The Wealth of Nations by Part 1 Chapter 5 Page 17

purchase or command more nearly the same quantity of the labour of other people. They will do this, I say, more nearly than equal quantities of almost any other commodity; for even equal quantities of corn will not do it exactly. The subsistence of the labourer, or the real price of labour, as I shall endeavour to show hereafter, is very different upon different occasions; more liberal in a society advancing to opulence than in one that is standing still; and in one that is standing still than in one that is going backwards.

Every other commodity, however, will at any particular time purchase a greater or smaller quantity of labour in proportion to the quantity of subsistence which it can purchase at that time. A rent therefore reserved in corn is liable only to the variations in the quantity of labour which a certain