Ts'ai-wu said, "I have heard the names kuei and shen, but I do not know what they mean." The Master said, "The [intelligent] spirit is of the shen nature, and shows that in fullest measure; the animal soul is of the kuei nature, and shows that in fullest measure. It is the union of kuei and shen that forms the highest exhibition of doctrine.
"All the living must die, and dying, return to the ground; this is what is called kuei. The bones and flesh molder below, and, hidden away, become the earth of the fields. But the spirit issues forth, and is displayed on high in a condition of glorious brightness. The vapors and odors which produce a feeling of sadness,[and arise from the decay of their substance], are the subtle essences of all things, and also a manifestation of the shen nature."
(The Book of Ritual)