There are two opposing schools in music aesthetics - autonomy and heteronomy. Their fundamental difference is: heteronomous theory believes that the laws and regulations that restrict music come from outside music, that is, music is determined by some external laws. This is because music always marks something beyond purely acoustic phenomena - human emotion, which is what music is about. It is the nature of this content that determines the structure, development, and form of a musical work. Autonomous aesthetics believes that the rules and regulations that restrict music do not come from outside music, but are within the music itself. The essence of music can only be understood in the sound structure itself, and can only be grasped from the music itself. The content of music can only be music itself. Music expresses nothing and means nothing but itself. It denies the duality of content and form in music, believing that the two are unified and that sound structure is everything.