The Wealth of Nations by Part 2 Chapter 2 Page 16

for a guinea, his revenue surely would not so properly consist in the piece of paper, as in what he could get for it. A guinea may be considered as a bill for a certain quantity of necessaries and conveniencies upon all the tradesmen in the neighbourhood. The revenue of the person to whom it is paid, does not so properly consist in the piece of gold, as in what he can get for it, or in what he can exchange it for.

If it could be exchanged for nothing, it would, like a bill upon a bankrupt, be of no more value than the most useless piece of paper.

Though the weekly or yearly revenue of all the different inhabitants of any country, in the same manner, may be, and in reality frequently is paid to them in money, their real riches, however, the real weekly or yearly revenue of all of them