Although the Arabs in the past were learned, maybe you imagined that the relics of their knowledge have been passed to their descendants. They are a hundred times more barbaric and ignorant than the Greeks in Athens today.
This is the line that made me stop and ask who gets to be the authority in these letters. Peiresc wants data about tides, mountains, and eclipses, and d’Arcos, who has lived in Tunis for years, answers by dismissing the people around him as unable to supply it. But his own explanation undercuts him: he says he is “ill and slow” and cannot do the fieldwork himself. So is the problem Tunisian ignorance, or his own limits, and why does he blame the society instead? He also admits that most people “do not know how old they are” and know only the “ceremony and law” of their religion, yet that is exactly the knowledge he must have gained by living among them. How can he claim to be an insider (“nos corsairs”) in one letter and an outside judge here?
I also wonder how much this is performance. Peiresc was horrified by the conversion, and d’Arcos may be reassuring him that he still shares European contempt for “barbarism.” If so, his survival in the archive may depend on that reassurance. This connects to the imbalance we keep returning to: the only convert who left a record did so by writing in French to a French scholar, and in doing so he describes the people who left no record as incapable of having knowledge. Do the voices that survive tend to flatter the readers they were written for? And what would the Tunisian observers he dismisses have said about the eclipse, the winds, or the sea, if anyone had asked them?