 For many animals, the ways in which physical movements vary with the varieties of visual cues is relatively fixed: cats pounce on mice (some of cloth), and swat at moths (or moving shadows on a sunlit wall). For human beings, thanks to the brain’s plasticity in forming “joint impressions,” there are many ways in which the “varieties” of one sensation “vary with the varieties” of another: at the sight of notes on a staff, fingers move determinately upon a piano keyboard. Expert musicians, at the sight of the notes, hear the tune in their mind’s ear.