Jingxi Zhanran 荊渓湛然 (711–782) interpreted the concept of Seeing the Inconceivable Realm (guan fousiyi jing 観不思議境) taught in the Mohe zhiguan 摩訶止観 as being comprised of Three Realms (san jing 三境). These Three Realms are the Realm in Principle (xingde jing 性徳境), the Realm of Practice (xiude jing 修徳境) and the Teaching Realm (huata jing 化他境). In the Song period, Tiantai monks developed various explanations of these Three Realms. Yugang Mengrun’s 玉岡蒙潤 (1275–1342) Tiantai sijiaoyi jizhu 天台四教儀集註,which was used widely in the Edo period, contains a unique interpretation of the Realm in Principle and the Realm of Practice. Mengrun declared that the place of practice exists only in the Realm of Practice. This view was criticized in the Edo period. Although Mengrun’s explanation resembles the dominant theory advanced in Northern Song period Tiantai circles, the two theories differ regarding their understanding of the Realm in Principle. The present paper shows that Mengrun’s explanation originates from Southern Song period opposition to Caotang Chuyuan’s 草堂処元 (–1103–) interpretation. Furthermore, the paper argues that one aspect of the strong theory in the Northern Song period emerged from Shenzhi Congyi’s 神智従義 (1042–1091) contribution to this debate.