Heaven is high, the earth is low, and thus ch'ien (Heaven) and Pun (Earth) are fixed. As high and low are thus made clear, the honorable and the humble have their places accordingly. As activity and tranquillity have their constancy, the strong and the weak are thus differentiated. Ways come together according to their kinds, and things are divided according to their classes. Hence good fortune and evil fortune emerge. In the heavens, forms (heavenly bodies) appear and on earth shapes (creatures) occur. In them change and transformation can be seen. Therefore the strong and the weak interact and the Eight Trigrams activate each other. Things are stimulated by thunder and lightning and enriched by the influence of wind and rain. Sun and moon revolve on their course and cold and hot seasons take their turn. The way of ch'ien constitutes the male, while the way of k'un constitutes the female. Ch'ien knows the great beginning, and k'un acts to bring things to completion. Ch'ien knows through the easy, and k'un accomplishes through the simple.
(The Books of Changes)