2. Sources and Contacts

The evidence of Leonardo's notes and correspondence is of a man with wide contacts and reading. Sometimes he cites oral reports. In the Madrid Codex he reports that Julius had seen a case of a wheel being worn out while in Germany.^[11] He also considers letters. In one of his prophetic statements he mentions "Writing letters from one country to another. Men will speak from very remote countries to one another, and reply."^[12] He writes business letters to his employers such as the Duke of Milan and the Pope. Letters are also one of the ways he searches for evidence, as when he reminds himself to write to Bartholemew the Turk on the question of tides and specifically about the Caspian sea.^[13]

Leonardo is also a reader of books and often he cites the evidence that he finds in these. Sometimes he refers generally to Aristotle's De caelo ^[14] or Euclid's Elements.^[15] Sometimes he refers to a specific book and chapter of Aristotle's Physics ^[16] or to given propositions in Euclid. At least twenty propositions ranging from books 1 to 5 of the Elements are cited explicitly^[17]. In his study of ancient weapons in the Manuscript B he cites a wide range of ancient authors: Pliny^[18], Virgil^[19], Lucretius^[20], Aulus Gellius^[21], Livy, Plautus, Flavius^[22], Lucan, Caesar^[23], Quintilian, Varro^[24], Plutarch^[25], Hermes Trismegistus, Pompeius Festus^[26], and Ammianus Marcellinusi^[27]. Elsewhere he cites Plato^[28] and Vitruvius^[29]. Mediaeval sources include Swineshead^[30], Thabit ibn Qurra^[31], Peckham^[32], a Treatise on the abacus ^[33], as well as Biagio Pelacani da Parma.^[34] Contemporary sources include Leon Battista Alberti.^[35] We know that he had a personal collection of 119 books.^[36] We know also that he did not limit himself to the books he owned himself. There are numerous references to books in other collections. For instance he takes note of a copy of Witelo which is said to be in the library at Pavia^[37] and a copy of Archimedes in from the Bishop of Padua^[38]. This is not exactly an intellectual vacuum, especially in 1494, four decades after the advent of printing.

In addition to books and correspondence there were his direct contacts with contemporaries. He studied and wrote in the margin on seven pages in one of Francesco di Giorgio Martini's manuscripts (Florence, Cod. Ashburnham 361). In 1490, he travelled with this engineer to Pavia to visit Fazio Cardan, the learned editor of Peckham's Optics and father of Jerome Cardan. In 1494 he bought a copy of Luca Pacioli's Summa and subsequently worked together with this mathematician for three years at the court of Milan (1496-1499). At the same court, Leonardo was "as a brother"^[39] with Jacopo Andrea da Ferrara, a leading architect and Vitruvian commentator of the time. As an employee of Cesare Borgia he was a colleague of Machiavelli. In the period 1508-1510 he appears to have worked with Marc Antonio della Torre a professor of anatomy at Milan. In Rome (1513-1515) he worked for the Pope on a sixteenth century version of the star wars project, using burning mirrors as described by Archimedes as a means of defence. The fame of his activities caught the attention of the King of France who persuaded Leonardo to move to France for an early "retirement". The curriculum vitae of a recluse would look somewhat different.