Ever the perfectionist, Leonardo turned to science in the quest to improve his artwork. His study of nature and anatomy emerged in his stunningly realistic paintings, and his dissections of the human body paved the way for remarkably accurate figures. He was the first artist to study the physical proportions of men, women and children and to use these studies to determine the "ideal" human figure. Unlike many of his contemporaries -- Michelangelo for example -- he didn't get carried away and paint ludicrously muscular bodies, which he referred to as "bags of nuts."
All in all, Leonardo believed that the artist must know not just the rules of perspective, but all the laws of nature. The eye, he believed, was the perfect instrument for learning these laws, and the artist the perfect person to illustrate them.