The concept of Non-obstruction (Skt. apratigha) between separate phenomena is often used to characterize the essence of Huayan One Vehicle thought. Generally, the concept of non-obstruction of separate phenomena implies that individual things resonate with each other, and that this is due to the fact that they are each based on a universal principle which unifies diversity. Zhi-yan expressed the experiential realm of the Huayan One Vehicle with the simile of Indra's Net. According to him, such a view of existence is not seen in the realm of the Three Vehicles, but only in the realm of Samantabhadra-bodhisattva. Succeeding Zhi-yan, Ui-sang revealed the Huayan experiential realm through the function of the principle of dharani. Such a world of the Huayan symbolized in dharani reveals a chaotic place, where not only mutual identity but also non-mutual identity is possible. Fa-zang emphasized the six types of dependent co-arising causality as the basis for understanding the non-obstruction theory. This means that all the experiential realms, which arise due to mutual relationships with each other, are formed by relations between causes and conditions. Weon-hyo often expressed the world of dependent origination by citing the relationship between the large and the small. That is, the large and the small are the same because of their lacking of substance. Hui-yuan held non-self-nature as the main theory for the basis of the view of non-obstruction, casting aside Fa-zang's explanation, which was based on the power of dependent co-arising as mutual causation. Furthermore, although Cheng-guan strongly criticized Hui-yuan's opinion, he also considered the principle of dharma-nature to be the basis for non-obstruction. Therefore, we can see the extent of Hui-yuan's influence on non-obstruction theory after Fa-zang. The influence of Weon-hyo and Fa-zang on Huayan studies of the Nara period in Japan is tremendous. In the Gokyousho-shiji, Jurei developed his dependent origination theory based on the approaches of Weon-hyo and Fa-zang. The specific feature of the Japanese Huayan School is seen in the Kegon-gokyo jushu-daii-ryakusho, which was written some time after 851. In this text, non-obstruction is expressed by the three kinds of perfect infusion theories -- the perfect infusion of phenomena and principle, separate phenomena, principle and principle are introduced as the perfect infusion of One Vehicle. Previous studies on non-obstruction theory in the Kegon School have not sufficiently examined this point, so it should be taken into account henceforth.
