… is from page 506 of the 2003 Liberty Fund collection, edited by Henry C. Clark, Commerce, Culture, and Liberty: Readings on Capitalism Before Adam Smith; specifically, it’s part of an excerpt from the Scotsman William Robertson’s 1766 book, A View of the Progress of Society in Europe:
Commerce tends to wear off those prejudices which maintain distinction and animosity between nations. It softens and polishes the manners of men. It unites them, by one of the strongest of all ties, the desire of supplying their mutual wants. It disposes them to peace, by establishing in every state an order of citizens bound by their interest to be the guardians of public tranquility.