The eight trigrams are symbols standing for changing transitional states; they are images that are constantly undergoing change.
These eight trigrams were conceived as images of all that happens in heaven and on earth. At the same time, they were held to be in a state of continual transition, one changing into another, just as transition from one phenomenon to another is continually taking place in the physical world. Here we have the fundamental concept of the Book of Changes. The eight trigrams are symbols standing for changing transitional states; they are images that are constantly undergoing change. Attention centers not on things in their state of being—as is chiefly the case in the Occident—but upon their movements in change. The eight trigrams therefore are not representations of things as such but of their tendencies in movement.
These eight images came to have manifold meanings. They represented certain processes in nature corresponding with their inherent character. Further, they represented a family consisting of father, mother, three sons, and three daughters, not in the mythological sense in which the Greek gods peopled Olympus, but in what might be called an abstract sense, that is, they represented not objective entities but functions.
The sons represent the principle of movement in its various stages—beginning of movement, danger in movement, rest and completion of movement. The daughters represent devotion in its various stages—gentle penetration, clarity and adaptability, and joyous tranquility.
Richard Wilhelm from Introduction to The Book of Changes