Some of the paintings could not be finished for practical reasons. For example, because Leonardo never learned the proper way to paint on a wall, the Battle of Anghiari was a failure from the start: the paint began to run off the wall. Leonardo was so disgusted by this failure that he didn't want to waste any more time on it, and quit. His work on Ludovico Sforza's bronze horse faced similar problems. Because the statue was to be so large, it required a huge amount of bronze, and because Milan was at war, this bronze was not available. The Adoration of the Magi was probably left incomplete because Leonardo left for Milan while still at work on it. But whether or not these are adequate explanations, there are some cases for which there is almost no explanation: Leonardo's first commission was an Adoration of the Shephards, which he made preliminary sketches for but never really began. Perhaps he lost interest. Similarly, we do not know why Saint Jerome and the Portrait of a Musician remain unfinished. One can speculate that Leonardo was fascinated by so many things, he found it hard to finish a painting he had lost interest in, even if he needed the money. After all, his wide-ranging interests indicate not only a broad talent, but probably a rather short attention span. More probably, some critics argue, Leonardo was interested in discovering the perfect designe, or composition for a painting, the solution to how to approach a given work; once having found these, he felt that his work was done; he was more interested in the puzzle of a work than in actually realizing the solution once found.

Leonardo made no note of his reasons for leaving, and we do not know exactly why he left the artistic capital of the quatrocento. He could have been embarassed by the accusations of sodomy; indeed, the accusations could have been part of a more extensive smear campaign. His failure to make good on his first two commissions, the Adoration of the Shephards and the Adoration of the Magi, could have further damaged his reputation there, making it hard for him to earn a living. When he went to Milan, he went with the intention of gaining the patronage of Ludovico Sforza. He composed a long letter to Sforza, detailing the military implements he was capable of devising, and also offering his services in the design of a bronze horse to commemorate Sforza's father. Although scholars suspect that Leonardo never mailed this letter, it is clear that he was looking for a patron who would be interested in funding more than painting. Milan attracted him not only with its influential inhabitants, but also with the city's own topology: Milan was a city on a plain, removed from the coast or any river, and it required a complex irrigation system, something that must have been tempting to Leonardo's scientific mind.

Leonardo believed in representing nature accurately. He did not want his paintings to by symbols; he wanted them to be pictures of what he saw, or of what he might have seen in another place and time. Compare his Last Supper to a "Last Supper" by any earlier artist to see how lifelike Leonardo's images were. His studies of anatomy, geology, and other subjects helped him represent life accurately, although he sometimes disregarded his knowledge, as in Saint John the Baptist. Leonardo wanted his paintings to be scientifically correct, but he was also careful to show emotions. He wanted to show both the inside and the outside of a figure. Thus, he chose evocative poses and faces to express feelings. And yet, we should not think of Leonardo as someone who loved only the brutal truth of appearance and emotion: he cast his subjects in shadow, using the technique of sfumato to avoid harsh outlines. He chose light that was warm. Although he strove for accuracy, he did not let his desire to be realistic interfere with his idea of the beautiful.