The Wealth of Nations by Part 1 Chapter 5 Page 10

liberty, and his happiness. The price which he pays must always be the same, whatever may be the quantity of goods which he receives in return for it.

Of these, indeed, it may sometimes purchase a greater and sometimes a smaller quantity; but it is their value which varies, not that of the labour which purchases them. At all times and places that is dear which it is difficult to come at, or which it costs much labour to acquire; and that cheap which is to be had easily, or with very little labour. Labour alone, therefore, never varying in its own value, is alone the ultimate and real standard by which the value of all commodities can at all times and places be estimated and compared. It is their real price; money is their nominal price only.

But though equal quantities of labour are