Poetry is an expression of a person's feelings and aspirations. When hidden in his heart, it is just his feelings and aspirations. When put forth through the medium of words, it becomes what is known as poetry. Feelings are expressed in language. Then if language is insufficient, he will start to hum and sigh. If even that fails to fully express himself, he will burst into song, accompanied sometimes by rhythmical bodily movements. When feelings surge up in his heart, they will naturally find expression via sounds. All five notes of varying heights thus form a harmonious melody. (Introductions to Mao’s Version of The Book of Songs)

Normally,when a person is moved by external objects or scenery, a feeling will spring up from deep within, thus producing the sound of humming and sighing. Then he will express such a feeling through chanting; hence a poem which can be sung out loud. (Bai Juyi: Collection of Essays in Preparation for the Final Round of the Imperial Examination)