… is from page 3 of the 1936 English-language edition (translated from German by Alfred Stonier and Frederic Benham) of Gottfried Haberler’s classic 1933 book, The Theory of International Trade With Its Application to Commercial Policy:
The classical theory starts from the fact that in international trade, as in all other economic activities, it is the individual economic subject who buys and sells, pays and is paid, grants and receives loans, and, in short, carries on the activities which, taken as a whole, constitute international trade. It is not, for example, Germany and England, but individuals or firms located in Germany and England, who carry on trade with one another.