The unique fame that Leonardo enjoyed in his lifetime and that, filtered by historical criticism, has remained undimmed to the present day rests largely on his unlimited desire for knowledge, which guided all his thinking and behaviour. An artist by disposition and endowment, he considered his eyes to be his main avenue to knowledge; to Leonardo, sight was man’s highest sense because it alone conveyed the facts of experience immediately, correctly, and with certainty. Hence, every phenomenon perceived became an object of knowledge, and saper vedere (“knowing how to see”) became the great theme of his studies. He applied his creativity to every realm in which graphic representation is used: he was a painter, sculptor, architect, and engineer. But he went even beyond that. He used his superb intellect, unusual powers of observation, and mastery of the art of drawing to study nature itself, a line of inquiry that allowed his dual pursuits of art and science to flourish.