Side by side with his drawings, sometimes on the same page, sometimes scrawled across a sketch of a man or a woman, a landscape or a machine, are the notes in which this insatiable mind puzzled over the laws and operations of nature. Perhaps the scientist grew out of the artist: Leonardo’s painting compelled him to study anatomy, the laws of proportion and perspective, the composition and reflection of light, the chemistry of pigments and oils; from these researches he was drawn to a more intimate investigation of structure and function in plants and animals; and from these inquiries he rose to a philosophical conception of universal and invariable natural law. Often the artist peered out again in the scientist; the scientific drawing might be itself a thing of beauty, or terminate in a graceful arabesque.
Like most scientists of his time, Leonardo tended to identify scientific method with experience rather than experiment.60 “Remember,” he counsels himself, “when discoursing about water, to adduce first experience and then reason.”61 Since any man’s experience can be no more than a microscopic fragment of reality, Leonardo supplemented his with reading, which can be experience by proxy. He studied carefully but critically the writings of Albert of Saxony,62 gained a partial acquaintance with the ideas of Roger Bacon, Albertus Magnus, and Nicholas of Cusa, and learned much from association with Luca Pacioli, Marcantonio della Torre, and other professors in the University of Pavia. But he tested everything with his own experience. “Whoever refers to authorities in disputing ideas works with his memory rather than with his reason.”63 He was the least occult of the thinkers of his age. He rejected alchemy and astrology, and hoped for a time when “all astrologers will be castrated.”64
He tried his hand at almost every science. He took enthusiastically to mathematics as the purest form of reasoning; he felt a certain beauty in geometrical figures, and drew some on the same page with a study for The Last Supper.65 He expressed vigorously one of the fundamental principles of science: “There is no certainty where one can neither apply any of the mathematical sciences nor any of those that are based upon them.”66 And he proudly echoed Plato: “Let no man who is not a mathematician read the elements of my work.”67
He was fascinated by astronomy. He proposed to “make glasses in order to see the moon large,”68 but apparently he did not make them. He writes: “The sun does not move… the earth is not in the center of the circle of the sun, nor in the center of the universe.”69“The moon has every month a winter and a summer.”70 He discusses acutely the causes of spots on the moon, and combats, on that matter, the views of Albert of Saxony.71 Taking a lead from the same Albert, he argues that since “every heavy substance presses downward, and cannot be upheld perpetually, the whole earth must become spherical,” and will ultimately be covered with water.72
He noticed on high elevations the fossil shells of marine animals, and concluded that the waters had once reached those altitudes73 (Boccaccio had suggested this about 1338 in his Filocopo74). He rejected the notion of a universal flood,75 and ascribed to the earth an antiquity that would have shocked the orthodoxy of his time. He assigned to the accumulations brought down by the Po a duration of 200,000 years. He made a map of Italy as he imagined it to have been in an early geological era. The Sahara Desert, he thought, had once been covered with salt water.76 Mountains have been formed through erosion by rain.77 The bottom of the sea is continually rising with the detritus of all the streams that flow into it. “Very great rivers flow underground”;78 and the movement of life-giving water in the body of the earth corresponds to the movement of the blood in the body of man.79 Sodom and Gomorrah were destroyed not by human wickedness but by slow geological action, probably the subsidence of their soil into the Dead Sea.80
Leonardo followed avidly the advances made in physics by Jean Buridan and Albert of Saxony in the fourteenth century. He wrote a hundred pages on motion and weight, and hundreds more on heat, acoustics, optics, color, hydraulics, and magnetism. “Mechanics is the paradise of the mathematical sciences, for by its means one comes to the fruit of mathematics” in useful work.81 He delighted in pulleys, cranes, and levers, and saw no end to what they could lift or move; but he laughed at seekers for perpetual motion. “Force with material movement, and weight with percussion, are the four accidental powers in which all the works of mortals have their being and their end.”82 Despite these lines he was not a materialist. On the contrary he defined force as “a spiritual capacity… spiritual because the life in it is invisible and without body… impalpable because the body in which it is produced is increased neither in size nor in weight.”83
He studied the transmission of sound, and reduced its medium to waves of air. “When the string of a lute is struck it… conveys a movement to a similar string of the same tone on another lute, as one may convince oneself by placing a straw on the string similar to the one struck.”84 He had his own notion of a telephone. “If you cause your ship to stop, and place the head of a long tube in the water, and place the other extremity to your ear, you will hear ships at a great distance from you. You can also do the same by placing the head of the tube upon the ground, and you will then hear anyone passing at a distance.”85
But sight and light interested him more than sound. He marveled at the eye: “Who would believe that so small a space could contain the images of all the universe?”86—and he wondered even more at the power of the mind to recall an image long past. He gave an excellent description of the means by which spectacles compensate for the weakening of the muscles of the eyes.87 He explained the operation of the eye by the principle of the camera obscura: in the camera and in the eye the image is inverted because of the pyramidal crossing of the light rays that come from the object into the camera or the eye.88 He analyzed the refraction of sunlight in the rainbow. Like Leon Battista Alberti he had a good notion of complementary colors four centuries before the definitive work of Michel Chevreul.89
He planned, began, and left countless notes for, a treatise on water. The movements of water captivated his eye and mind; he studied placid and turbulent streams, springs and falls, bubbles and foam, torrents and cloudbursts, and the simultaneous fury of wind and rain. “Without water,” he wrote, repeating Thales after twenty-one hundred years, “nothing can exist among us.”90 He anticipated Pascal’s fundamental principle of hydrostatics—that the pressure exerted upon a fluid is transmitted by it.91 He noted that the liquids in communicating vessels keep the same level.92 Inheriting Milan’s tradition of hydraulic engineering, he designed and built canals, suggested ways of conducting navigable canals under or over the rivers that crossed them, and proposed to free Florence from her need of Pisa as a port by canalizing the Arno from Florence to the sea.93 Leonardo was not a Utopian dreamer, but he planned his studies and works as if he had a dozen lives to live.
Armed with the great text of Theophrastus on plants, he turned his alert mind to “natural history.” He examined the system on which leaves are arranged about their stalks, and formulated its laws. He observed that the rings in a cross section of a tree trunk record the years of its growth by their number, and the moisture of the year by their width.94 He seems to have shared several delusions of his time as to the power of certain animals to heal some human diseases by their presence or their touch.95 He atoned for this uncharacteristic lapse into superstition by investigating the anatomy of the horse with a thoroughness to which recorded history had no precedent. He prepared a special treatise on the subject, but it was lost in the French occupation of Milan. He almost inaugurated modern comparative anatomy by studying the limbs of men and animals in juxtaposition. He set aside the superannuated authority of Galen, and worked with actual bodies. The anatomy of man he described not only in words but in drawings that excelled anything yet done in that field. He planned a book on the subject, and left for it hundreds of illustrations and notes. He claimed to “have dissected more than thirty human cadavers,”96 and his countless drawings of the foetus, the heart, lungs, skeleton, musculature, viscera, eye, skull, and brain, and the principal organs in woman, support his claim. He was the first to give—in remarkable drawings and notes—a scientific representation of the uterus, and he described accurately the three membranes enclosing the foetus. He was the first to delineate the cavity of the bone that supports the cheek, now known as the antrum of Highmore. He poured wax into the valves of the heart of a dead bull to get an exact impression of the chambers. He was the first to characterize the moderator band (catena) of the right ventricle.97 He was fascinated by the network of bloodvessels; he divined the circulation of the blood, but did not quite grasp its mechanism. “The heart,” he wrote, “is much stronger than the other muscles… The blood that returns when the heart opens is not the same as that which closes the valves.”98 He traced the bloodvessels, nerves, and muscles of the body with fair accuracy. He attributed old age to arteriosclerosis, and this to lack of exercise.99 He began a volume De figura umana, on the proper proportions of the human figure as an aid to artists, and some of his ideas were incorporated in his friend Pacioli’s treatise De divina proportione. He analyzed the physical life of man from birth to decay, and then planned a survey of mental life. “Oh, that it may please God to let me also expound the psychology of the habits of man in such fashion as I am describing his body!”100
Fig. 15 —ANDREA DEL SARTO: Madonna delle Arpie; Uffizi Gallery, Florence PAGE 167
Fig. 16 —CRISTOFORO SOLARI: Tomb Effigies of Lodovico il Moro and Beatrice d’Este; Certosa di Pavia PAGE 190
Fig. 17—AMBROGIO DA PREDIS or LEONARDO DA VINCI: Portrait of Bianca Sforza; Pinacoteca Ambrosiana, Milan PAGE 197
Fig. 18—LEONARDO DA VINCI: Virgin of the Rocks; Louvre, Paris PAGE 204
Fig. 19—LEONARDO DA VINCI: Self-portrait, red chalk; Turin Gallery PAGE 215
Fig. 20—LEONARDO DA VINCI: Mona Lisa; Louvre, Paris PAGE 211
Fig. 21—PIERO DELLA FRANCESCA: Portrait of Duke Federigo da Montefeltro; Uffizi Gallery, Florence PAGE 232
Fig. 22—LUCA SIGNORELLI: The End of the World (detail), fresco; Cathedral of Orvieto, Chapel of San Brizio PAGE 234
Fig. 23—IACOPO DELLA QUERCIA: The Nativity, one of four reliefs from the main portal; San Petronio, Bologna PAGE 237
Fig. 24—IACOPO DELLA QUERCIA: Noah’s Ark, relief; San Petronio, Bologna PAGE 237
Fig. 25—PERUGINO: Self-portrait; Sistine Chapel, Rome PAGE 245
Fig. 26 —PINTURICCHIO: The Nativity; Santa Maria del Popolo, Rome PAGE 244
Fig. 27—ANDREA MANTEGNA: Lodovico Gonzaga and His Family; Castello, Mantua PAGE 253
Fig. 28—ANDREA MANTEGNA: Adoration of the Shepherds; Metropolitan Museum of Art, New York PAGE 253
Fig. 29—LEONARDO DA VINCI: Portrait of Isabella d’Este; Louvre, Paris PAGE 255
Fig. 30—TITIAN: Portrait of Isabella d’Este; Kunsthistorische Museum, Vienna PAGE 256
Was Leonardo a great scientist? Alexander von Humboldt considered him “the greatest physicist of the fifteenth century,”101 and William Hunter ranked him as “the greatest anatomist of his epoch.”102 He was not as original as Humboldt supposed; many of his ideas in physics had come down to him from Jean Buridan, Albert of Saxony, and other predecessors. He was capable of egregious errors, as when he wrote that “no surface of water that borders upon the air will ever be lower than that of the sea”;103 but such slips are remarkably few in so vast a production of notes on almost everything on the earth or in the sky. His theoretical mechanics were those of a highly intelligent amateur; he lacked training, instruments, and time. That he achieved so much in science, despite these handicaps and his labors in art, is among the miracles of a miraculous age.
From his studies in so many fields Leonardo rose at times to philosophy. “O marvelous Necessity! Thou with supreme reason constrainest all effects to be the direct result of their causes, and by a supreme and irrevocable law every natural action obeys thee by the shortest possible process.”104 This has all the proud ring of nineteenth-century science, and suggests that Leonardo had shed some theology. Vasari, in the first edition of his life of the artist, wrote that he was of “so heretical a cast of mind that he conformed to no religion whatever, accounting it perchance better to be a philosopher than a Christian”105—but Vasari omitted this passage in later editions. Like many Christians of the time, Leonardo took a fling now and then at the clergy; he called them Pharisees, accused them of deceiving the simple with bogus miracles, and smiled at the “false coin” of celestial promissory notes which they exchanged for the coinage of this world.106 On one Good Friday he wrote: “Today all the world is in mourning because one man died in the Orient.”107He seems to have thought that dead saints were incapable of hearing the prayers addressed to them.108 “I could wish that I had such power of language as should avail me to censure those who would extol the worship of men above that of the sun… Those who have wished to worship men as gods have made a very grave error.”109 He took more liberties with Christian iconography than any other Renaissance artist: he suppressed halos, put the Virgin across her mother’s knee, and made the infant Jesus try to bestride the symbolic lamb. He saw mind in matter, and believed in a spiritual soul, but apparently thought that the soul could act only through matter, and only in harmony with invariable laws.110 He wrote that “the soul can never be corrupted with the corruption of the body,”111but he added that “death destroys memory as well as life,”112 and “without the body the soul can neither act nor feel.”113 He addressed the Deity with humility and fervor in some passages;114 but at other times he identified God with Nature, Natural Law, and “Necessity.”115 A mystic pantheism was his religion until his final years.