De insulis et earum proprietatibus can be described as a sort of universal insular encyclopedia of its time, with distinctly medieval formal characteristics, such as its encyclopedism and organization as a dictionary in alphabetical order. It is also interspersed with more or less recent information, such as the account of Nicoloso da Recco’s expedition to the Canary Islands or details about Ireland obtained firsthand from Bishop Milo. These elements foreshadow future geographical discoveries and represent the first steps of the impending cultural renewal of 14th-century Italy.
The text consists of a preface, a true statement of intent in which Domenico Silvestri outlines the method to be followed, along with other questions, and the insular text itself, where 900 references to islands are listed in alphabetical order throughout the treatise.
The preface of De insulis begins with Silvestri's open declaration of his main purpose in composing his work: to complete Giovanni Boccaccio’s De montibus et silvis, de fontibus et lacubus et fluminibus ac etiam de stagnis et paludibus nec non et de maribus seu diversis maris nominibus. Boccaccio, in turn, had modeled and sourced his work on the near-homonymous work by Vibius Sequester. The renowned Florentine poet had omitted (or perhaps "avoided") including a section on islands in his geographical treatise, an oversight not unnoticed by Silvestri. Silvestri expressed his opinion on the importance of researching and classifying islands, stating:
"More remarkable events worthy of admiration, both reported and witnessed, are read about on islands than in marshes, lagoons, lakes, or forests."
He also stated that he intended to adopt the same method used by Boccaccio in his treatise: collecting in a single work all the information about islands scattered across ancient texts, much as others had done with rivers, lagoons, marshes, and so on. However, Silvestri's work ultimately diverged significantly from the Boccaccian model it was based on—not only in the number of islands and sources cited, which are far more numerous and heterogeneous, but primarily in theme. Unlike Boccaccio, Silvestri included islands known only in modern times in his manuscript.
The body of the work consists of 900 entries corresponding to as many island names, under which all available information about each island is organized. The arrangement follows a fairly strict alphabetical order, with only slight modifications. The enumeration of islands was not an original idea at the end of the 14th century (especially after the spread of Vibius Sequester’s repertoire), but it was undoubtedly a challenging task. In general, De insulis includes the islands of classical geography, those of the medieval encyclopedic tradition, and the latest findings from the most credible contemporary sources. Silvestri compiled the greatest amount of information available for each, driven by his passionate search for data and vast erudition.
There is no doubt that Giovanni Boccaccio (1313–1375) profoundly influenced Silvestri for many reasons: their close friendship, their teacher-student relationship, their shared intellectual pursuits, and hobbies. Focusing on Boccaccio’s significance as a source for Silvestri’s treatise, it is worth noting that Silvestri drew material from five of Boccaccio’s works:
- De casibus virorum illustrium, a nine-book collection of the fates of illustrious men, initially favored and later abandoned by fortune. It is mentioned only once.
- De montibus, silvis, fontibus, lacubus, stagnis seu paludibus, modeled on and sourced from the near-homonymous work of Vibius Sequester, is a small repertoire of geographical names found in classical texts. This work is frequently cited in De insulis, in addition to being the inspiration for Silvestri’s treatise, as stated in his preface.
- In his Zibaldone, Boccaccio transcribed a geographical extract from the voluminous chronography of Friar Paolino Minorita and the treatise De Mappa Mundi, known as Descriptio maritime Syrie, which Silvestri cites only once (s.v. Aradia).
- Silvestri also utilized Boccaccio’s De Canaria et insulis reliquis ultra Hispaniam noviter repertis for information about the Canary Islands, a document of exceptional importance for the history of the islands.
- Finally, Genealogia deorum gentilium, a treatise compiling ancient myths with moral and allegorical interpretations, is consistently used by Silvestri for mythological topics. This work played a crucial role in the transmission of classical mythology to the Renaissance.
In summary, De insulis et earum proprietatibus reflects Silvestri’s comprehensive and innovative approach to insular geography, marking a significant departure from his Boccaccian predecessor while contributing to the burgeoning geographical knowledge of the late Middle Ages.