The Wealth of Nations by Part 2 Chapter 2 Page 22

But in what manner this operation is performed, and in what manner it tends to increase either the gross or the net revenue of the society, is not altogether so obvious, and may therefore require some further explication.

There are several different sorts of paper money; but the circulating notes of banks and bankers are the species which is best known, and which seems best adapted for this purpose.

When the people of any particular country have such confidence in the fortune, probity, and prudence of a particular banker, as to believe that he is always ready to pay upon demand such of his promissory notes as are likely to be at any time presented to him; those notes come to have the same currency as gold and silver money, from the confidence that such money can at any time be had for them.