"The good painter has to paint two principal things, man and the intention of his mind," he wrote. "The first is easy and the second is difficult, because the latter has to be represented through gestures and movements of the limbs."44 He expanded on this concept in a long passage in his notes for his planned treatise on painting: "The movement which is depicted must be appropriate to the mental state of the figure. The motions and postures of figures should display the true mental state of the originator of these motions, in such a way they can mean nothing else. Movements should announce the motions of the mind."45 Leonardo's dedication to portraying the outward manifestations of inner emotions would end up driving not only his art but some of his anatomical studies. He needed to know which nerves emanated from the brain and which from the spinal cord, which muscles they activated, and which facial movements were connected to others. He would even try, when dissecting the brain, to figure out the precise location where the connections were made between sensory perceptions, emotions, and motions. By the end of his career, his pursuit of how the brain and nerves turned emotions into motions became almost obsessive. It was enough to make the Mona Lisa smile."