… is from page 434 of Book IV, Chapter 1 of the 1981 Liberty Fund edition of Adam Smith’s timeless 1776 book, An Inquiry Into the Nature and Causes of the Wealth of Nations:
That foreign trade enriched the country, experience demonstrated to the nobles and country gentlemen as well as to the merchants; but how, or in what manner, none of them well knew. The merchants knew perfectly in what manner it enriched themselves. It was their business to know it. But to know in what manner it enriched the country was no part of their business.