In my opinion, Langer is trying to distinguish between the actual music and what our minds create of it. Music is often thought of as something without substance, it does not have a physical forms. Though today, you could consider instruments, sheet music, or cd to be a physical form of music, the primary focus is on the sound, the non-physical occurrence of hearing music and not physically being able to touch it or hold it. I think this is what Langer is trying to focus on. When we hear music, something happens in our minds. We associate the music with emotions, experiences, and images. This is the illusion, the thing we create in response to the music. Music inspires this illusion but it is not the music itself thus we create something outside of the music itself. This experience is completely within our imagination and perspective and that is why believe Langer chooses the work illusion.