《文心雕龙·声律》
在外的声音容易掌握，是由于可以用手决定琴弦；内心的声音不好控制，则由于声音和心思纷乱不一。这只能从掌握音律技巧来求得解决，是难以用文辞说明白的。
故外听之易，弦以手定，内听之难，声与心纷；可以数求，难以辞逐。
The outside sound is easy to control because one can use their hand to determine the strings; the inner sound is not easy to control because the sound and the thoughts are chaotic and inconsistent. This can only be solved by mastering the skills of melody, and it is difficult to explain it clearly with words.