The Books of Changes,
Books Of Changes, APPENDED REMARKS," PT. 1, Ch. 4, in Wing-Tsit Chan, Chinese Philosophy, Chapter 13.
Exactly what is meant by "spirit" is not clear, but it is surely not the spirit of a deceased person that influences human affairs. Traditionally kuei-shen means either simply spirits of ancestors or spiritual beings. In the latter case, it may mean either good or evil spirits or the positive and negative aspects of the soul, respectively. But here it is simply the unfathomable force behind all transformations. Later in Neo-Confucianism, it is to be understood purely as the spontaneous activity of yin and yang.