Here is a brief excerpt from an article by Parul Sehgal for The New York Times. To read the complete article, check out others, and obtain information about deep-discount subscriptions, please click here.

* * *

When testing a new pen, Leonardo da Vinci was in the habit of scrawling the phrase dimmi — tell me.

The doodles appear in the margins of his notebooks, those records of his ravenous, almost carnal curiosity about the natural world. Dimmi was his animating question. Dimmi, he wrote between scribbled shopping lists (eels, velvet, wine) and sketches of inventions, instructing himself: “Describe what sneezing is, what yawning is, the falling sickness, spasm.” Dimmi — tell me your secrets — he implored in his studies of the movement of water and the working of the woodpecker’s tongue.

Painter, engineer, anatomist, the designer of torture devices as well as machines to break men out of prison, Leonardo is heralded as the “real Renaissance man.” Never mind that this notion is reductive and plain wrong — or so argues the art historian Francesca Fiorani in her new book, The Shadow Drawing.

Leonardo’s interests were not as dizzyingly disparate as they seem. His mind sought synthesis. He was hunting basic principles, the fundamental laws of all nature. “Write of swimming under water,” he declared, “and you will have the flight of birds through the air.” His artistic and scientific interests were conjoined. In painting, Leonardo could apply all he learned about geometry, shadow and light, about the interplay of the eye and mind in perception.

By no means is Fiorani the first to make this case (the historian Sydney Freedberg elegantly described how knowledge was indivisible for Leonardo), but she makes it with fresh force and pitches it against the misconception that Leonardo abandoned painting for science in his later years.

Where did that idea come from? Sure enough: The unmistakable, chaos-sowing figure of Freud skitters in the shadows.

It was from Freud’s influential, and exuberantly fictionalized, study of Leonardo that many of our false impressions spring, including the idea of the “dual Leonardo” — the artist turned scientist. (This depiction is buried beneath the essay’s most memorable claim: Freud, in full sail, laments that Leonardo was excessively cuddled by his mother.) True, Leonardo’s output seems to support Freud’s case — the first charge at least. He was one of the least prolific painters of his era. In 40 years, he completed, at most, 15 paintings and left much work — including the “Mona Lisa” — deliberately unfinished. In contrast, a kind of graphomania seemed to seize him. By some counts, the notebooks run to 16,000 pages — only a fraction of which have been viewed.

In The Shadow Drawing, Fiorani argues that Leonardo’s artistic and scientific preoccupations likely shared their root in the seven-volume 11th-century manuscript Book of Optics, by the Arab philosopher Abu Ali al-Hasan Ibn al-Haytham, known in the West at the time as Alhacen.

There’s no proof that Leonardo read the text, but translations circulated among Italian painters. Born near Florence in 1452, the illegitimate child of a notary and a household servant, Leonardo was apprenticed to the artist Andrea del Verrocchio, one of the few professional avenues open to him. From his shadow drawings in his notebooks, we can see that he was already exploring optical effects in his 20s, perhaps ones he first encountered in Alhacen’s work.

Book of Optics gathered all the available knowledge of the field at the time: Galen’s study of the eye, Euclid’s and Ptolemy’s treatises on geometry, Aristotle’s suppositions about the soul. What Alhacen theorized, Leonardo put into practice.

Roland Barthes once wrote that the photographs that compel us never overpower or coerce; they attract us because they are pensive, they think. Leonardo’s paintings are stuffed with ideas, with suppositions on perspective and distortion, on how the rounded surface of the eye influences perception. They were living laboratories — which is Fiorani’s explanation for the unfinished paintings. Process simply became more alluring to Leonardo than a final product.

* * *

Here is a direct link to the complete article.

Parul Sehgal is a book critic for The New York Times. She was previously a columnist and senior editor at the Book Review. Her work has also appeared in the Atlantic, Slate, Bookforum and The New Yorker, and she was awarded the Nona Balakian Award from the National Book Critics Circle for her criticism.