twenty to fifty shillings the quarter. But when corn is at the latter price, not only the nominal, but the real value of a corn rent will be double of what it is when at the former, or will command double the quantity either of labour or of the greater part of other commodities; the money price of labour, and along with it that of most other things, continuing the same during all these fluctuations.

Labour, therefore, it appears evidently, is the only universal, as well as the only accurate measure of value, or the only standard by which we can compare the values of different commodities at all times, and at all places.