Leonardo’s fascination with anatomical studies reveals a prevailing artistic interest of the time. In his own treatise Della pittura (1435; “On Painting”), theorist Leon Battista Alberti urged painters to construct the human figure as it exists in nature, supported by the skeleton and musculature, and only then clothed in skin. Although the date of Leonardo’s initial involvement with anatomical study is not known, it is sound to speculate that his anatomical interest was sparked during his apprenticeship in Verrocchio’s workshop, either in response to his master’s interest or to that of Verrocchio’s neighbor Pollaiuolo, who was renowned for his fascination with the workings of the human body. It cannot be determined exactly when Leonardo began to perform dissections, but it might have been several years after he first moved to Milan, at the time a centre of medical investigation. His study of anatomy, originally pursued for his training as an artist, had grown by the 1490s into an independent area of research. As his sharp eye uncovered the structure of the human body, Leonardo became fascinated by the figura istrumentale dell’ omo (“man’s instrumental figure”), and he sought to comprehend its physical working as a creation of nature. Over the following two decades, he did practical work in anatomy on the dissection table in Milan, then at hospitals in Florence and Rome, and in Pavia, where he collaborated with the physician-anatomist Marcantonio della Torre. By his own count Leonardo dissected 30 corpses in his lifetime.

Leonardo’s early anatomical studies dealt chiefly with the skeleton and muscles; yet even at the outset, Leonardo combined anatomical with physiological research. From observing the static structure of the body, Leonardo proceeded to study the role of individual parts of the body in mechanical activity. This led him finally to the study of the internal organs; among them he probed most deeply into the brain, heart, and lungs as the “motors” of the senses and of life. His findings from these studies were recorded in the famous anatomical drawings, which are among the most significant achievements of Renaissance science. The drawings are based on a connection between natural and abstract representation; he represented parts of the body in transparent layers that afford an “insight” into the organ by using sections in perspective, reproducing muscles as “strings,” indicating hidden parts by dotted lines, and devising a hatching system. The genuine value of these dimostrazione lay in their ability to synthesize a multiplicity of individual experiences at the dissecting table and make the data immediately and accurately visible; as Leonardo proudly emphasized, these drawings were superior to descriptive words. The wealth of Leonardo’s anatomical studies that have survived forged the basic principles of modern scientific illustration. It is worth noting, however, that during his lifetime, Leonardo’s medical investigations remained private. He did not consider himself a professional in the field of anatomy, and he neither taught nor published his findings.

Although he kept his anatomical studies to himself, Leonardo did publish some of his observations on human proportion. Working with the mathematician Luca Pacioli, Leonardo considered the proportional theories of Vitruvius, the 1st-century-bce Roman architect, as presented in his treatise De architectura (“On Architecture”). Imposing the principles of geometry on the configuration of the human body, Leonardo demonstrated that the ideal proportion of the human figure corresponds with the forms of the circle and the square. In his illustration of this theory, the so-called Vitruvian Man, Leonardo demonstrated that when a man places his feet firmly on the ground and stretches out his arms, he can be contained within the four lines of a square, but when in a spread-eagle position, he can be inscribed in a circle.

Leonardo envisaged the great picture chart of the human body he had produced through his anatomical drawings and Vitruvian Man as a cosmografia del minor mondo (“cosmography of the microcosm”). He believed the workings of the human body to be an analogy, in microcosm, for the workings of the universe. Leonardo wrote: “Man has been called by the ancients a lesser world, and indeed the name is well applied; because, as man is composed of earth, water, air, and fire…this body of the earth is similar.” He compared the human skeleton to rocks (“supports of the earth”) and the expansion of the lungs in breathing to the ebb and flow of the oceans.

Mechanics and cosmology

According to Leonardo’s observations, the study of mechanics, with which he became quite familiar as an architect and engineer, also reflected the workings of nature. Throughout his life Leonardo was an inventive builder; he thoroughly understood the principles of mechanics of his time and contributed in many ways to advancing them. The two Madrid notebooks deal extensively with his theory of mechanics; the first was written in the 1490s, and the second was written between 1503 and 1505. Their importance lay less in their description of specific machines or work tools than in their use of demonstration models to explain the basic mechanical principles and functions employed in building machinery. As in his anatomical drawings, Leonardo developed definite principles of graphic representation—stylization, patterns, and diagrams—that offer a precise demonstration of the object in question.

Leonardo was also quite active as a military engineer, beginning with his stay in Milan. But no definitive examples of his work can be adduced. The Madrid notebooks revealed that, in 1504, probably sent by the Florentine governing council, he stood at the side of the lord of Piombino when the city’s fortifications system was repaired and suggested a detailed plan for overhauling it. His studies for large-scale canal projects in the Arno region and in Lombardy show that he was also an expert in hydraulic engineering.

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Leonardo was especially intrigued by problems of friction and resistance, and with each of the mechanical elements he presented—such as screw threads, gears, hydraulic jacks, swiveling devices, and transmission gears—drawings took precedence over the written word. Throughout his career he also was intrigued by the mechanical potential of motion. This led him to design a machine with a differential transmission, a moving fortress that resembles a modern tank, and a flying machine. His “helical airscrew” (c. 1487) almost seems a prototype for the modern helicopter, but, like the other vehicles Leonardo designed, it presented a singular problem: it lacked an adequate source of power to provide propulsion and lift.

Wherever Leonardo probed the phenomena of nature, he recognized the existence of primal mechanical forces that govern the shape and function of the universe. This is seen in his studies of the flight of birds, in which his youthful idea of the feasibility of a flying apparatus took shape and which led to exhaustive research into the element of air; in his studies of water, the vetturale della natura (“conveyor of nature”), in which he was as much concerned with the physical properties of water as with its laws of motion and currents; in his research on the laws of growth of plants and trees, as well as the geologic structure of earth and hill formations; and, finally, in his observation of air currents, which evoked the image of the flame of a candle or the picture of a wisp of cloud and smoke. In his drawings based on the numerous experiments he undertook, Leonardo found a stylized form of representation that was uniquely his own, especially in his studies of whirlpools. He managed to break down a phenomenon into its component parts—the traces of water or eddies of the whirlpool—yet at the same time preserve the total picture, creating both an analytic and a synthetic vision.