Complex sentence: Bullialdus was an active member of the Republic of Letters, the long-distance correspondence of many different intellectuals or "philosophes" as they were called in France, during the 17th and 18th centuries.
Split into simpler sentences: Bullialdus was an active member of the Republic of Letters. The Republic of Letters was the long-distance correspondence of many different intellectuals or "philosophes" as they were called in France, during the 17th and 18th centuries.