Yang Cheng Fu's introduction

​Taijiquan is based on the Taiji and Bagua of the Book of Changes, and the ideas of Li (principles), Qi (energy), and Xiang (image) help to give shape to these concepts. Confucius said of the Changes, “Modeled on heaven and earth’s transformations, it never goes beyond them.

​”How does this come out of Li, Qi, and Xiang? It is only that Li, Qi, and Xiang are the unfinished beginnings of Taijiquan. When one is able to realize all three, then the form and function are integrated. Thus, Xiang takes its model from the Taiji and the Bagua. Qi does not depart from yin and yang, hard and soft. Li orders what is changeless in change so that one may thoroughly know transformation. 

Students must especially begin by seeking the Xiang in order to nurture the Qi. Given time, they will come to understand the Li spontaneously.

Source: The Essence and Applications of Taijiquan (p. 20). North Atlantic Books

Yang Cheng Fu (click to enlarge)