Kemp has devised a taxonomy of disinformed or eccentric ideas about Leonardo. There are the mystic theorists (who believe that secret messages about the nature of the cosmos are concealed in Leonardo’s work); heresy theorists (who believe that Leonardo was involved in some sort of religious cabal); geo theorists (who fall over themselves trying to identify the background landscape in the Mona Lisa and other paintings); attribution theorists (who keep wanting to put Leonardo’s name on work that isn’t his); drag theorists (who believe that the Mona Lisa depicts either Leonardo or one of his pupils dressed as a woman); and sci-fi theorists (pretty much exactly what you’d imagine).

Kemp calls Dan Brown, the author of The Da Vinci Code, the “godfather” of some of these speculations. “He is responsible for the idea that there are hidden codes, messages, mystic geometries, disguised words, and esoteric numbers in Renaissance paintings,” he said. Carmen C. Bambach, a curator in the department of drawings and prints at the Metropolitan Museum of Art and the author of an upcoming four-volume study, Leonardo da Vinci Rediscovered, hears from Leonardo disciples on the fringe at least once a week. A recent example: “Two men said that in Leonardo’s earliest dated drawing, The Arno Valley, from 1473, they saw elephants, camels, and birds in what is essentially a landscape. I commended them for their love of Leonardo, but said there are no elephants or other animals in the drawing.”