Xin and Yi: Two Minds
The Wuji state exists inside each of us. It is the state from which all creative impulses grow. Taiji is generated out of Wuji and is the mother of Yin and Yang. Thus, Taiji is the cause of the Yin and Yang division, and is itself neither Wuji nor Yin and Yang, but the cause of the separation of Yin and Yang. In this sense it is a part of the divine aspect of the Dao. All things can be classified as either Yin or Yang. Taijiquan was created according to this theory. In the beginning posture of the Taijiquan sequence, the mind is calm and empty, and the weight is evenly distributed on both feet. This state is Wuji. When your mind starts to lead the body into the posture of Grasp Sparrow’s Tail , internal (Yin) and external (Yang) aspects of Taijiquan features start to be discriminated. Moreover, the hands and feet are differentiated into insubstantial (Yin) and substantial (Yang). This is the state of Two Polarities. Through interaction of substantial (Yang) and insubstantial (Yin), all of Taijiquan’s fighting strategies and techniques are generated. From this, you can see that the Taiji (i.e., the Dao) in Taijiquan is actually the mind. It is the mind that makes the body move and divides the Wuji state into Yin and Yang two polarities. We can conclude from this that Taijiquan is actually a martial art of the mind. Taiji means "grand ultimate". In the body, the mind is the grand ultimate that initiates movement, Qi circulation, and discrimination between yin and yang.
Xin and Yi are able to reach anywhere in the universe without being restricted by time and space. From Xin and Yi, the Yin and Yang are initiated and continue to move into unlimited variations. This is the theory of millions of divisions and creations of Taiji. Therefore, those who practice Taijiquan must begin from the training of Xin and Yi. Though our physical bodies are restricted by our three-dimensional reality, our minds are free to travel and reach anywhere in the universe, unrestricted by time, or even beyond this universe. All human creations, from shovels to airplanes, arose first in our imaginations. From our thoughts, new ideas are created. It is the same for Taijiquan. It was created from the mind, and its creation will continue without an end. Since it is an active, living, and creative art, Taijiquan is a product of spiritual enlightenment and an understanding of life.
The emotional mind and wisdom mind are contained internally, which belongs to Yin. The movements (actions) of Taijiquan are manifested externally, which belongs to Yang. When the functions of Xin and Yi are applied to our spiritual feeling, they direct us into the correct Dao of cultivating our human nature, through efforts toward strengthening the mind, raise up the spirit, and comprehend the real meaning of human life, and from this, further to comprehend the meaning and relationship among humans, between humans and objects around us, and also to search for the truth of nature in heaven and earth. When the function of Xin and Yi is applied to our physical body, it is the great Dao of cultivating the physical life for self-defense, nourishing the physical life, and strengthening the physical body. This is the foundation for extending our lives and establishing a firm root of health.
When Xin and Yi are acting on internal spiritual feeling, it serves to cultivate our human temperaments and helps us to understand the meaning of our lives. When Xin and Yi are acting and manifested externally, it promotes physical health and self-defense. Therefore, when we practice Taijiquan, we should cultivate both our spiritual beings (Yin) and train our physical bodies (Yang). Taijiquan originated from the Daoist family. Its ultimate goal is to reach enlightenment and so as to achieve the Dao of unification between heaven and human. Therefore, the final goal of practicing Taijiquan is to reach the unified harmonious Wuji state of heaven and human. From practicing Taijiquan, we are able to further comprehend the meaning of human life and the universe.
This subject is discussed in the book "Taijiquan Theory of Dr. Yang, Jwing-Ming".