Trade and exchange across cultural lines have played a crucial role in human history, being perhaps the most important external stimuli to change, leaving aside the unmeasurable and less-benign influence of military conquest. External stimulation, in turn, has been the most important single source of change and development in art, science, and technology. Perhaps this goes without saying, since no human group could invent by itself more than a small part of its cultural and technical heritage. Take as simple an example as the manufacture of this book. The English language is derived from one of those that came into Western Europe with German immigrants, combined with elements of Latin, originally imposed by imperial conquerors from the south, plus other borrowings. The alphabet came from the Phoenicians. The page numbers are “Arabic,” which actually means that the Europeans learned about them from the Arabs, who had borrowed them in turn from the Indians, who invented positional notation in the first place. A few years ago, a book like this would have been printed with movable type, which would have given it a possible Chinese heritage as well.

On the negative side, cross-cultural trade and communication pose special problems. People with a different way of life are strangers by definition; their ways seem unpredictable, and the unpredictable is probably dangerous as well. Communication itself is difficult. Even after an appropriate medium comes into existence, like a second language in common, understanding is hard to come by. Strangers may appear not to be hostile, but they are still not to be trusted in the same full sense that neighbors and kinfolk can be trusted.