Zhu Xi maintained that all things are brought into being by the union of two universal aspects of reality: qi, sometimes translated as vital (or physical, material) force; and li, 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 (fire, water, wood, metal, and earth) evolve. 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 (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 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.

Zhu Xi considered the earlier Confucian Xun Zi to be a heretic for departing from Mencius' idea of innate human goodness. Even if people displayed immoral behavior, the supreme regulative principle was good. The cause of immoral actions is qi. Zhu Xi's metaphysics is that everything contains li and qi. Li is the principle that is in everything and governs the universe. Each person has a perfect li. As such, individuals should act in perfect accordance with morality. However, while li is the underlying structure, qi is also part of everything. Qi obscures our perfect moral nature. The task of moral cultivation is to clear our qi. If our qi is clear and balanced, then we will act in a perfectly moral way.

According to Zhu Xi's epistemology, knowledge and action were indivisible components of truly intelligent activity. Although he did distinguish between the priority of knowing, since intelligent action requires forethought, and the importance of action, as it produces a discernible effect, Zhu Xi said "Knowledge and action always require each other. It is like a person who cannot walk without legs although he has eyes, and who cannot see without eyes although he has legs. With respect to order, knowledge comes first, and with respect to importance, action is more important."
Zhu Xi advocated gewu, the investigation of the things. How to investigate and what these things are is the source of much debate. To Zhu Xi, the things are moral principles and the investigation involves paying attention to everything in both books and affairs because "moral principles are quite inexhaustible".

Zhu Xi did not hold to traditional ideas of God or Heaven (Tian), though he discussed how his own ideas mirrored the traditional concepts. He encouraged an agnostic tendency within Confucianism, because he believed that the Supreme Ultimate was a rational principle, and he discussed it as an intelligent and ordering will behind the universe (while stating that "Heaven and Earth have no mind of their own" and promoting their only function was to produce things. Whether this can be considered a conscious or intelligent will is clearly up to debate). He did not promote the worship of spirits and offerings to images. Although he practiced some forms of ancestor worship, he disagreed that the souls of ancestors existed, believing instead that ancestor worship is a form of remembrance and gratitude.

Zhu Xi practiced a form of daily meditation called jingzuo similar to, but not the same as, Buddhist dhyana or chan ding. His meditation did not require the cessation of all thinking as in Buddhism; rather, it was characterized by quiet introspection that helped to balance various aspects of one's personality and allowed for focused thought and concentration.

His form of meditation was by nature Confucian in the sense that it was concerned with morality. His meditation attempted to reason and feel in harmony with the universe. He believed that this type of meditation brought humanity closer together and more into harmony.

Zhu Xi heavily focused his energy on teaching, claiming that learning is the only way to sagehood. He wished to make the pursuit of sagehood attainable to all men.

He lamented more modern printing techniques and the proliferation of books that ensued. This, he believed, made students less appreciative and focused on books, simply because there were more books to read than before. Therefore, he attempted to redefine how students should learn and read. In fact, disappointed by local schools in China, he established his own academy, White Deer Hollow Academy, to instruct students properly and in the proper fashion.

Zhu Xi wrote what was to became the orthodox Confucian interpretation of a number of concepts in Taoism and Buddhism. While he appeared to have adopted some ideas from these competing systems of thought, unlike previous Neo - Confucians he strictly abided by the Confucian doctrine of active moral cultivation. He found Buddhist principles to be darkening and deluding the original mind as well as destroying human relations.

From 1313 to 1905, Zhu Xi's commentaries on the Four Books formed the basis of civil service examinations in China. His teachings were to dominate Neo - Confucians such as Wang Fuzhi, though dissenters would later emerge such as Wang Yangming and the School of Mind two and a half centuries later.

His philosophy survived the Intellectual Revolution of 1917, and later Feng Youlan would interpret his conception of li, qi, and taiji into a new metaphysical theory.

He was also influential in Japan known as Shushigaku (朱子学, School of Zhu Xi), and in Korea known as Jujahak (주자학), where it became an orthodoxy.

Life magazine ranked Zhu Xi as the forty - fifth most important person in the last millennium.

This renowned neo - Confucianist, educator and thinker from Southern Sung dynasty had, from an early age, followed his father and a number of great calligraphers at the time in practicing this art. At first he learned the style of Cao Cao, but later specialized in the regular script of Zhong Yao and the running cursive script of Yan Zhenqing. As he never ceased practicing, he reached a superb level in the art characterized by overpowering strength. Since then, though his manuscripts left to the world are piecemeal and incomplete, they have been regarded as invaluable for collection. While he bequeathed to posterity quite a bit of calligraphy which has been highly acclaimed in history, it is regrettable that most of it has been lost. Moreover, since the Yuan dynasty, his school of philosophy has been adopted as the official ideology of China. His philosophy not only profoundly affected traditional Chinese thinking and culture, but also spread outside China with tremendous influence. He has been hailed as one of the ten key philosophers of the Confucian School. His fame in the realm of philosophy was so great that even his brilliance in calligraphy was overshadowed. He was skillful in both running and cursive scripts, and more especially in large characters. His extant artworks consist mainly of short written notes in running script and rarely of large characters. His authentic manuscripts are collected by Nanjing Museum, Beijing Palace Museum, Liao Ning Province Museum, China; Taipei Palace Museum and the National Museum of Tokyo, Japan. Some pieces are in private collections in China and overseas. The Thatched Hut Hand Scroll, one of Zhu Xi's masterpieces in running - cursive script, is in an overseas private collection.

Thatched Hut Hand Scroll contains three separate parts:

1. Title
2. 102 characters by Zhu Xi in running cursive scripts
3. The postscripts by Wen Tianxiang (1236 ～ 1283) of Sung dynasty, Fang Xiaoru (1375 ～ 1402), Zhu Yunming (1460 – 1526), Tang Yin (1470 ～ 1523) and Hai Rui (1514 ～ 1587) of the Ming dynasty.

The calligraphy of Zhu Xi had been acclaimed as acquiring the style of the Han and Wei dynasties. He was skillful in the central tip, and his brush strokes are smooth and round, steady yet flowing in the movements without any trace of frivolity and abruptness. Indeed, his calligraphy possesses stability and elegance in construction with a continuous flow of energy. Without trying to be pretentious or intentional, his written characters are well balanced, natural and unconventional. As he was a patriarch of Confucianism philosophy, it is understandable that his learning permeated in all his writings with due respect for traditional standards. He maintained that while rules had to be observed for each word, there should be room for tolerance, multiplicity and naturalness. In other words, calligraphy had to observe rules and at the same time not be bound by them so as to express the quality of naturalness. It's small wonder that his calligraphy had been highly esteemed throughout the centuries, by great personages as follows:

Tao Chung Yi (around 1329 ～ 1412) of Ming dynasty: Whilst Master Zhu inherited the orthodox teaching and propagated it to the realm of sages and yet he was also proficient in running and cursive scripts, especially in large characters. His execution of brush was well - poised and elegant. However piecemeal or isolated his manuscripts, they were eagerly sought after and treasured.

Wang Sai Ching (1526 – 1590) of Ming dynasty: The brush strokes in his calligraphy were swift without attempting at formality, yet none of his strokes and dots were not in conformity with the rules of calligraphy.

Wen Tianxiang of Sung dynasty in his postscript for the Thatched Hut Hand Scroll by Zhu Xi: People in the olden days said that there was embedded the bones of loyal subject in the calligraphy of Yan Zhenqing. Observing the execution of brush strokes by Zhu Xi, I am indeed convinced of the truth of this opinion.

Zhu Yunming of Ming dynasty in his postscript for the Thatched Hut Hand Scroll by Zhu Xi: Master Zhu was loyal, learned and a great scholar through out ages . He was superb in calligraphy although he did not write much in his lifetime and hence they were rarely seen in later ages. This roll had been collected by Wong Sze Ma for a long time and of late, it appeared in the world. I chanced to see it once and whilst I regretted that I did not try to study it extensively until now, in the study room of my friend, I was so lucky to see it again. This showed that I am destined to see the manuscripts of master Zhu. I therefore wrote this preface for my intention.

Hai Rui of Ming dynasty in n his postscript for the Thatched Hut Hand Scroll by Zhu Xi: The writings are enticing, delicate, elegant and outstanding. Truly such calligraphy piece is the wonder of nature.