Early modern Europe's most extensive commonwealth - the Republic of Letters - could not be found on any map. This republic had patriotic citizens, but no army; it had its own language, but no frontiers. From its birth during the Renaissance, the Republic of Letters long remained a small and close-knit elite community, linked by international networks of correspondence, sharing an erudite neo-Latin culture. In the late seventeenth century, however, it confronted fundamental challenges that influenced its transition to the more public, inclusive, and vernacular discourse of the Enlightenment.