Ancient Chinese philosophers usually explore humans through the concept of "nature." The discussions in pre-Qin and Han dynasties Confucianism on human nature are based upon a common thesis: nature is "what human beings receive to live." This approach is criticized after the introduction of Buddhism to China, as Sanskrit users meditate on human nature from the perspective of invariability. Buddhists think that the ancient Chinese concept of human nature is samskrtadharma (conditioned dharmas) for it is developed from phenomenal recognition, while the true nature should be asamskrta-dharma (unconditioned dharmas) or even Buddha, which can be furthermore comprehended in terms of dharmata (dharmanature) and buddha-dhatu (Buddha-nature). Therefore, the search of buddha-dhatu becomes the core of Buddhism from the Southern and Northern dynasties to the Tang dynasty. During the incorporation of Buddhism into central mainland China, the translation of Sanskrit cosmology and thought into Chinese bring about the fusion of Chinese and Sanskrit thoughts. Amid this context originates Zhang Zai's theory of heaven-earth nature and psycho-physical nature. The psycho-physical nature means the nature which human beings receive to live, which conforms to ancient Chinese concepts; the heaven-earth nature refers to the true and unchanging nature, which is influenced by Sanskrit thought. Zhang Zai uses the structure of "inherent nature" and "realistic nature" to relate the two concepts of nature. As a synthesis of Sanskrit and Chinese thought, this structure resolves the pre-Qin and Han dynasties debate over human nature and the predicament of buddha-dhatu practice. Hence very soon it is accepted by other Neo-Confucianists and the Daoist school Neidan (internal alchemy) and becomes the core theory of modern Confucianism and Daoism.