Jang, back-dan achieved a synthesis of Naedan() theory. By his work, Naedan theory was accepted by people in a comprehensive way and it became the orthodox theory in the practice of Taoism. Basically, his Naedan theory can be understood by the three important discourses or by the theory and technique of Dan. The three important discourses are cauldrons(), medical material() and heating control (). The cauldrons() are the specific parts of human body where the medical materials are gained and processed. The medical materials() are the sexual energy, vital energy and spiritual energy() of human body. They are divided into the pure and the impure ones. The heating control() is gathering and processing of the medical materials. There, the spiritual energy and the pure will take the leading roles. This is the process of harmonizing the energy of human body. The theory of Naedan is based on the principle of the cosmic dual forces(). Human body is a copy of the universe and the rule of creation is the rule of life. Thus, understanding and adapting the rule of creation leads a man to perpetual youth. The rule of creation is a forward movement from nonexistence to existence, and there is life and death. To practice and become a Sinseun() is to reverse this movement. That is the way of going from three() to two(), from two to one(), from one to nonexistence().