How did Francesco Melzi create the Treatise on Painting from Leonardo da Vinci’s notes?

After inheriting Leonardo da Vinci’s vast collection of manuscripts in 1519, Francesco Melzi created the Trattato della pittura (or Treatise on Painting) by extracting and organizing passages from those notebooks. This work, known as the Codex Urbinas and housed in the Vatican Library, was compiled sometime before 1542.1

It remains impossible to determine exactly how closely Melzi’s organization reflected Leonardo’s original intentions, as only about one-quarter of the sources used in the manuscript have been identified within Leonardo's surviving notebooks. Despite these uncertainties, the compilation serves as a significant record of the theoretical and practical ideas Leonardo intended to present, particularly his argument that painting is a science rooted in the liberal arts and the direct observation of nature.1