The Wealth of Nations by Part 4 Chapter 3 Page 49

By advantage or gain, I understand not the increase of the quantity of gold and silver, but that of the exchangeable value of the annual produce of the land and labour of the country, or the increase of the annual revenue of its inhabitants.

If the balance be even, and if the trade between the two places consist altogether in the exchange of their native commodities, they will, upon most occasions, not only both gain, but they will gain equally, or very near equally; each will in this case afford a market for a part of the surplus produce of the other; each will replace a capital which had been employed in raising and preparing for the market this part of the surplus produce of the other, and which had been distributed among, and given revenue and maintenance to a certain number of its inhabitants. Some part of the