by The Science of Leonardo: Inside the Mind of the Great Genius of the Renaissance
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.37 Unfortunately, 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 the senso 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 knowin
g.
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.
EPILOGUE
“Read me, O reader, if in my words you find delight”
Leonardo’s science cannot be understood within the mechanistic paradigm of Galileo, Descartes, and Newton. Although he was a mechanical genius who designed countless machines, his science was not mechanistic. He fully recognized and extensively studied the mechanical aspects of the human and animal bodies, but he always saw them as instruments, used by the soul for the organism’s self-organization. Trying to understand those processes of self-organization—the growth, movements, and transformations of nature’s living forms—was at the very core of Leonardo’s science. It was a science of qualities and proportions, of organic forms shaped and transformed by underlying processes. Nature as a whole was alive and animated for Leonardo, a world in continual flux and development, in the macrocosm of the Earth as in the microcosm of the human body.
