Li （理）is a concept found in Neo-Confucian Chinese philosophy. It refers to the underlying reason and order of nature as reflected in its organic forms.
It may be translated as rational principle or law. It was central to Zhu Xi's integration of Buddhism into Confucianism. Zhu Xi held that li, together with qi (氣: vital, material force), depend on each other to create structures of nature and matter. The sum of li is the Taiji.
This idea resembles the Buddhist notion of li, which also means principle. Zhu Xi maintained, however, that his notion is found in I Ching (Book of Changes), a classic source of Chinese philosophy. Zhu Xi's school came to be known as the School of Li, which is comparable to rationalism. To an even greater extent than Confucius, Zhu Xi had a naturalistic world-view. His world-view contained two primary ideas: qi and li. Zhu Xi further believed that the conduct of the two of these took places according to Tai Ji.
Holding to Confucius and Mencius' conception of humanity as innately good, Zhu Xi articulated an understanding of li as the basic pattern of the universe, stating that it was by understanding these principles that one could live with li and live an exemplary life. In this sense, li according to Zhu Xi is often seen as similar to the in Daoism or to in Platonic philosophy. Wang Yangming, a philosopher who opposed Zhu Xi's ideas, held that li was to be found not in the world but within oneself. Wang Yangming was thus more of an idealist with a different epistemic approach.
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