Didascalicon, Book II: the sciences, their parts and their discipline
Chapter XXIV. On navigation
Navigation includes all commerce in buying, selling and exchanging goods, domestic or foreign. This is rightly a kind of rhetoric of its own sort, because eloquence is most necessary to this profession. Hence he who is said to preside over eloquence, Mercury, is called as it were merkaton kyrios, lord of merchants. It penetrates the secrets of the world, visits unseen shores, traverses horrid deserts, and exercises the commerce of humanity with barbarous nations and unknown tongues. Its study unites peoples, quiets wars, confirms peace, and turns private goods to the common use of all.