3
Body and Soul
Why is it important to establish that Leonardo read optical manuals early in his life? And why is it important that he read this specific tract written in Arabic by an eleventh-century philosopher? Because the author of this book, Abu Ali al-Hasan Ibn al-Haytham (about 965–1041), conceived of sensory experience differently from other optical writers—and that conception, it seems likely, was fundamental to Leonardo’s art from the very beginning. This celebrated philosopher was known in Leonardo’s time as “Alhacen” (or Alhazen), a Latinized form of his given name al-Hasan. In his Book of Optics (Kitab al-Manazir), he argued that sensory experience was the foundation of our knowledge of the world. More important, he provided experience-based examples of optical illusions related to the air that light must travel through to reach an observer. No other optical writer explored the subject in such detail.
But let’s start from the beginning.
Why optics? And why Alhacen?
As odd as it may seem to us today, optics was a hot topic in the Renaissance. It was considered a science that stood between natural philosophy and physics and was taught in most European universities, together with astronomy, as part of the medical curriculum. In fact, thanks to optics, astronomers learned how to analyze the shadows celestial bodies cast on one another and to use those shadows to better calculate their orbits and shapes; as Leonardo himself wrote, “the science of astronomy […] is merely optics [prospettiva] since it consists of visual lines and intersected pyramids.” Astronomy was also seen as essential to the practice of medicine: doctors consulted the stars to diagnose illnesses, and it was believed that cures were most effective when the stars were favorably aligned. This was the reason Renaissance doctors like Toscanelli and Marsilio Ficino were often authors of optical books.
For preachers, optics provided a way to talk about the divine. As Antoninus, archbishop of Florence from 1446 to 1459, wrote in one of his sermons, “The light of grace pervades the world according to optical principles.” As we have seen, merchants were keen readers of optical manuals, as this science taught them how to measure distances indirectly through the position of stars and planets in the sky and to find new trade routes.
Craftsmen used optics to make eyeglasses and mirrors. Architects who followed the approach of the Roman architect Vitruvius, whose architectural manual was the only book on any art that survived from antiquity, found it helpful to solve “difficult questions involving symmetry” and to find “light in buildings,” by which Vitruvius meant orienting buildings to maximize sunlight. Painters learned the basic rules of optics in order to represent three-dimensional objects on a flat surface—rules that by the second half of the fifteenth century had been codified into simple workshop procedures.
Like his fellow citizens, Leonardo learned about the most basic applications of optics by simply carrying on with his life. Whether by mingling with patrons and fellow artists, strolling past Florence’s cathedral, or going to Mass, chances are he heard conversations about light, vision, mirrors, reflections, the length of the solar year, and the size of the ocean: these were all topics that were discussed in the Florence of his day.
He may have also read about optics in popular vernacular books that were taught in abacus schools. Cecco d’Ascoli’s The Bitter Age included cursory discussions on the propagation of light and its reflection in mirrors. Dante’s Comedy offered a learned discussion on light. He might well have read The Composition of the World, a cosmology book written by a thirteenth-century scholar named Ristoro d’Arezzo, who, like Leonardo, was proficient in “drawing things of the world [le cose del mondo]”—although by Ristoro’s own admission nothing delighted him more than “the science of the stars.” Leonardo might have borrowed this rarer text from one of his acquaintances, perhaps from Toscanelli, or from Antonio Manetti, who was an influential “master of optics [maestro di prospettiva]” in Florence in the 1470s. He could even have potentially borrowed it from the Benci brothers, who were distant relatives of his friend Giovanni d’Amerigo Benci and active in Neoplatonic circles, the groups that defined elite culture in late fifteenth-century Florence. And, of course, he had been exposed to the optics of burning mirrors in Verrocchio’s bottega.
But, since antiquity, optics had been much more than a set of rules by which to make eyeglasses, cure maladies, orient buildings, find new trade routes, and study astronomy. Optics, in its most refined formulation, was natural philosophy tout court.
When it was invented in antiquity, optics was the science that explained how vision works: the English word “optics” is derived from the Greek verb opteuo, which means “to see,” and from the noun ops, which means “eye.” But explaining how vision works entailed understanding how the soul—psyche in Greek and anima in Latin—processes information gathered by the eye. This, in turn, meant wrestling with one of the fundamental questions of natural philosophy: How do we know the world?
Aristotle had discussed sensory experience as the basis of our knowledge of the world in his book On the Soul (De anima), a text widely read in the Renaissance, and had regarded sight as “the most highly developed sense.” He was unequivocal in stating that “sight can never be in error” but that our judgment “may be mistaken”—as, for instance, when we perceive something white in front of us, but our judgment of what this white thing is “may be false.” Optics, for Aristotle and later philosophers, was a philosophical investigation of the soul and, more specifically, the study of the role of visual experience in the acquisition of knowledge.
Michele Savonarola, a scholar from Padua and the father of the famous preacher Girolamo Savonarola, understood this: he wrote in 1445 that “the optics [perspectiva] of artist, which deals with the projection of rays,” is in fact “part of philosophy.” Decades later, Leonardo would share this view: “without optics [prospettiva] nothing can be done well in painting,” he wrote in a note he meant to include in the book on painting he planned to write.
Gentlemen of the time—of which Leonardo was not one—read the canonical textbooks written by three clergymen in the thirteenth century: Roger Bacon’s Perspectiva (c. 1265), Witelo’s Perspectiva (c. 1275), and John Pecham’s Perspectiva communis (c. 1280). These authors were priests—Bacon and Pecham were Franciscans—and they wrote their optical books while residing at the papal court, which was then in the small town of Viterbo, about fifty miles from Rome. The court had become a center for the study of human anatomy. Their books were written in Latin for university students, not artists, and they were modeled on Alhacen’s Book of Optics, which had been written in Arabic, a language that was widely read and spoken around the Mediterranean in the Middle Ages. The Book of Optics was nonetheless translated into Latin sometime in the first half of the thirteenth century—most likely in Andalusia, where Arab and Latin scholars routinely worked side by side. The title of the Latin translation—De aspectibus (On Visual Appearances)—acknowledged the fundamental role optical illusions played in Alhacen’s philosophical system.
Young Leonardo knew little Latin, although he was a sophisticated visual thinker and may have learned quite a bit from the illustrations and diagrams of these high-end optical books. He could have accessed them in Florence either through an acquaintance or a patron, or in the library of San Marco, one of the very few public libraries in Europe at the time.
The library had been established by the Medici, and it contained over four hundred books ranging from various editions of and commentaries on the Holy Scriptures to an enviable corpus of volumes by ancient authors on philosophy, astronomy, mathematics, optics, geometry, history, poetry, literature, and even profane topics. Cosimo de’ Medici had created this remarkable library from the books he had acquired wholesale from an inspired but financially inept Florentine bookseller, who had spent his life gathering texts by ancient authors and copying them for collectors across Europe, but who had also accumulated a staggering debt of over six thousand florins. Cosimo paid the debt and got the books in exchange, but instead of keeping them for himself he donated them to the Dominican monastery of San Marco, a religious institution near his palace that he supported. He even kept a personal cell there, as he liked to break his banking routine and move to the convent for a few days to meditate with the monks. The books themselves were kept in a church-like space commissioned from his favorite architect, Michelozzo.
Sixty-four wooden benches were lined up in a beautiful hall with a vaulted ceiling and slender columns. Tall windows maximized the availability of sunlight for reading. The books were stored flat on the benches’ built-in shelves—religious books in the benches on the east side, philosophy and science books in the benches on the west side. A metal chain attached each book to its bench. The chain was long enough for readers to position the book comfortably on the desk, but the books could not be borrowed. Theft was a real issue.
These chained books (libri incatenati) made an impression on Leonardo, who mentioned them in his notes on burning mirrors. In the library of San Marco, in his twenties, he read an illustrated work on shadows and later jotted down a note to remind himself to copy some of the drawings: “reserve until the end of my book on shadows the figures which could be seen on the writing desk of Gherardo the illuminator at San Marco in Florence.” The “illuminator” was the painter Gherardo di Giovanni di Miniato, who lived as a lay brother in the convent, but the work on shadows that Leonardo saw on his desk cannot be identified, although the historian of science Dominique Raynaud has suggested recently that it was a “Latin optical source, apparently a scholastic disputation, contained in three books of numbered propositions, from the late fourteenth-century Paris school.” We do not know if Leonardo read any of the optical books that were stored in a bench marked with the Roman numeral XIX on the west side of the library of San Marco. But we do know that he searched for the book by “Roger Bacon in print” and another by “Witelo in San Marco” in his early fifties and read Pecham’s book in his forties. Those were the three canonical books on optics.
But these books were all written in Latin, and Alhacen’s book was the only major optical book available in Italian.
It had been translated in the fourteenth century with the title De li aspetti.
One copy of this Italian translation—or at the very least a large portion of it—was in the hands of an artist who lived near Verrocchio’s workshop and who was in contact with Leonardo. In addition to being written in a language artists could read, Alhacen’s book dealt extensively with visual illusions (Alhacen called them deceptiones visus) that were potentially of great interest to artists but that his medieval followers—Bacon, Witelo, and Pecham—had dealt with more cursorily in their books.
Alhacen was the philosopher who had redefined the field of optics, the auctor perspectivae, as Pecham called him reverentially. But he was just one of many Arab scholars whose work flourished in Spain, northern Africa, Iran, and Persia in the years between 800 and 1258. These scholars translated into Arabic works from ancient Greek and Roman authors as well as from authors who had lived in regions of the Islamic empire, which in its largest area extended from Spain and northern Africa to the Arabian peninsula, Iraq, and Persia. Books on astronomy, geography, medicine, physics, anatomy, philosophy, and mathematics that had originally been written in Greek, Latin, Sanskrit, Assyrian, and Persian were made available in Arabic and, through these translations, were preserved for posterity. In the course of the centuries, those Arabic translations made their way to the Latin West, where they were retranslated, first into Latin and later into vernacular languages, contributing to the advancement of modern philosophy and science.
A native of the city of Basra on the Persian Gulf, in modern-day Iraq, Alhacen moved to the al-Azhar Mosque in Cairo sometime after 1021 to devote his life to learning. He was one of the first philosophers to stress the importance of experiments and mathematical demonstrations to confirm hypotheses. Some regard him as the initiator of experimental science, and while this may be an exaggeration, there is no question that, as the historian of science and Alhacen expert A. Mark Smith puts it, “he took an overwhelmingly empirical, or inductive, tack in analyzing light and vision.” Alhacen was an extraordinarily prolific author, and over 180 texts are credited to him, most of which are now lost. He made his name with the Book of Optics, the highly learned, seven-volume tract about direct, reflected, and refracted light that he wrote between the years 1028 and 1036.
The novelty of Alhacen’s book was that it presented the mechanics of vision—literally how the eye works—alongside a philosophical study of the soul and its ability to “process” visual data, while also offering a geometrical explanation of optical phenomena. It combined for the first time in one single philosophical system different strands of optical research that had been separated in previous centuries: anatomy, natural philosophy, and geometry. After Alhacen, one no longer needed to read what Aristotle, Euclid, Ptolemy, and Galen each had to say about optics. Alhacen brought their views into a single understanding of the visual world. One could go directly to Alhacen’s Book of Optics and find there a philosophical system that combined Galen’s anatomy of the eye, Aristotle’s philosophical study of sensory experience and the soul, and Euclid’s and Ptolemy’s geometry, all illustrated by a plethora of examples and diagrams. The book was not “a mere agglomeration of past ideas” but rather “a synthesis” that, according to Smith’s assessment, incorporated ideas of the past “into a seamless whole.”
Alhacen began his book with the anatomy of the eye, which he based almost entirely on the work of Galen of Pergamum, the second-century C.E. Greek physician who had written the most important book on human anatomy. From Galen he lifted two fundamental notions: that the soul resides in the brain and not in the heart, as Aristotle had believed; and that the eyes and the soul are physically connected by optical nerves. His medieval followers adopted the same view, and centuries later Leonardo set for himself the task of fixing the exact location of the soul in the brain. For Alhacen and other philosophers, it was enough to simply talk about the soul, but for Leonardo such knowledge had to be visualized and mapped with cartographic precision—which is what he did by way of stunning drawings of the human skull.
Building on Galen’s anatomy of the eye, Alhacen reasoned that visual data that travel along optical nerves are more reliable than sensory data coming from other senses—smell, touch, taste, and hearing—because they had to travel the shortest distance inside the body to get to the soul. This meant that sight was the prince of the senses—a notion Leonardo would adopt for the book on painting he planned to write.
The eye never errs: this was the central tenet of Alhacen’s philosophical system—and one of Leonardo’s foundational beliefs when he wrote that “experience does not err, but rather your judgements err when they hope to exact effects that are not within her power.”
In truth, Aristotle had briefly mentioned the infallibility of the eye, and Galen had explained its anatomy, but legions of other philosophers, inspired by Plato, challenged this notion as they considered visual experience to be false, deceptive, and unreliable. No philosopher had previously built an entire philosophical system on the notion that the eye never errs.
It never errs, Alhacen reasoned, because it is the perfect organ to capture the only aspect of the physical world that we “see”: color (Aristotle had made the same argument). In other words, we do not see the shape or motion of things, only their colors. From color and its variations due to light and shadow, we deduce everything else about the world: shapes, distances, textures, sizes, motion, etc. Color is inseparable from light: “The form of color is always mingled with the form of light and is not separable from it,” Alhacen wrote, adding that “sight senses light only when it is mingled with color.”
Today, thanks to Isaac Newton, we know that light is color, but Alhacen and his medieval followers thought they were different entities: color belonged to the object, whereas light was the condition that made sight possible. Leonardo repeated these views almost verbatim in many of his writings, but he was an artist writing for other artists, and so he transformed Alhacen’s teachings into instructions for painters: “the quality of colors is revealed by means of light […] Hence, painter, remember to display the true qualities of color in the illuminated parts.”
All the eye sees are luminous colors, which Alhacen called “forms” (sura in Arabic, forma in Latin). Thousands of these forms radiate as rays from all the points of an object and scatter in all directions. These forms travel to the observer’s eye through air and other transparent bodies, bringing information only about the particular points of the object from which they originate. This means that for Alhacen, forms were only radiations of color mixed with light from one of its points. Later Leonardo illustrated these radiating forms in a beautiful drawing.
To study the trajectory of forms in a mathematically precise way, Alhacen adopted Euclid’s geometry, which Euclid himself had applied to optics. He used Euclid’s notion of visual rays to analyze the paths through which forms “travel” to reach the eye, but he did not share Euclid’s and Ptolemy’s view that these visual rays are mathematical abstractions and that forms are immaterial. For Alhacen, forms were physical objects: they radiated outward through a continuous, transparent medium and traveled along imaginary lines—visual rays—to the eye, upon which they made a physical impression.
He subscribed to what was known as “intromission theory,” which argues that “vision cannot be due to some physical substance that comes out of the eye and travels to the visible object” but rather to forms of luminous colors that travel to the eye through air and other transparent bodies. In other words, from the infinite variations of forms that strike the eye we learn everything about the world.
Alhacen’s philosophy of the soul was complex, his prose convoluted. It is difficult to summarize, but it is based on several guiding principles:
Contrary to Plato and other influential philosophers, but in agreement with Aristotle, Alhacen maintained that we do not have preexisting mental ideas of the world fixed in the soul that correspond with what we see in the world. Instead, we collect data through the senses, primarily the sense of sight, and transmit these sensory data to the soul via optical nerves for further elaboration. In other words, we form our judgments of the world from intuitive perceptions, not from preexisting ideas, a notion that had a lasting influence on Leonardo.
It is equally important to understand that for Alhacen, we do not form our judgments solely based on the eye, which is part of our physical body, or solely based on the soul, which is immaterial but nonetheless exists only within a material body. Instead, body and soul work together in a constant back-and-forth exchange, comparing new sensory data with previous experiences stored in the memory, which is part of the soul. Aristotle had previously talked about the interaction of body and soul, but Alhacen expanded his understanding of such interactions; for him, the exchange of soul and body brings us knowledge of an object’s characteristics, or its “visual intentions [intentiones sensibiles],” as Alhacen called them, such as an object’s size and shape, its spatial relation to other objects, its distance from the observer, if it is in motion or at rest, if its surface is rough or smooth, if its body is transparent or opaque, if it is made of one continuous part or of separated parts, if it stands in darkness or in shadow, if it is beautiful or ugly. Some “visual intentions” we identify quickly, for instance shape or color. Others require a slower process of repeated visual scrutiny and judgment, such as an object’s distance from us, or whether it is beautiful or ugly. The perceived characteristics of objects are continuously adjusted, modified, and perfected based on new intuitive perceptions. This means that knowledge is always temporary, and time is required to fine-tune our knowledge of the world—for instance, Alhacen argues that it takes longer to distinguish two identical twins than two unrelated men.
This was precisely the way Leonardo went about examining the world and painting it. The downside of his approach was that it made it very difficult to determine when a portrait of an object or person was “complete”—and in fact Leonardo was never done with anything, not a painting, not a book, not a building, not a machine.
While Alhacen believed the eye never errs, he was nonetheless fascinated by “visual illusions,” as he called them.
Like modern neuroscientists who learn how the brain works by looking at its abnormal behavior, Alhacen focused on “why, when, and how, sight happens to be deceived.” In his account, the never-erring eye will perceive “a visible object correctly” when the eight conditions of sight are met, which are, again, distance between the eye and the object, a facing orientation, light, size, opacity, transparency in the air, time, and a healthy eye. But “if an object lacks any of them it is still perceived by sight, but it will not be perceived correctly.” In addition, because he was a systematic and experience-based thinker, and also because he was so deeply concerned with a philosophical study of the soul, Alhacen established for each condition a “proper range of moderation” according to which “the visible object will appear as it actually exists,” but “if one or more of these conditions falls very far outside that range, sight will not perceive the object as it actually exists.”
He filled his tract with many examples, all based on firsthand observations, analyzing in excruciating detail and demonstrating with intricate geometrical diagrams how unfavorable conditions send contradictory sensory data to the soul. He went through this detailed analysis because he wanted to explain his main claim: “[the] only reason sight errs in perceiving forms is because one or more of the aforementioned conditions has fallen outside [the range of] moderation.”
His observations on human flesh seen through transparent fabrics are exemplary of the attention to detail he displayed. “If a sheer cloth is placed in front of the eye and a body is seen behind that cloth, the color of the body will appear mixed [with that of the cloth],” he wrote. He then proceeded to differentiate “the color of the cloth [that] reaches the eye only from the threads” and “the colored spots on the body [that] only reach the eye through the interstices in the cloth.” He reasoned that if there is no “perceptible separation” between the threads and the interstices through which the body is seen, cloth and flesh “appear to coalesce, so their colors appear as a perfect blend.” The effect of blended color between body and fabric is even more evident if a body is seen through woolen cloth, which has “the threads […] covered with hair” and its “interstices […] made even narrower.”
One could not ask for a more detailed explanation of the mingling of colors and complexion, effects that Leonardo was keenly aware of: “Do not make the boundaries around your figures of any color other than its adjoining background. That is, do not make dark outlines between the background and your figure,” he told painters.
Because light affects the way we see, Alhacen analyzed every possible kind of light—candlelight, moonlight, sunlight, daylight, firelight, starlight, and even light coming from a window or through a pinhole inside “dark chambers” he created appositely to study the trajectories of light rays at different moments of the day. Later, Leonardo would study in great detail daylight coming into a dark room from a window.
But, as Aristotle had explained, color and light can be perceived only if they travel through a continuous substance with some level of opacity, such as air or water. This explains why Alhacen submitted the “transparency of the air” to close scrutiny: “an inordinate decrease in the transparency of the air is a cause of error,” especially “when the air is hazy.”
Interestingly, the example he used to describe the “transparency of the air” is the exact same example Leonardo would use to explain aerial perspective. “If the air is misty or dusky, as usually happens in the morning, and if there is a tower facing the eye at a moderate distance,” Alhacen wrote, “it will be judged by sight to lie further away than it actually is” because the misty air hides the ground “according to which the distance of the tower is gauged.” Centuries later, Leonardo wrote about aerial perspective in painting:
There is another perspective which we call aerial, because through variations in the air we are made aware of the different distances of various buildings whose bases apparently arise from a single line, as might be seen with many buildings beyond a wall, all of which appear the same size over the top of the said wall. And if you wish in painting to make one appear more distant than the other, you should represent the air as rather dense.
Alhacen’s goal was to show that regardless of their source and the medium through which they travel—air or water—color and light follow the same identical laws of reflection and refraction. But what he ended up most notably providing was an unprecedentedly exhaustive set of observations on reflected colors generated by different light sources.
Often, Alhacen illustrated “visual illusions” with examples taken from the art of painting. For instance, the soul judges “the hair of somebody who is depicted in a painting” as having “texture because that texture is represented by the painting.” But it arrives at this conclusion “by resemblance” as it knows that “real hair has texture” and it supposes that “there is texture in the painted hairs according to the way their form is represented.” Similarly, the soul operates by resemblance in the case of “clothing with designs and with the hair of animals that are represented in paintings”: it sees texture in a painting where “instead of actual texture there is utter smoothness.” This was a philosophical explanation of trompe l’oeil: the soul operates “by resemblance” when it looks at a painting.
And then there were Alhacen’s ideas on beauty—the hardest “visual intention” of an object to detect. Unlike color, shape, or size, which eye and soul apprehend quickly, beauty requires a slow, repeated process of back-and-forth between eye and soul. It resides in “the configuration [compositio] of different intentions,” in their “proportionality or harmony,” and in the way the parts relate to the whole.
The ultimate example of this composite idea of beauty is the human face, which is beautiful when facial features are “proportionate among each other as well as to the breadth of the face,” not when individual features are beautiful. His Renaissance readers did not hesitate to apply his thoughts on proportion to art: “Proportions create beauty more than any other quality [intenzione],” the sculptor Lorenzo Ghiberti wrote in the early fifteenth century, quoting Alhacen almost literally.
To read Alhacen on visual illusions was to never again look at light and color the same way.
The only problem with Alhacen’s catalog of “visual illusions” was that it was excruciating in details and, thus, an absolute bore to read.
Understandably, his medieval followers found it unpalatable and cut it to the bone. From their point of view, there was no need to have university students, astronomers, and doctors go through so many examples to make the simple point that light and color behave the same way regardless of their generating source, or that the medium through which they travel affects the way we perceive things.
Even the writer and architect Leon Battista Alberti, who was deeply knowledgeable about optics and who in 1435 had written a seminal essay titled “On Painting” that was steeped in this field of study, thought that scientific discussions of color and light were of no interest to painters. Artists and patrons read Alberti’s essay widely, as did Leonardo, who was highly influenced by it; scholars have detected many references to Alberti’s essay in his writing, even though the artist did not list it among the books in his library or mention it explicitly in any of his surviving notes. Alberti writes that he had purposefully left out of his essay “all the functions of the eye [officj degli occhi] in relation to vision,” as he thought they were not essential to painting, as well as a discussion of “whether sight rests at the juncture of the inner nerve of the eye, or whether images are represented on the surface of the eye, as it were in an animated mirror [speculo animato].” For Alberti, it was “enough in these books to describe briefly those things that are essential to the present purpose”: that is, basic rules of linear perspective to diminish the size of objects mathematically. He omitted “the disputes of philosophers regarding the origins of colors” because artists did not need to know “how color is made from the mixture of rare and dense, or hot and dry and cold and wet.”
At least two artists felt differently, and they went back to the medieval optical sources to write books that would teach artists precisely what Alberti had left out.
Leonardo was one, but before him there was the sculptor Lorenzo Ghiberti.
That Ghiberti read the Italian translation of Alhacen’s Book of Optics is documented. He copied extensive passages from it for his own art book: “Note that Aristotle and Alhacen say that there is difference between light, lumen, and splendor […]” and “The reason of this derives from Alhacen, as we said earlier […]” are typical sentences scattered throughout his book. When Leonardo trained in Verrocchio’s bottega, Ghiberti’s notes from Alhacen were in the hands of Lorenzo’s grandson and heir, Bonaccorso Ghiberti, one of Leonardo’s acquaintances.
Lorenzo Ghiberti is best known as Brunelleschi’s rival and as the creator of the Gates of Paradise, a series of reliefs adorning the doors of the Florentine Baptistery. But he was also the promoter of a novel view on the arts. Before the Renaissance, artists were regarded as mere craftsmen since they worked in noisy workshops and dirtied themselves with dust and pigments, their manual labor deemed inferior to the intellectual work of writers. Lorenzo Ghiberti was among the first to revise this way of thinking.
He himself behaved like a man of letters.
He befriended the protagonists of Florentine intellectual life: the financially inept book dealer Niccolò Niccoli, whose stock formed the library of San Marco; Leon Battista Alberti; the historian and chancellor Leonardo Bruni; and the theologian and expert on Greek literature Ambrogio Traversari.
Ghiberti read widely.
Learning that Vitruvius recommended a liberal arts education for architects, Ghiberti advocated the same for artists: “painters and sculptors should be learned in all the liberal arts: Grammar, Geometry, Philosophy, Medicine, Astrology, Optics [Prospettiva], History, Anatomy, Theory of Drawing [Teorica disegno], Arithmetic.”
Ghiberti also, of course, wrote a book of his own.
This book was meant to be about his own art, but it is not an art book—at least not as we understand it today. Ghiberti did not give it a title, so today the book is known simply as Ghiberti’s Commentaries, which is a misnomer because it is not a commentary at all. It is an autobiography, but even as an autobiography it is odd. It does not start with Ghiberti’s birth, childhood, and early training, but with Vitruvius’s recommendation that architects be trained in the liberal arts. It continues with short biographies of ancient artists, which Ghiberti copied from Pliny’s Natural History, and of medieval artists from Florence, Siena, and Rome, which he himself wrote. Only after about twenty pages of talking about others does he finally come around to his own biography. In so doing, Lorenzo created an illustrious genealogy for himself that connected him directly to the greatest artists of the past. The very idea of an artist writing about his own place in the history of art was a novelty, but one that was fitted to Ghiberti’s conception of the artist.
But the oddest thing of all about Lorenzo’s book is the third section, the so-called Third Commentary, which at over one hundred pages is by far the longest. He never finished this section, and it is hard to imagine how he would have, given that it was nothing else than a set of apparently disorganized notes on optics taken from Alhacen, Bacon, Witelo, and Pecham. But a more careful reading reveals that Ghiberti mastered medieval optics, which is in itself astonishing, as no other artist had ever read optical books of that complexity so thoroughly.
He wrote that since childhood, “I followed art with great study and discipline [studio e disciplina]” and always wished to understand “the basic concepts [primi precetti]” so as to “investigate how nature operates in art and how I can relate to it.” He wished to know “how the species [specie] come to the eye, what is the role of visual acuity [virtú visiva], and how visual rays [raggi visuali] behave.” “Species,” “visual acuity,” and “visual rays” are all technical terms Ghiberti lifted from optical books. Other terms he invented himself, such as light ray (razzo luminoso) and shadow ray (razzo ombroso); later, Leonardo himself used Ghiberti’s terms—“razzo luminoso” and “razzo ombroso”—which is an indication that he was familiar with the sculptor’s writings. Ghiberti’s main goal was to understand “how the theory of the arts of sculpture and painting have to be carried out.”
What is even more astonishing is that over half of Ghiberti’s Third Commentary was based on Alhacen’s Book of Optics. One may think that this was due to the fact that he could read Italian better than Latin, and Alhacen’s work—unlike the later works based on it—was available in the vernacular. But there seems to have been another reason as well: he copied from Alhacen mainly passages on “visual illusions” that were not present in the books written by his Latin followers.
Ghiberti never published his work, and at his death his grandson Bonaccorso inherited it together with the workshop’s prized stock of sketches and drawings and the materials Ghiberti had consulted to compile his book: a complete copy of Vitruvius’s On Architecture, various optical texts, and either a copy of the Italian translation of Alhacen’s book or extensive excerpts from it (the Italian translation Ghiberti used was very similar to the only surviving Italian translation now kept in the Vatican Library).
Bonaccorso and Leonardo had much in common. Born only a year apart, they received similar artistic educations, though Leonardo trained in Verrocchio’s workshop and Bonaccorso in his grandfather’s foundry. Both developed similar interests in casting, metals, and furnaces. They were deeply fascinated by Brunelleschi’s castello and made sketches of it that are so similar that it is conceivable they drew them while working near each other or, at the very least, showed each other their work.
Eventually, however, their interests diverged. Leonardo learned how to paint, and Bonaccorso became a talented gun maker, blacksmith, and military architect. But they shared a passion for writing, and although Bonaccorso never wrote as much as Leonardo—very few artists did—he kept a fat volume of 239 folios, each about twenty by fourteen centimeters, in which he gathered notes on subjects ranging from architecture and geometry to weaponry, metal casting, and the building of furnaces. Bonaccorso compiled his book between 1472 and 1483, when Leonardo was in Florence, and added notes to it in the 1490s.
Leonardo’s own writings from this period testify to his preoccupation with optics, and later in life he himself quoted directly from Pecham and looked for Witelo’s book in libraries in Florence and Pavia.
Some notes from later years seem to echo the words of Alhacen: “Men wrongly complain of experience,” Leonardo wrote, “which with great abuse they accuse of falsity […] Experience does not err, but rather your judgements err when they hope to exact effects that are not within her power.” He described painting as an art that “embraces all the ten functions [offizi] of the eye,” adapting to art Alhacen’s conditions of sight. Even more revealing is Leonardo’s explanation that his “little work [the book on painting he planned to write] will comprise an interweaving of these functions.” He revealed that in a way he thought of his book as a sort of simplified “Alhacen for Artists” to teach artists to imitate with their art “the works by which nature adorns the world,” including the most distinguished of all the world’s ornaments—“man and the intentions of his mind.”
Even by his twenties, he had picked up fundamental notions of optics. As we have seen, he sketched the anatomy of the eye on a folio relating to the palla and wrote a note that strongly suggests he had fully grasped the most complex aspects of optical theory in his Florentine years: “surface” of the eye, “pupil as a convex mirror,” and “eye as a glass ball” are all expressions that come from optical literature. His earliest shadow drawings, made to study penumbras cast by a round object, or projected onto it by various light sources, show just how important it was for him to grapple scientifically with forms of luminous colors that disperse in the air and to find a way to represent their true behavior in his works. It was not just a matter of learning how to draw from nature the lights and shadows on each muscle, which is what Verrocchio and his fellow artists did. Rather, it was a matter of learning how to capture the effect of particle-filled air on the luminous forms of people’s faces and their bodies.
Considering that Alhacen’s book was the only optical text translated into Italian, that it included long descriptions of “visual illusions,” that Leonardo had painted such “visual illusions” since his youth, and that Alhacen’s book was in the hands of an artist Leonardo knew, it is not inconceivable that Leonardo did actually read Alhacen’s Book of Optics in the 1470s.
If Leonardo read Alhacen’s Book of Optics in his Florentine years, he would have found there detailed descriptions of “visual illusions” and exhaustive explanations of the science behind them. Buildings in a landscape filled with humid air, the texture of people’s hair, the fur of animals, the brocaded decorations of precious textiles from Damascus, skin seen through a transparent fabric—these were among the excellent examples Alhacen had painstakingly described and that Leonardo had excelled in painting since his earliest works. As the art historian Martin Kemp remarked a long time ago, Leonardo “chased problematical perceptions at least as avidly as Alhazen, making his own telling observations, even if his exposition never achieves the orderly logic of book III of De aspectibus.”
From optical literature, and Alhacen in particular, he would have learned that air is “in continuous contact” with faces and bodies, and that it was air that made visible the smallest and most imperceptible penumbras as people moved their limbs, faces, and expressions. Toward the end of his life, he wrote that “the boundary of a thing is a surface, which is not part of the body clothed in that surface, nor is it part of the air surrounding these bodies, but is the division interposed between the air and the body […] Therefore, painter, do not surround your bodies with lines.” But the depiction of air was the defining characteristic of his paintings from the very beginning.
Other artists had attempted to render the effect of air as well, but none with Leonardo’s sustained focus, nor with his skills, let alone with his outcomes.
Brunelleschi designed two perspectival panels (now lost) in which the architect applied the medieval theory of vision to painting and in which—it is widely accepted—he invented linear perspective, a mathematically accurate procedure to represent three-dimensional objects on a flat surface. Less often considered is the fact that he was also deeply engaged in representing the effects of air. In the first panel representing the Florentine Baptistery, which he rendered with mathematical precision, he chose not to paint the sky above the building. Instead, he applied a layer of silver so that the background of the painting was actually a mirror. The scope of this odd arrangement was clear to his first biographer, Antonio Manetti, who explained that “the air and natural heavens are seen reflected there, and also the clouds that are seen in that silver to be stirred by the wind when it blows.” In other words, the painted image of the Baptistery was completed by the scattered reflections of natural light from the real sky on a layer of silver. In the second panel, representing the Signoria palace, which was also rendered with mathematical precision, Brunelleschi cut the panel along the palace’s skyline so that when the painting was displayed outside, effectively like a cardboard cutout, real sky and real air completed the painted building. Not knowing how to represent in painting the effect of air, which Alhacen had shown was fundamental to perception, Brunelleschi in both panels had real sky and air do the work.
Donatello was also very attentive to optical matters. He created the illusion of great depth with carvings that were only millimeters deep, and with minimal variations of light and shadow between the carved figures and their background; but these shadows were so masterfully done that they gave the illusion of fully round objects situated in an enveloping atmosphere. It was known as flattened-out relief, or rilievo schiacciato, and it seemed like a realization of the “subtle engravings that are not of a different color but rather of the same color as the body” Alhacen mentioned to explain how light affects the perception of small details. These “subtle engravings” are visible when “moderate light shines upon that body” but are invisible if they “are illuminated by intense light” that faces the object. Ghiberti later described them as “sculture sottili,” commenting similarly that the effect was visible in moderate light but that it was invisible “in feeble light or in the dark.”
Optical literature and Alhacen offered Leonardo only a starting point, however. In his writings and drawings some fifteen years later, there remained some very basic unanswered questions.
What, for example, was the soul exactly? Where did it reside in the body? And how—by what mechanism—did it interact with the body to reveal its form to us? Young Leonardo had of course heard the “soul” referenced in a religious context. And he had also encountered Aristotle’s interpretation—that the soul and the body are inseparable. But such knowledge took him only so far. If he wanted to create a definitive book on painting, if he wanted to codify operational rules for art, then he would need to take a religious and philosophical statement about the interaction of body and soul and combine it with geometrical information about light and shadow.
Optical books, including Alhacen’s, could not do that for him: For all his interest in painting, Alhacen knew far more about natural philosophy than about art. Only a painter could extract from his analysis a set of rules for painting.
Put more clearly, what Aristotle understood (the body-soul connection), what Alhacen explained scientifically (optical effects as markers of how we know the world), what Brunelleschi was unable to capture with pigments, what Alberti refused to discuss, and what Ghiberti attempted, and failed, to assemble into an art book—all this a young Leonardo took upon himself to synthesize into rules for a new type of painting that, like natural philosophy, aimed to reveal the truth about the world and, above all, the truth about people’s soul.
The influence of optical literature and of Alhacen in particular on a young Leonardo becomes even more apparent in light of something equally remarkable: the particular way young Leonardo painted. In the first works he painted entirely by himself, he devoted great attention to atmospheric effects derived from “the air between the eye and the visible object,” which were nowhere to be found in the art of his peers. Perhaps Leonardo developed this style simply through practice. But perhaps he painted differently than his peers because he had read and digested books they had not, including Alhacen’s Book of Optics.