The Wealth of Nations by Part 1 Chapter 5 Page 31

If the regulated value of a guinea, for example, was either reduced to twenty, or raised to two-and-twenty shillings, all accounts being kept and almost all obligations for debt being expressed in silver money, the greater part of payments could in either case be made with the same quantity of silver money as before; but would require very different quantities of gold money; a greater in the one case, and a smaller in the other. Silver would appear to be more invariable in its value than gold. Silver would appear to measure the value of gold, and gold would not appear to measure the value of silver. The value of gold would seem to depend upon the quantity of silver which it would exchange for; and the value of silver would not seem to depend upon the quantity of gold which it would exchange for.

This difference, however,