information:classification, linguistic:translation, structural:expansion
Vital force, principle, and the Supreme Ultimate

Zhu Xi maintained that all things are brought into being by the union of two universal aspects of reality: qi (氣, sometimes translated as vital – or physical, material – force); and li (理, sometimes translated as rational principle or law). The source and sum of li is the taiji, meaning the Supreme Ultimate. The source of qi is not so clearly stated by Zhu Xi, leading some authorities to maintain that he was a metaphysical monist and others to maintain that he was a metaphysical dualist.

According to Zhu Xi's theory, every physical object and every person has its li and therefore has contact in its metaphysical core with the taiji. What is referred to as the human soul, mind, or spirit is understood as the taiji, or the supreme creative principle, as it works its way out in a person.

Qi and li operate together in mutual dependence. They are mutually aspective in all creatures in the universe. These two aspects are manifested in the creation of substantial entities. When their activity is waxing (rapid or expansive), that is the yang energy mode. When their activity is waning (slow or contractive), that is the yin energy mode. The yang and yin phases constantly interact, each gaining and losing dominance over the other. In the process of the waxing and waning, the alternation of these fundamental vibrations, the so-called five elements evolve (fire, water, wood, metal, and earth). Zhu Xi argues that li existed even before Heaven and Earth.

In terms of li and qi, Zhu Xi's system strongly resembles Buddhist ideas of li (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 already present long before in the I Ching.

Zhu Xi discussed how he saw the Supreme 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 or empty but was dynamic, and that the Supreme Ultimate is itself in constant creative activity.