Fazang (Fa-tsang, 643–712), one of the most important Huayan thinkers, wrote 'Essay on the Golden Lion' and 'Treatise on the Five Teachings', which contain other metaphors for the interpenetration of reality. He also used the metaphor of a House of mirrors. Fazang introduced the distinction of "the Realm of Principle" and "the Realm of Things". This theory was further developed by Cheng-guan (738–839) into the major Huayan thesis of "the fourfold Dharmadhatu" (dharma realm): the Realm of Principle, the Realm of Things, the Realm of the Noninterference between Principle and Things, and the Realm of the Noninterference of All Things. The first two are the universal and the particular, the third is the interpenetration of universal and particular, and the fourth is the interpenetration of all particulars. The third truth was explained by the metaphor of a golden lion: the gold is the universal and the particular is the shape and features of the lion.