The Three Elements
The Three Elements refer to heaven, earth, and man. When explaining the trigrams, Commentary on The Book of Changes proposes the idea of “the Three Elements.” In a trigram which consists of six undivided and divided lines, the first and second lines at the bottom represent earth, the third and fourth lines in the center represent man who lives between earth and heaven, and the fifth and sixth lines at the upper part represent heaven. Collectively, the six lines united in one diagram signify the whole of heaven, earth, and man. The three elements share the same rules but have different manifestations of rules in their each field.
So the law of heaven is governed by yin and yang; the law of earth is governed by softness and hardness; and the law of man is governed by benevolence and righteousness. Each trigram, described in The Book of Changes, consists of six lines with each two being a unit representing heaven, earth and man. (The Book of Changes)
Each trigram in The Book of Changes consists of three elements: the qi of yin and yang representing heaven, the quality of softness and hardness representing earth, and the virtue of benevolence and righteousness representing man. (Zhang Zai: Zhang Zai’s Explanation of The Book of Changes)