NINE
The structure of the eye and the process of vision were natural wonders for Leonardo that never ceased to amaze him. “What language can express this marvel?” he writes about the eyeball, before continuing with a rare expression of religious awe: “Certainly none. This is where human discourse turns directly to the contemplation of the divine.”1 In the Treatise on Painting, Leonardo waxes enthusiastic about the human eye:
Don’t you see that the eye embraces the beauty of the whole world? It is the master of astronomy, it practices cosmography, it counsels and corrects all human arts; it transports man to different parts of the world. [The eye] is the prince of mathematics; its sciences are most certain. It has measured the heights and sizes of the stars, it has discovered the elements and their locations…. It has created architecture, perspective, and divine painting…. [The eye] is the window of the human body, through which [the soul] contemplates and enjoys the beauty of the world.2
It is not surprising that Leonardo spent more than twenty years investigating the anatomy and physiology of the eye by carefully dissecting the eyeball and associated muscles and nerves. One of his earliest drawings, made around 1487, shows the human head and brain surrounded by several membranes, like layers of an onion (Fig. 9-1). In fact, this onion analogy was widely used by leading medieval anatomists.3 Beneath the layers of the scalp Leonardo shows two membranes (known today as dura mater and pia mater) surrounding the brain and then extending to form the eyeball, which contains a round lens. The pupil is formed by a transparent gap in the membranes in front of the lens, which appears to lie unattached, presumably floating in some clear fluid. This crude drawing is a faithful illustration of the medieval view of the eye, which was based almost entirely on imagination rather than on empirical knowledge.
With his own anatomical dissections, Leonardo soon progressed far beyond these traditional ideas. The “onion drawing” already shows one of his discoveries, the frontal sinus above the eyeball, and in the subsequent years he would gradually add many fine details concerning the anatomy of the eye and the pathways of visual perception.
He was well aware of the novelty of his discoveries. “The eye has until now been defined by countless writers in a certain way,” he noted in the Codex Atlanticus, “but I find through experience that it works in a different manner.”4
LEONARDO’S ANATOMY OF THE EYE
Leonardo’s study of visual perception was an extraordinary program of scientific investigation, combining optics, anatomy of the eye, and neuroscience. He explored these fields without any inhibitions, applying the same meticulous empirical method to them that he used to explore everything else in nature, never fearing that some phenomenon might be beyond his grasp.
Figure 9-1: Leonardo’s illustration of the medieval view of the scalp, brain, and eyeball, Anatomical Studies, folio 32r
One of the first things Leonardo noticed when he studied the structure of the eye in detail was its ability to change the size of the pupil according to its exposure to light. He first observed this phenomenon while painting a portrait, and then tested it in a series of experiments in which he exposed subjects to varying amounts of light. “The pupil of the eye,” he concluded, “changes to as many different sizes as there are differences in the degrees of brightness and darkness of the objects which present themselves before it…. Nature has equipped the visual faculty, when irritated by excessive light, with the contraction of the pupil…, and here nature works like someone who, having too much light in his house, closes half of a window, and more or less according to necessity.” And then he added: “You can observe that in nocturnal animals such as cats, screech owls, tawny owls and others, which have the pupil small at midday and very large at night.”5
When he investigated the mechanism of these contractions and dilations in his dissections of the eyeball, Leonardo discovered the delicate sphincter of the pupil. “I find by experiment,” he recorded, “that the black, or nearly black, crinkled rough color, which appears around the pupil, serves no other function than to increase or decrease the size of that pupil.”6 In another passage, he likened the action of the radial folds of the sphincter to the closing of a purse with a string.7 Leonardo’s detailed description of the “nearly black, crinkled rough color” of the pupillary muscles is amazingly accurate. Indeed, it is almost identical to that of modern medical textbooks, in which the muscles on the central opening of the iris, the so-called “pupillary ruff,” are described as a dark brown, wrinkled rim.8
In the Middle Ages and the Renaissance, most natural philosophers believed that vision involved the emission of “visual rays” by the eye, which were then reflected back by the perceived objects. This view was first proposed by Plato and was supported by Euclid, Ptolemy, and Galen. Only the great experimental philosopher Alhazen expounded the opposite view—that vision was triggered when visual images, carried by light rays, entered the eye.
Leonardo debated the merits of both points of view at great length before agreeing with Alhazen.9 His principal argument in favor of the theory of “intromission” was based on his discovery of the pupil’s adaptation to changing illumination. In particular, he saw the fact that sudden bright sunlight produces pain in the eye as decisive proof that light not only enters the eye, but can also cause harm to it and, in extreme cases, even its destruction. An additional argument for the entry of light into the eye was Leonardo’s observation of afterimages. “If you look at the sun or another luminous body and then shut your eyes,” he noted, “you will see it similarly inside your eye for a long space of time. This is evidence that images enter the eye.”10
After a hiatus of almost twenty years, Leonardo returned to the study of vision around 1508 to explore further details of the eye’s anatomy and its visual pathways.11 This time, he also made use of his new technique of embedding the eyeball in egg white during dissections.12 He recognized the cornea as a transparent membrane and noticed its prominent curvature, concluding correctly that this extends the visual field beyond 180 degrees: “Nature made the surface of the cornea in the eye convex in order to allow surrounding objects to imprint their images at greater angles.”13
Leonardo realized that the extension of the visual field by the prominence of the cornea’s curvature is due to the refraction of light rays when they pass from the air into the denser medium of the cornea, and he carefully illustrated this phenomenon in several sketches. In addition, he tested the refractions experimentally by building a crystal model of the cornea.14
Leonardo was quite familiar with lenses from his optical experiments as well as from his own use of spectacles, which he had to wear by the time he studied the lens of the eye.15 Naturally, he applied his knowledge of refraction to his investigations of both the cornea and the lens. However, he always presented the lens, which he called the “crystalline humor,” as spherical and located in the center of the eyeball, suspended in a clear fluid, rather than right behind the pupil. Kenneth Keele has pointed out that Leonardo’s sophisticated technique of dissection of the eyeball, developed around 1509, would certainly have enabled him to recognize the true shape and location of the lens, and Keele has speculated that Leonardo either did not continue his dissections of the eye after that time, or that more accurate drawings have been lost.16
The detailed optics of the light rays inside the eyeball presented great difficulties for Leonardo, as they did for all his contemporaries. Today we know that the rays are refracted by the convex lens in such a way that they cross each other behind the lens and form an inverted image of the perceived object on the retina. How the brain then corrects the inversion to produce normal vision is still not fully understood.
Since Leonardo could not know that a second inversion of the image is performed in the brain, he had to construct two consecutive inversions of the light rays within the eyeball to produce an upright image. He came up with a brilliant though incorrect idea. The first inversion of the rays, he postulated, occurs between the pupil and the lens, caused by the small opening of the pupil, which turns the image upside down like a camera obscura.17
The inverted rays then enter the lens where they are inverted a second time, resulting in an upright image at the end. Leonardo built a simple but very ingenious model of the eye to test this idea and illustrated it clearly with a charming drawing in Manuscript D (Fig. 9-2). In the lower part of the drawing, he has sketched the visual pathways according to his theory. The light rays, entering the eye from below, are slightly refracted by the cornea (except for the central ray), proceed through the small opening of the pupil and, as in a camera obscura, produce an inverted image on the spherical lens. There, the rays are inverted again before they form a proper image on the back of the lens, from where they would enter the optic nerve.
The upper part of the drawing shows Leonardo’s model. He has filled a transparent globe, representing the eyeball, with water and at the front has fitted a plate with a small hole in the middle, representing the pupil. Suspended in the center of the globe is a “ball of thin glass,” representing the lens, behind which Leonardo places his own eye underwater in the position of the optic nerve. “Such an instrument,” he explains in the accompanying text, “will send the images…to the eye just as the eye sends them to the visual faculty.”18
Leonardo’s construction of the visual pathways was certainly ingenious, but it also had some serious problems. The camera-obscura effect would work only if the size of the pupil were much smaller and its distance from the lens greater than they actually are. And even if that were the case, the images of objects on the retina would be affected by the contractions and dilations of the pupil in response to varying exposures to light. Leonardo considered that possibility and also experimented with alternative visual paths, but he was never able to resolve the inconsistencies inherent in his construction.19 Nevertheless, his discoveries of many fine details of the eye’s anatomy are truly remarkable.
Leonardo was the first to distinguish between central and peripheral vision. “The eye has a single central line,” he observed, “and all the things that come to the eye along this line are seen well. Around this central line, there are an infinite number of other lines, which are of less value the further they are from the central line.”20 He was also the first to explain binocular vision—the way in which we see things stereoscopically by fusing the separate images of the visual field formed in each eye. To explore the details of binocular vision, he placed objects of various sizes at varying distances from the eyes, from very close to very far, and looked at them alternatively with the right and left eye and with both eyes. His conclusion was unequivocal and correct: “One and the same object is clearly comprehended when seen with two concordant eyes. These eyes refer it to one and the same point inside the head…. But if you displace one of those eyes with the finger, you will see one perceived object converted into two.”21
Figure 9-2: Model of the eye and diagram of visual pathways, Ms. D, folio 3v
FROM THE OPTIC NERVE TO THE SEAT OF THE SOUL
From his earliest studies of sensory perception, Leonardo did not limit his investigations of vision to the optics of the eye, but followed the paths of sensory impressions through the nerves into the brain. Indeed, even his early “onion drawing” of the scalp and eyeball (Fig. 9-1), which represents the medieval conception of the eye, shows the optic nerve leading to the center of the brain, where the vague outlines of three cavities can be seen. According to Aristotelian and medieval philosophy, these were the areas in the brain where different stages of perception took place. The first cavity, named sensus communis (common sense) by Aristotle, was the place where all the senses came together to produce an integrated perception of the world, which was then interpreted and partly committed to memory in the other two cavities.
These hollow spaces do exist in the central portion of the brain, but their shapes and functions are quite different from those imagined by medieval natural philosophers. They are called cerebral ventricles by today’s neuroscientists; there are actually four of them, all interconnected. They support and cushion the brain and produce a clear, colorless fluid that circulates over the surfaces of the brain and spinal cord, transporting hormones and removing metabolic waste products.
Leonardo embraced the Aristotelian idea of the ventricles as centers of sensory perception, expanded it, and, by employing his skills as an anatomist and empirical scientist, integrated it with his ideas about the nature of light and the physiology of vision. To begin with, he determined the exact shape of the cerebral ventricles by carefully injecting wax into them.22
He recorded his results in several drawings, for example, the one shown in Figure 9-3, which also exhibits the pathways of several sensory nerves to the brain. Comparison of this drawing (which is based on the dissection of the brain of an ox) with those in a modern medical textbook makes it evident that Leonardo reproduced the shapes and locations of the cerebral ventricles with tremendous accuracy. The two anterior, so-called lateral ventricles, the third (central) ventricle, and the fourth (posterior) ventricle can easily be recognized.
Leonardo’s neurological theory of visual perception must be ranked as one of his greatest scientific achievements. It has been analyzed in admirable detail by the eminent Leonardo scholar and physician Kenneth Keele.23
In Leonardo’s anatomy, the optic nerve is pictured as expanding gradually where it enters the eyeball and attaching itself directly to the back of the spherical lens, forming a kind of restricted retina. This is where the visual images are transformed into nerve impulses. He saw this process as a percussion of the optic nerve by the light rays, which triggers sensory impulses (sentimenti) that travel through the nerves in the form of waves, just as the “tremors” triggered by stones thrown into a pond propagate in the form of water waves.24 However, Leonardo specified that the sensory, or nervous, impulses are not material. He called them “spiritual,” by which he simply meant that they were incorporeal and invisible. Following Galen, he thought that the optic nerve, like all nerves, was hollow, “perforated” by a small central tube through which the wave fronts formed by sensory impulses travel toward the center of the brain.
Figure 9-3: Cerebral ventricles and pathways of cranial nerves, “Weimar Blatt,” in Anatomical Studies, between folios 54 and 55
Kenneth Keele concludes that Leonardo’s physiology of sensory perception is “thoroughly mechanistic,” because it prominently features movement and percussion.25 I disagree with this assessment in view of Leonardo’s explicit emphasis on the nonmaterial nature of the nervous impulses. According to modern neuroscience, the nerve impulses are of electromagnetic nature, wave fronts of ions moving along the nerves—and, as Leonardo stated, invisible to the naked eye. The neurons form long, thin fibers (called axons), surrounded by cell membranes, for which Leonardo’s term “perforated tubes” does not seem a bad description. Inside these tubes, the wave fronts of ions move in the fluid of the nerve cells. These are phenomena in the realms of microbiology and biochemistry, which were inaccessible to Leonardo. As a good empiricist, he simply stated that the sensory impulses are invisible and did not further speculate about their nature. No scientist could have done better before the development of the microscope and the theory of electromagnetism several centuries later.
From his very first anatomical studies, Leonardo paid special attention to the pathways of the sensory nerves in the human skull, in particular the optic nerve. Indeed, as Keele points out, “Leonardo’s personal investigations of the anatomy of the eye and optic nerves…formed the central motive for his beautiful perspectival demonstrations of the structure of the human skull.”26 These stunning pictures of the skull are famous for their delicate renderings of light and shade and their masterful application of visual perspective (see Fig. 8-2 on Chapter 8). In addition, the trained eye of the physician sees in them amazingly accurate depictions of the skull’s cavities and nerve openings—the eye socket, its neighboring sinuses, the tear ducts, and the openings (foramina) for the optic and auditory nerves.27
When Leonardo followed the optic nerves from each eyeball into the brain, he noticed that they intersect in an area now known as the optic chiasma (“crossing”).28 He documented this discovery in all his drawings of the optic and cranial nerves (see Fig. 9-3). Leonardo speculated that the crossing of the optic nerves served to facilitate “the equal movement of the eyes” in the process of visual perception.29 He was on the right track, but he did not know that the process of synchronizing the visual perception of the two eyes is much more complex, involving the subtle interplay of several sets of muscles and nerves.
By the time Leonardo drew the so-called Weimar Blatt (Fig. 9-3), around 1508, his knowledge of the nature and course of the cranial nerves had reached its peak. He still maintained that all the nerves carrying the sensory impressions converge in the anterior ventricle,30 but he departed from Aristotle by shifting the location of the senso comune to the central cavity of the brain.31 In the anterior ventricle, Leonardo located a special organ not mentioned by anyone before him, which he called the receptor of impressions(impressiva).32 He saw it as a relay station that collects the wave patterns of sensory impressions, makes selections by some process of resonance, and organizes them into harmonious rhythmic forms that are then passed on to the senso comune, where they enter consciousness.
HEARING AND THE OTHER SENSES
Although Leonardo considered sight “the best and most noble of the senses,”33 he investigated the other senses as well, paying particular attention to the pathways of their cranial nerves. From his earliest drawings of the head, he consistently delineated the auditory and olfactory nerves, as well as the optic nerve, and showed how they all converge toward the senso comune.
In his famous drawings of the skull in perspective, Leonardo clearly depicted the auditory canal, but in his known manuscripts there is no detailed description of the anatomy of the ear. He was aware of the eardrum and recognized that its percussion by sound waves produces sensory impulses in the auditory nerve. However, he did not document any of the intermediary processes, having convinced himself, perhaps, that the generation of auditory nervous impulses by means of percussion was analogous to that of the impulses in the optic nerve, and that both of them ended up in thesenso comune.
Leonardo may or may not have recorded more detailed studies of the human perception of sound in manuscripts that have been lost, but we know for certain that he spent considerable time studying the production of sound by the human voice. He not only investigated the anatomy and physiology of the entire vocal apparatus to understand the formation of the voice, but extended his studies to phonetics, musical theory, and the functioning and design of musical instruments.34
The larynx, or voice box, which contains the vocal cords, is a notoriously complicated organ, and it is not surprising that Leonardo did not fully understand its functioning. However, he produced astonishingly accurate drawings of its detailed anatomy, far beyond anything known in his time, and he also realized that many other parts of the body are involved in the formation of the human voice. In the words of Kenneth Keele, Leonardo realized that
voice production involved the integrated function of structures ranging from the thoracic cage, through lungs, bronchi, trachea, larynx, pharynx, nasal and mouth cavities to the teeth, lips and tongue; and he considers all these structures, producing unprecedentedly accurate drawings of them all.35
In his studies of the human voice, Leonardo frequently used the mechanisms of sound production in flutes and trumpets as models. In fact, he always used the word voce (voice) for the sounds produced by these instruments. His investigations of the variation of pitch in wind instruments naturally led him to study scales and develop elements of musical theory.
Leonardo’s musical talent was well known by his contemporaries and played an important role in his early success at the Sforza court in Milan.36 We also have contemporary reports that he composed pieces of music for the theatrical performances and other spectacles he produced at court.37Unfortunately, no musical score by Leonardo has been preserved. On the other hand, we can find numerous drawings of musical instruments in his Notebooks, most of them with designs for improving existing instruments. These designs include keyboards for wind instruments, tuned drums, glissando flutes (like Swanee whistles), and a viola organista (organ violin), a kind of organ with timbre similar to a string instrument.38
Leonardo’s dissections of the cranial nerves and the central nervous system convinced him that all five senses are associated with special nerves that carry sensory impressions to the brain, where they are selected and organized by the receptor of impressions(impressiva) and passed on to thesenso comune. There, in the central ventricle of the brain, the integrated sensory impressions are judged by the intellect and influenced by the imagination and memory.
In several of his drawings of the human skull, Leonardo indicated the position of the third cerebral ventricle by three intersecting coordinates, with complete spatial accuracy in three dimensions (see Fig. 8-2). This cavity in the center of the brain he identified not only as the location of the senso comune, but also as the seat of the soul. “The soul appears to reside in the judicial part,” he concluded, “and the judicial part appears to be in the place where all the senses come together, which is called senso comune…. The senso comune is the seat of the soul, the memory is its store, and the receptor of impressions is its informant.”39 With this statement, Leonardo links his elaborate theory of sensory perception to the ancient idea of the soul.
COGNITION AND THE SOUL
In early Greek philosophy, the soul was conceived as the ultimate moving force and source of all life.40 Closely associated with this moving force, which leaves the body at death, was the idea of knowing. From the beginning of Greek philosophy, the concept of the soul had a cognitive dimension. The process of animation was also a process of knowing. Thus Anaxagoras, in the fifth century B.C., called the soul nous (reason) and saw it as a world-moving rational substance.
During the period of Hellenistic-Roman philosophy, Alexandrian thought gradually separated the two characteristics that had originally been united in the Greek conception of the soul—that of a vital force and that of the activity of consciousness. Side by side with the soul, which moves the body, now appears “spirit” as an independent principle expressing the essence of the individual, and also of the divine personality. Alexandrian philosophers introduced the triple division of the human being into body, soul, and spirit, but the boundaries between “soul” and “spirit” were fluctuating. The soul was situated somewhere between the two extremes, matter and spirit.
Leonardo adopted the integrated view of the soul that was held by Aristotle and the early Greek philosophers, who saw it both as the agent of perception and knowing and as the force underlying the body’s formation and movements. Unlike the Greek philosophers, however, he did not merely speculate about the nature of the soul, but tested the ancient views empirically. In his delicate dissections of the brain and the nervous system, he traced the sensory perceptions from the initial impressions on the sense organs, especially the eye, through the sensory nerves to the center of the brain. He also followed the nerve impulses for voluntary movement from the brain down the spinal cord, and through the peripheral motor nerves out to the muscles, tendons, and bones; and he illustrated all these pathways in precise anatomical drawings (see, e.g., Fig. 9-4).41
From his thorough investigations of the brain and the nervous system, Leonardo concluded that the soul evaluated sensory impressions and transferred them to the memory, and that it was also the origin of voluntary bodily movement, which he associated with reason and judgment.
In Leonardo’s view, all material movement originated in the immaterial and invisible movements of the soul. “Spiritual movement,” he reasoned, “flowing through the limbs of sentient animals, broadens their muscles. Thus broadened, these muscles become shortened and draw back the tendons that are connected to them. This is the origin of force in the human limbs…. Material movement arises from theimmaterial.”42 With this concept of the soul, Leonardo expanded the traditional Aristotelian idea according to his empirical evidence. In this, he was far ahead of his time.
During the subsequent centuries, Leonardo’s Notebooks remained hidden in ancient European libraries and many of them were lost, and the integrated Aristotelian view of the soul gradually disappeared from philosophy. The idea of spirit as a disembodied divine principle became the dominant theme of religious metaphysics, and the soul, accordingly, was seen as being independent from the body and endowed with immortality. For other philosophers, the concept of the soul became increasingly synonymous with that of the rational mind, and in the seventeenth century, René Descartes postulated the fundamental division of reality into two independent and separate realms—that of mind, the “thinking thing” (res cogitans), and that of matter, the “extended thing” (res extensa).
This conceptual split between mind and matter has haunted Western science and philosophy for more than three hundred years. Following Descartes, scientists and philosophers continued to think of the mind as an intangible entity and were unable to imagine how this “thinking thing” is related to the body. In particular, the exact relationship between mind and brain is still a mystery to most psychologists and neuroscientists.
During the last two decades of the twentieth century, however, a novel conception of the nature of mind and consciousness emerged in the life sciences, which finally overcame the Cartesian division between mind and body. The decisive advance has been to reject the view of mind as a thing; to realize that mind and consciousness are not entities but processes. In the past twenty-five years the study of mind from this new perspective has blossomed into a rich interdisciplinary field known as cognitive science, which transcends the traditional frameworks of biology, psychology, and epistemology.43
Figure 9-4: Study of the anterior muscles of the leg, c. 1510, Anatomical Studies, folio 151r
One of the central insights of cognitive science is the identification of cognition, the process of knowing, with the process of life. Cognition, according to this view, is the organizing activity of living systems at all levels of life. Accordingly, the interactions of a living organism—plant, animal, or human—with its environment are understood as cognitive interactions. Thus life and cognition become inseparably connected. Mind—or, more accurately, mental activity—is immanent in matter at all levels of life. This new conception represents a radical expansion of the concept of cognition and, implicitly, the concept of mind. In the new view, cognition involves the entire process of life—including perception, emotion, and behavior—and does not even necessarily require a brain and a nervous system.
It is evident that the identification of mind, or cognition, with the process of life, although a novel idea in science, comes very close to Leonardo’s concept of the soul. Like Leonardo, modern cognitive scientists see cognition (or the soul) both as the process of perception and knowing and as the process that animates the movements and organization of the body. There is a conceptual difference. Whereas cognitive scientists understand cognition clearly as a process, Leonardo saw the soul as an entity. However, when he wrote about it, he always described it in terms of its activities.
How close Leonardo’s conception of the soul comes to the modern concept of cognition can be seen in his notes on the flight of birds, in which he compares the movements of the living bird with those of the flying machine he is designing. Over many hours of intense observations of birds in flight in the hills surrounding Florence, Leonardo became thoroughly familiar with their instinctive capacity to maneuver in the wind, keeping their equilibrium by responding to changing air currents with subtle movements of their wings and tails.44
In his notes, he explained that this capacity was a sign of the bird’s intelligence—a reflection of the actions of its soul.45 In modern scientific language, we would say that a bird’s interactions with the air currents and its delicate maneuvers in the wind are cognitive processes, as Leonardo clearly recognized and accurately described. He also realized that these delicate cognitive processes of a bird in flight would always be superior to those of a human pilot steering a mechanical device:
It could be said that such an instrument designed by man is lacking only the soul of the bird, which must be counterfeited with the soul of the man…. [However], the soul of the bird will certainly respond better to the needs of its limbs than would the soul of the man, separated from them and especially from their almost imperceptible balancing movements.46
Following Aristotle, Leonardo saw the soul not only as the source of all bodily movements, but also as the force underlying the body’s formation. He called it “the composer of the body.”47 This is completely consistent with the views of today’s cognitive scientists who understand cognition as a process involving the self-generation and self-organization of living organisms.
The main difference between Leonardo’s concept of the soul and modern cognitive science seems to be that Leonardo gave the human soul a specific location in the brain. Today we know that reflective consciousness—the special kind of cognition that is characteristic of the great apes and humans—is a widely distributed process involving complex layers of neural networks. Without access to the brain’s microscopic structures, chemistry, and electromagnetic signals, Leonardo had no way of discovering these extended networks of neurons; and since he observed that the pathways of various sensory nerves seem to converge toward the brain’s central ventricle, he decided that this had to be the seat of the soul.
At the time of the Renaissance, there was no agreement about the soul’s location. Whereas Democritus and Plato had recognized the importance of the brain, Aristotle regarded the heart as the seat of the sensus communis. Averroës, the great Arab commentator on Aristotle whose teachings were very influential in Italy during the Renaissance,48 had expounded yet another view. He identified the soul with the form of the entire living body, which meant that it did not have a specific location. Leonardo, after considering such opinions, in view of the empirical evidence he had gathered, confidently located the soul in the central cavity of the brain.
Body and soul formed one indivisible whole for Leonardo. “The soul desires to stay with its body,” he explained, “because without the organic instruments of that body it can neither carry out nor feel anything.”49 Again, this is completely consistent with modern cognitive science, where we have come to understand the relationship between mind and body as one between (cognitive) process and (living) structure, which represent two complementary aspects of the phenomenon of life. Indeed, as Leonardo wrote of the soul, so cognitive scientists today speak of the mind as being fundamentally embodied. On the one hand, cognitive processes continually shape our bodily forms, and on the other, the very structure of reason arises from our bodies and brains.50
Remarkably, for his time, Leonardo repeatedly argued against the existence of disembodied spirits. “A spirit can have neither voice, nor form, nor force,” he declared. “And if anyone should say that, through air collected together and compressed, a spirit assumes bodies of various forms, and by such instrument speaks and moves with force, to that I reply that, where there are neither nerves nor bones, there can be no force exerted in any movement made by such imaginary spirits.”51
In Leonardo’s view, the essential unity of body and soul arises at the very beginning of life, and it dissolves with the demise of both at death. On the two folios that contain his most beautiful drawings of the human embryo in the womb (Fig. E-1), we find the following inspired thoughts on the relationship between the souls of mother and child:
One and the same soul governs these two bodies; and the desires, fears and pains are common to this creature as to all other animated parts…. The soul of the mother…in due time awakens the soul which is to be its inhabitant. This at first remains asleep under the guardianship of the soul of the mother who nourishes and vivifies it through the umbilical vein.52
This extraordinary passage is completely compatible with modern cognitive science. In poetical language, the artist and scientist describes the gradual development of the embryo’s mental life together with its body. At the end of life, the reverse process takes place. “While I thought I was learning how to live, I have been learning how to die,” Leonardo wrote movingly late in his life.53 In a striking departure from Christian doctrine, Leonardo da Vinci never expressed a belief that the soul would survive the body after death.
A THEORY OF KNOWLEDGE
My last two chapters outline what amounts to an extensive theory of knowledge, testifying to Leonardo’s genius as an integrative, systemic thinker. Approaching perception and knowledge as a painter, he began by exploring the appearance of things to the eye, the nature of perspective, the phenomena of optics, and the nature of light. He not only used the ancient metaphor of the eye as the window of the soul, but took it seriously and subjected it to his empirical investigations, following the rays of the “pyramids of light” into the eye, tracing them through the lens and the eyeball to the optic nerve. He described how in that area, known today as the retina, the percussion of light rays generates sensory impulses, and he followed these sensory impulses along the optic nerve all the way to the “seat of the soul” in the central cavity of the brain.
Leonardo also developed a detailed theory of how the sensory impressions enter consciousness. He remained vague on how exactly the nerve impulses come under the influence of the intellect, memory, and imagination, glossing over the relationship between conscious experience and neurological processes. However, even today our leading neuroscientists can do no better.54
That Leonardo was able to develop a sophisticated and coherent theory of perception and knowledge based on empirical evidence but without any knowledge of cells, molecules, biochemistry, or electromagnetism is certainly extraordinary. Many facets of his explanations later became separate scientific disciplines, including optics, cranial anatomy, neurology, brain physiology, and epistemology. During the last decade of the twentieth century, these subjects began to converge again within the interdisciplinary field of cognitive science, showing striking similarities to Leonardo’s systemic conception of the process of knowing.
Once again, I cannot help but wonder how differently Western science would have developed if Leonardo had published his treatises during his lifetime, as he had intended. Galileo, Descartes, Bacon, and Newton—the giants of the Scientific Revolution—lived and worked in intellectual milieus that were much closer to that of the Renaissance than ours. I believe they would have understood Leonardo’s language and reasoning much better than we do today. These natural philosophers, as they were still called, struggled with the very same problems that occupied and fascinated Leonardo during his life, and for which he often found original solutions. How would they have incorporated his insights into their theories?
Alas, such questions have no answers. While Leonardo’s paintings had a decisive influence on European art, his scientific treatises remained hidden for centuries, disconnected from the development of modern science.