Vital force (qi), principle (li), and the Great Ultimate (taiji)
He argued that all things are brought into being by two universal elements:
qi, translated as vital (or physical, material) force; and
li, translated as rational principle (or law). The source and sum of
li is the
Taiji (
Wade-Giles: Tai Chi), meaning the Great Ultimate.
According to Zhu Xi's theory, every physical object and every person contains
li and therefore has contact with the
Taiji. What is referred to as the human soul, mind, or spirit is defined as the
Taiji, or the supreme regulative principle at work in a person.
Qi and
li operate together in mutual dependence. These are not entirely non-physical forces; one result of their interaction is the creation of matter. When their activity is rapid the
yang energy mode is generated, and when their activity is slow, the
yin energy mode is generated. The yang and yin constantly interact, gaining and losing dominance over the other. This results in the structures of nature known as the five elements (fire, water, wood, metal, and earth).
In terms of
li and
qi, Zhu Xi's system strongly resembles
Buddhist ideas of li (again, principle) and shi (affairs, matters), though Zhu Xi and his followers strongly argued that they were not copying Buddhist ideas. Instead, they held, they were using concepts present in the Book of Changes.
Zhu Xi discussed how he saw the Great Ultimate concept to be compatible with principle of Taoism, but his concept of
Taiji was different from the understanding of
Tao in Daoism. Where
Taiji is a differentiating principle that results in the emergence of something new, Dao is still and silent, operating to reduce all things to equality and indistinguishability. He argued that there is a central harmony that is not static, empty but dynamic, and that the Great Ultimate is in constant movement.