… is from page 543 of Douglas Irwin’s essay “Adam Smith and Free Trade,” which is chapter 32 in the 2016 volume, edited by Ryan Patrick Hanley, Adam Smith: His Life, Thought, and Legacy:
As notes from his lectures at Glasgow University in the early 1760s attest, Smith was a convinced advocate of free trade from the very start of his career. Smith held that trade between nations was similar to trade between individuals. Human beings, he noted, have a natural propensity to truck, barter, and exchange goods with one other [sic]. If the exchange was voluntary, trade would not take place unless both parties were better off. And just as individuals were better off with trade, so were countries.