Around the time that he reached the unnerving milestone of turning thirty, Leonardo da Vinci wrote a letter to the ruler of Milan listing the reasons he should be given a job. He had been moderately successful as a painter in Florence, but he had trouble finishing his commissions and was searching for new horizons. In the first ten paragraphs, he touted his engineering skills, including his ability to design bridges, waterways, cannons, armored vehicles, and public buildings. Only in the eleventh paragraph, at the end, did he add that he was also an artist.

He had a reverence for the wholeness of nature and a feel for the harmony of its patterns, which he saw replicated in phenomena large and small. In his notebooks he would record curls of hair, eddies of water, and whirls of air, along with some stabs at the math that might underlie such spirals.

I asked […] Martin Clayton, whether he thought [ Maestro Da Vinci ] had done them as works of art or of science. Even as I spoke, I realized it was a dumb question. “I do not think that Leonardo would have made that distinction,” he replied.

I embarked on this book because [ Maestro Da Vinci ] is the ultimate example of the main theme of my previous biographies: how the ability to make connections across disciplines—arts and sciences, humanities and technology—is a key to innovation, imagination, and genius. Benjamin Franklin, a previous subject of mine, was a Leonardo of his era: with no formal education, he taught himself to become an imaginative polymath who was Enlightenment ███████’s best scientist, inventor, diplomat, writer, and business strategist. He proved by flying a kite that lightning is electricity, and he invented a rod to tame it. He devised bifocal glasses, enchanting musical instruments, clean-burning stoves, charts of the Gulf Stream, and ███████’s unique style of homespun humor. Albert Einstein, when he was stymied in his pursuit of his theory of relativity, would pull out his violin and play Mozart, which helped him reconnect with the harmonies of the cosmos. [ Maestro Lovelace ], whom I profiled in a book on innovators, combined the poetic sensibility of her father, Lord Byron, with her mother’s love of the beauty of math to envision a general-purpose computer. And [ Steve Jobs ] climaxed his product launches with an image of street signs showing the intersection of the liberal arts and technology. Leonardo was his hero. “He saw beauty in both art and engineering,” Jobs said, “and his ability to combine them was what made him a genius.

In fact, [ Maestro Da Vinci ’s] genius was a human one, wrought by his own will and ambition. It did not come from being the divine recipient, like Isaac Newton or Einstein, of a mind with so much processing power that we mere mortals cannot fathom it. Leonardo had almost no schooling and could barely read Latin or do long division. His genius was of the type we can understand, even take lessons from. It was based on skills we can aspire to improve in ourselves, such as curiosity and intense observation. He had an imagination so excitable that it flirted with the edges of fantasy,

At first I thought that [ Maestro Da Vinci ’s] susceptibility to fantasia was a failing, revealing a lack of discipline and diligence that was related to his propensity to abandon artworks and treatises unfinished. To some extent, that is true. Vision without execution is hallucination. But I also came to believe that his ability to blur the line between reality and fantasy, just like his sfumato techniques for blurring the lines of a painting, was a key to his creativity.

[ Maestro Da Vinci ] crammed every inch of his pages with miscellaneous drawings and looking-glass jottings that seem random but provide intimations of his mental leaps. Scribbled alongside each other, with rhyme if not reason, are math calculations, sketches of his devilish young boyfriend, birds, flying machines, theater props, eddies of water, blood valves, grotesque heads, angels, siphons, plant stems, sawed-apart skulls, tips for painters, notes on the eye and optics, weapons of war, fables, riddles, and studies for paintings.

My favorite gems in [ Maestro Da Vinci ’s] notebooks are his to-do lists, which sparkle with his curiosity. One of them, dating from the 1490s in Milan, is that day’s list of things he wants to learn. “The measurement of Milan and its suburbs,” is the first entry. This has a practical purpose, as revealed by an item later in the list: “Draw Milan.” Others show him relentlessly seeking out people whose brains he could pick: “Get the master of arithmetic to show you how to square a triangle…Ask Giannino the Bombardier about how the tower of Ferrara is walled…Ask Benedetto Protinari by what means they walk on ice in Flanders…Get a master of hydraulics to tell you how to repair a lock, canal and mill in the Lombard manner…Get the measurement of the sun promised me by Maestro Giovanni Francese, the Frenchman.” He is insatiable.

That followed [ Maestro Da Vinci ’s] injunction to begin any investigation by going to the source: “The one who can go to the fountain does not go to the water-jar."

The sexual act of coitus and the body parts employed for it are so repulsive that, if it were not for the beauty of the faces and the adornment of the actors and the pent-up impulse, nature would lose the human species.

Dante [Alighieri], whose Divine Comedy was beloved by [ Maestro Da Vinci ] and illustrated by Botticelli, consigned sodomites, along with blasphemers and usurers, to the seventh circle of hell. However, Dante displayed Florence’s conflicted feelings about homosexuals by praising in the poem one of the denizens he put into this circle, his own mentor, Brunaetto Latini.

[ Maestro Da Vinci ’s] homosexuality seems to have been manifest in his sense of himself as somewhat different, an outsider who didn’t quite fit in. By the time he was thirty, his increasingly successful father was an establishment insider and a legal adviser to the Medici, the top guilds, and churches. He was also an exemplar of traditional masculinity; by then he’d had at least one mistress, three wives, and five children. Leonardo, on the contrary, was essentially an outsider. The birth of his step siblings reinforced the fact that he was not considered legitimate. As a gay, illegitimate artist twice accused of sodomy, he knew what it was like to be regarded, and to regard yourself, as different. But as with many artists, that turned out to be more an asset than a hindrance.

According to Lomazzo, the other early biographer, “[ Maestro Da Vinci ] never finished any of the works he began because, so sublime was his idea of art, he saw faults even in the things that to others seemed miracles.

Perfecting the Adoration of the Magi must have been especially daunting. There were originally more than sixty characters in his underdrawing. As he went along, he reduced this number by turning some groups of fighters or builders in the background into fewer large-scale characters, but that still left more than thirty to be rendered. He was intent on making sure each one reacted emotionally to the others so that the painting would feel like a coherent narrative and not a random assortment of isolated characters.

Even more complex were the lighting challenges, made all the more difficult by his obsession with optics. On the bottom of a notebook page from around 1480 that shows the mechanisms of the crane that Brunelleschi used to erect Florence’s cathedral dome, [ Maestro Da Vinci ] sketched a diagram of how light rays hit the surface of a human eye and are focused inside the eyeball. In painting the Adoration of the Magi, he wanted to convey the power of the light that shone down from heaven with the Epiphany and how each rebound of reflected light affected the coloration and gradation of each shadow. “He must have faltered at the thought of how to balance the reflections that bounce from one figure to another and to control “that bounce from one figure to another and to control the myriad variables of light, shade, and emotions for such a multitude,” according to the art historian Francesca Fiorani. “Unlike any other artist, he could not ignore an optical problem.”

I do not know what to say or what to do, for everywhere I seem to find myself swimming head downwards through that mighty throat and remaining buried in that huge belly, in the confusion of death.

Many of the prophecy-riddles [A type of literary amusement that Leonardo pioneered in the 1490s and called them “prophecies,”. They were often little riddles or trick questions. He was particularly fond of describing some scene of darkness and destruction, in a style that mocked the prophets and doomsayers who hung around the court, then revealing that he was actually referring to something far less apocalyptic.] reflect [ Maestro Da Vinci ’s] love for animals. “Countless numbers will have their little children taken away and their throats shall be cut,” is one prophecy, as if describing a brutal act of war and genocide. But then Leonardo, who had become a vegetarian, reveals that this prophecy refers to the sheep and cows that humans eat.