Cosimo’s friendship with Gian dei Brughi instills in him a lifelong passion for reading and learning. After the bandit’s death, Biagio often finds Cosimo with a book, taking notes. Cosimo begins to seek out the Abbé Fauchelafleur with questions and for lessons, but the Abbé often isn’t much use. Instead, Cosimo regales the old man with tales about Rousseau and Benjamin Franklin, when he’s not hunting to pay for books. Gradually, Cosimo becomes the teacher and the Abbé the student, and Cosimo convinces the Abbé to spend more and more time in the trees arguing about monarchies, republics, religion, and empiricism—often for so long that the Abbé misses Biagio’s lessons.
Again, the ideas that Cosimo introduces to the Abbé Fauchelafleur, while normal by today’s standards (many democratic governments of today were built on Enlightenment theories of government), were new and scary for monarchs and church officials in charge at the time the novel is set. This means that the Abbé is undergoing a change his higher-ups won’t appreciate, as he’s learning about ideas that would jeopardize his role in the di Rondò family.
The old Abbé vacillates between passive acceptance and a latent passion for spiritual rigor. He absorbs all of Cosimo’s new ideas, likes them at first, and then passionately denounces everything. The Abbé begins purchasing books from Cosimo’s bookseller and eventually, the rumor circulates that a priest is purchasing the wickedest publications in Europe. The local religious tribunal comes to investigate, finds the works of Bayle in his room, and carries the man away. Cosimo is hunting and doesn’t know anything until later. Fearing an assassination attempt by the Jesuits, Baron Arminio locks himself in his room, and the Abbé spends the rest of his life unsure of what he believes in, but trying until the end to believe.
The Abbé’s genuine interest in Cosimo’s new ideas offers the possibility that even among individuals who, in a sense, shouldn’t be interested in Enlightenment ideals, those ideas are still interesting and worth considering. This validates the righteousness of Enlightenment ideas as a whole, while the Abbé’s arrest shows how dangerous it can be for someone like him to engage in this kind of academic rigor. Religious bodies, at this point, will only stay in power if they can stamp out these new ways of thinking that seek to take power away from religion.
The Abbé’s arrest doesn’t stop Cosimo’s education. He begins writing to the greatest scientists and philosophers of the age, and Biagio laments that he hasn’t been able to find his brother’s papers. Cosimo constructs shelves to hold his books and orders the entire set of Diderot and D’Alembert’s Encyclopedia. He begins to send for books about trades and professions and soon decides he wants to be useful to his neighbors (Biagio insists that Cosimo learned this from Gian dei Brughi). Cosimo learns to prune trees and so prunes orchards and gardens in winter. As he prunes, he trains the trees to grow into bridges for his own use, which prove helpful in his old age. Later generations, however, make it so no one can ever again live like Cosimo.
In this moment, Cosimo begins to come of age in several ways. He first begins to write and correspond with Enlightenment philosophers, which suggests that he’s joining the ranks of the philosophical elite and sharing his ideas. Then, when Biagio mentions that Cosimo wants books on trades, it falls in again with Rousseau’s beliefs regarding human development. This desire to learn a trade represents Cosimo’s leap to the next step of development, in which he begins to make himself useful to the community by providing work for it.