… is from page 1 of the July 1961 edition of Harvard economist Gottfried Haberler’s A Survey of International Trade Theory  (footnote deleted):
Strictly speaking, it is neither possible nor essential to draw a sharp distinction between the problems of foreign and domestic trade. If we examine the alleged peculiarities of foreign trade, we find that we are dealing with differences in degree rather than with such basic differences of a qualitative nature as would warrant sharp theoretical divisions.
Trade is trade is trade. Any economic disadvantages that you can identify as arising from Americans’ trade with non-Americans, I can identify as arising also from some Americans’ trade with other Americans. Likewise, any economic advantages that you can identify as arising from some Americans’ trade with other Americans, I can identify as arising also from Americans’ trade with non-Americans.
If the key conclusion of the economic analysis of international trade – from the analyses of David Hume , Adam Smith , Jean-Baptiste Say , and David Ricardo  through the likes of much more modern scholars such as Haberler , Fritz Machlup , Leland Yeager , Jagdish Bhagwati , and Doug Irwin  – must be reduced to a sentence of no more than ten words, I propose this sentence: Political borders carry no natural economic significance; none – none whatsoever.